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Antennas and You Non Technical Advice for the Non-Ham
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Understanding Antennas For the Non-technical Ham by N4JA
The reason the polarization of antennas is most important is that it determines the angle of radiation. Horizontally polarized antennas at ordinary heights used …
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Understanding Antennas For The Non-Technical Ham
a book by jim abercrombie n4ja reason for writing this book is to educate you so you can make an informed choice concerning the best antenna for you another …
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Understanding Antennas for the Non-Technical Ham A Book By Jim Abercrombie, N4JA (Jim Abercrombie [email protected]) Illustrations by Frank …
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FREE!! – UNDERSTANDING ANTENNAS FOR THE NON …
Written by the late Jim Abercrombie, N4JA and with illustrations by Frank Wamsley, K4EFW, Understanding Antennas for the Non-Technical Ham …
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Understanding Antennas for The Non-Technical Ham
It was written primarily for the newcomer and the non-technical old-timer. This book is about common medium-wave and high-frequency (short wave) …
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Understanding Antennas for the Non-Technical Hamhttps://www.survivalistboards.com/d3/downloads/95248-basicantennas.pdf.
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Understanding Antennas For The Non-Technical Ham. Leave a reply. A free PDF antenna book, available from Hamuniverse at: …
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Antenna Basics for Non Technical Hams – HAM … – DOCUMENTS
Download Antenna Basics for Non Technical Hams – HAM · PDF fileUnderstanding Antennas For The Non -Technical Ham A Book By Jim Abercrombie, N4JA (Jim…
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Understanding Antennas For the Non-technical Ham by N4JA
Understanding antennae for the non-technical ham
A book by Jim Abercrombie, N4JA (SK)
Illustrations by Frank Wamsley, K4EFW Edited by Judy Haynes, KC4NOR Copyright July 2005. Second Edition
Edited for the Web, N4UJW Editor’s Note: This is a book-length web article made available FREE by the author to all radio amateurs.
This is copyrighted material and property of Hauniverse.com and/or the author of the article and may be used for personal, non-profit educational use only.
You can download a PDF copy of it here….74 pages!
It’s huge!
Bookmark this page for later reading, or check out more save options at the bottom of the page!
The original book contained 60 pages and illustrations.
They are all here!
Many of the antennas described here are in project form on this website.
Here are some of the main themes of the book that you will learn more about. Antenna systems, antennas, simple antenna formulas, basic antenna theory, feed lines, matching units, how antennas work, polarization of electromagnetic waves, frequency, ionosphere and modes of propagation, ground wave propagation, direct wave or line-of-sight propagation, propagation by refraction, skywave propagation, greyline propagation, long-path propagation , Ham Radio Propagation, Antenna Myths, Standing Wave Ratio, Real Antenna Systems, Flat-Top Dipole, Inverted-V Dipole, Dipole Shape Variations, Calculating the Length of a Half-Wave Resonant Dipole, The Decibel, Resistances and Reactance, Efficient Feeding of Dipoles, Cause of Feedline Radiation , Baluns, Other types of dipoles, truncated charged dipole, all-band dipoles, tilted dipole, folded dipole, double bazooka dipole, broadband coax-fed fan dipole, two-element collinear dipole, four-element collinear dipole, coaxial-fed dipoles designed with odd h harmonic frequencies, three-half-wave dipole, all-band random-length dipole, center-fed all-band random-length dipole, two-band fan-fold dipole, trapped dipole for 75 and 40 meters, extended double Zepp dipole, G5RV dipole, off-center fed dipoles, single wavelength off-center fed dipole, Carolina windom, windom dipole (Fritzel type), end-fed antennas, end-fed zepp, alternative method of feeding an end-fed zepp, random-length end-fed antenna, the half-sloper Antenna, vertical antennas, ground mounted trapped verticals, disadvantages of using quarter wave verticals, long and short verticals, unscientific observations of verticals, the inverted L vertical, vertical mobile antennas, HF mobile antenna comparisons, single wavelength single loop antennas, horizontally oriented loop , vertically oriented single loop for 40 and 80 meters, single element vertical delta loop, Ri Light Beam Antennas, Monoband Yagi, Three Element Yagi, Trapped Multiband Yagis, SteppIR Antenna, Log-Periodic Array, Cubic Quad and Delta Loop Directional Antennas, Cubic Single Band AntennaQuad, Field Meter, The Quagi, Gain vs. Front-to-Back Radio, feeder lines, antenna security, erecting antennas on poles, tower security, quarter wave matching sections of 70 ohm coax charts and much more! The book starts here!
Enjoy!
FOREWORD One reason for writing this book is to inform you so you can make an informed choice about the best antenna for you. Another reason is to dispel the many antenna myths circulating in the amateur community. The third reason is the desire to teach the average radio amateur basic antenna theory. Therefore, to achieve that goal, you should read this book cover to cover. It was written primarily for the newbie and non-technical old-timer. This book covers common medium wave and radio frequency (shortwave) antennas, but the theory presented here applies to antennas of any frequency. It is in condensed form and antenna theory is explained in a way that most radio amateurs can understand. Since many radio amateurs are mathematically challenged, only simple mathematical procedures are used. If you can add, subtract, and divide with a calculator, you’ll have no trouble reading this book. Some principles here are based on inferences from the laws of physics. Everything else in this book can be found scattered throughout The A.R.R.L. antenna book and nothing here contradicts what is written there. I. WHY ALL THE FUCK ABOUT ANTENNAS
Definition: An antenna is a piece of metal, an electrical conductor, to which you connect the radio. It broadcasts your signal and receives the signals you want to hear.
Definition: An antenna system consists of the antenna, the feed line and a possible matching unit. Most antennas are made of copper or aluminum, while most cellular antennas are made of stainless steel. A feedline consists of two conductors that carry the signal to and from the radio and to and from the antenna. A matching unit can be an antenna tuner, a series matching section, or one of several different types of matching circuits at the feed point. Does the type of antenna make a big difference? Here’s an example: In 1959, two of us were involved in testing two antennas at 15 meters. The late R. Lynn Kalmbach, W4IW, with one antenna received a 30 dB better signal report on his antenna from a station in England than we did on our antenna. (Decibels or dB will be explained later). Thirty dB means that his signal appeared, that he had 1000 times more transmit power than us. We didn’t live that far apart back then, so we couldn’t blame it on spread. We both ran about the same. Both antennas were at 50 feet. The comparison proved that a good antenna can make a difference. Lynn used a homemade G4 ZU mini beam; We used a 15 meter two element Mosely Mini-Beam that had short loaded elements. Obviously it had a lot of loss. Another example: today we hear people breaking into our ragchews with signals that are almost the same as the sound level. Why is that? The reason is that they use the wrong antennas. Their signals are twenty to thirty decibels below everyone else’s. They make contacts, but only just. Our group’s first question is “What kind of antenna are you using?” Experienced amateurs know that the antenna can make all the difference. The guy with the bad signal will sometimes blame his bad signal report on band conditions or the lack of a linear amplifier. He just sticks his head in the sand. What we’re trying to prove is next to your radio, the most important part of your transmitter is the antenna. Many years ago a vintage car said, “For every dollar you spend on a radio, you should spend two dollars on your antenna.” That also applies today. With antennas, you can do more to improve your signal strength than you ever could by increasing your power. The ability to make contacts on a particular antenna does not mean it will work well! Each antenna makes contacts, but your signals will be stronger on some antennas than others. Also, some antennas hear better than others.
II. HOW ANTENNAS WORK. For the antenna system to work properly, it must first be matched to the transmitter. That is, all modern transmitters have an output impedance of 50 ohms. Antenna systems have an impedance range from a few ohms to several thousand ohms. There are several ways to match them: trimming the length of the antenna, using an antenna tuner, matching the antenna to a length of transmission line called a matching section, or using one of several matching systems at the antenna feed point. Antenna tuning is beyond the scope of the material in this book and it is recommended that you consult a more comprehensive antenna manual. Simple half-wave dipoles eliminate the need for a matching system, since a resonant half-wave dipole has an impedance close to 50 ohms. You need to understand electromagnetism to understand how antennas work. When you connect the two poles of a direct current (DC) voltage source to the two ends of a coil of wire, current flows through the coil of wire and it becomes magnetized. The magnetized coil is known as an electromagnet. Its magnetism extends to infinity, weakening with distance. Remove the voltage and the magnetic field breaks back into the coil. When alternating current (AC) is connected to the coil, the magnetism moves out and collapses in the coil in step with the frequency of the AC source. The north and south poles of the electromagnet reverse each half cycle of AC voltage. If voltage and current can cause a coil to become magnetized, it’s the other way around: a magnetic field can create a voltage and current in a coil. This is known as Faraday’s principle of magnetic induction. A voltage is created at the ends of the coil of wire when you move any permanent magnet close to and parallel to the coil. The difference in this case is that the magnet must be kept moving. Move the magnet in one direction and the current will flow in one direction. Reverse the direction the magnet moves and the current will flow in the opposite direction. Alternating current is created by moving the magnet back and forth. An alternator rotates a coil of wire between the two poles of a magnetic field. It doesn’t matter which one moves. The coil or magnet can move. Any moving magnetic field can induce current in another coil. It doesn’t have to be a piece of metal, which we call a magnet. Imagine a moving magnetic field created by alternating current circulating in and out of a coil. When this moving magnetic field passes through a second coil nearby, it induces an alternating current in the second coil. A transformer uses this method to work. Transformers have a continuous iron core that runs from the inside of one coil through the inside of the second coil to confine magnetism inside the iron core. As a result, the transformer achieves almost 100% efficiency, since little magnetic energy escapes. A straight wire through which an alternating current flows is also surrounded by a magnetic field. But it’s a weaker field than that produced by a coil. The wire’s magnetic field radiates into space and gets weaker with distance. The magnetic field radiated by a wire is known as “electromagnetic radiation,” and a radio wave is one type of it. The radiating wire becomes the transmitting antenna. Some distance away, in a second wire in the path of these waves, current is induced by the passing electromagnetic waves. This second wire is the receiving antenna. The voltage in the receiving antenna is many times weaker than the voltage in the transmitting antenna. It can be as weak as a millionth of a volt or less and still be useful. The receiving antenna feeds this voltage to the amplifiers in the front end of the receiver, where it is amplified many thousands or millions of times. The dipole antenna consists of a wire that is broken in the middle and where it is broken each half of the wire is connected to an insulator that splits the wire in two. Two wires from the voltage source, which is the transmitter, are connected across the insulator. On one side of the dipole, current in the form of moving electrons first flows from the voltage source to one end of the dipole. In the end it reflects towards the voltage source. The same thing happens on the other half of the wire for the other half cycle of the alternating current. An antenna that is the right length for current to reach the other end of the cable when polarity changes is said to be resonant. Because electricity travels in a wire at 95% the speed of light, the number of polarity changes in one second (frequency) determines how long the wire must be to be resonant. III. POLARIZATION OF ELECTROMAGNETIC WAVES Electromagnetic waves propagate away from the wire in horizontal, vertical, oblique or circular waves. If the antenna wire is horizontal or parallel to the earth, the radiation will be horizontally polarized. A wire or conductor perpendicular to earth produces vertical radiation. A tilted wire has both horizontal and vertical components of radiation. Crossed wires connected by appropriate phase lines that shift the phase from one wire to the other by 90 degrees produce circular polarization. Amateurs operating orbiting satellites at VHF, UHF, and microwave frequencies use circular polarization. If your radio frequency signals are reflected off the ionosphere, it doesn’t matter if the other station’s antenna has the opposite polarization to yours (polarization matters for line-of-sight communication). The reflected polarized waves passing through the ionosphere are rotated slowly, resulting in fading signals (QSB). The reason polarization of antennas is most important is that it determines the beam angle. Horizontally polarized antennas at normal heights, used by radio amateurs, produce mainly high-angle radiation and weaker low-angle radiation, but that doesn’t mean that there is no low-angle radiation. It is present but weaker than high-angle radiation. However, you must place a horizontally polarized antenna more than one wavelength high to get strong, low-angle radiation. A wavelength is 280 feet at 80 meters, 140 feet at 40 meters and 70 feet at 20 meters. High angle radiation works best near stations and low angle radiation works best for distant stations (DX). A vertically polarized antenna produces mainly low-angle radiation, with its high-angle radiation being weak. For this reason, vertical antennas do not work as well as horizontal antennas at ordinary elevations for workstations less than about 500 miles away. FREQUENCY The number of times an AC voltage changes polarity per second determines its frequency. Frequency is measured in cycles per second, or Hertz (Hz). A thousand cycles per second is a kilohertz (kHz). A million hertz is one megahertz (MHz). The only difference between the 60 Hz power in your home and radio frequency (RF) is the frequency, but 60 Hz power in a wire creates electromagnetic radiation as well as radio waves. Useful radio waves start at 30 kHz and go up in frequency until you reach infrared light waves. Light is the same type of waves as RF, except light is a much higher frequency. Light waves are used like radio waves when encased in fiber optic cables. X-rays and gamma rays are found above the frequencies of light. The radio bands: The long wave band (LW) starts at 30 kHz and goes up to 300 kHz. The medium wave (MW) band ranges from 300 kHz to 3000 kHz or 3 MHz. The radio frequency (HF) band ranges from 3 MHz to 30 MHz. The very high frequency band (VHF) ranges from 30 MHz to 300 MHz. The Ultra High Frequency (UHF) band ranges from 300 MHz to 3000 MHz or 3 GHz. Above these frequencies are several microwave bands defined as the Super High Frequency Band (SHF).
V. THE IONOSPHERE AND PATHWAYS OF RF PROPAGATION The Ionosphere In the upper air about 80 km and higher, where the air molecules are far apart, radiation from the sun strips electrons from the oxygen molecules, ionizing the molecules and forming the ionosphere. The ionized oxygen molecules and their free electrons float in space and form radiation-reflecting layers. Ionosphere ionization varies with time of day, season, and sunspot cycle. The strength of the ionization also varies from day to day and hour to hour. Because the height of the ionosphere varies, the higher the ionized layer gets, the farther away the jump will be. We will define skip in Section 5 of Part V. The part of the earth’s atmosphere called the ionosphere is divided into three layers. The three layers are, from lowest to highest, the D layer, the E layer, and the F layer. Each layer affects RF radio propagation differently. At lower altitudes, the molecules of the D layer are more tightly squeezed by gravity than those at higher layers, and the free electrons easily redistribute to the molecules. The D layer requires constant radiation from the sun to maintain its ionization. Radio waves with lower frequencies, such as the frequencies of the AM broadcast band, cannot penetrate this layer and are absorbed. The higher frequency signals can pass through the D layer. The D layer disappears at night, causing AM stations to reflect off the higher layers. Because of this, AM broadcast signals travel only over bumps during the day and can be received over long distances at night. Like the broadcast band, the D-layer absorbs signals on 160 and to a lesser extent 80 meters during the day, making those bands dead. During solar flares, the D-layer becomes so heavily ionized that all high-frequency radio waves are absorbed, resulting in radio blackout. The propagation of the E layer is not well understood. Being at a lower elevation than the F layer, the E layer is responsible for the short summer jump propagation in the higher high frequency bands. The jump zone is at about 1000 miles, but double jumps can be seen at times when the E cloud covers a wide area in summer. A double jump occurs when the signal reflects off the ionosphere, then returns to the ground, is reflected off the ground back to the ionosphere, where it is reflected back to the ground. A double jump can spread the signal 2000 miles or more. The E layer forms mainly during the day and has the highest degree of ionization at noon. The E layer, like the D layer, disappears at night. Nevertheless, sporadic E propagation can and will form at night. There is a minor occurrence of sporadic E propagation during the wintertime. On rare occasions, sporadic E propagation may surprise you by occurring at any time, regardless of sunspot cycle or time of year. The F layer is the highest layer and is divided into two levels: F1 and F2. At night, F1 and F2 merge into one layer. During the day, the F1 layer plays no role in radio propagation, but F2 does. It is responsible for most long-distance radio frequency propagations at 20 meters and beyond. However, the F shift allows you to work on the lower bands DX at night. Sunspots are responsible for the ionization layers and in years with high sunspot numbers, F2 layer propagation can easily make worldwide contacts at 10-20 meters. In years with low sunspot counts, it is difficult to operate remote stations on these bands. Consequently, ten and fifteen yards will be totally dead most days and twenty yards will be dead at night. In years with a low number of sunspots, DX contacts can easily be made at night at 160, 80 and 40 meters. The number of sunspots increases and decreases in average 11-year cycles. Since the curvature of the earth averages about 16 feet every 5 miles, an object 5 miles away from you on a perfectly flat earth is 16 feet below the horizon. Because light travels in straight lines, you cannot see objects beyond the horizon. Radio waves travel in straight lines, but there are ways to send them over the horizon. This is called propagation. 2. Propagation of the ground wave The ground wave only works with vertical polarization. One side of the antenna is the vertical metal radiator and the other side of the antenna is the ground. The surface wave in the air travels faster than the part of the wave that travels through the ground. The surface of the earth is curved like the curved part of a race track. On the curved track, a car on the outside of the track has to go faster than the car on the inside lane to stay even, and the two cars travel on a curved path. Although the wave travels faster in the air than the wave on the ground, the two parts of the wave cannot be separated. Because of this, the radio wave also travels in a curved path that follows the curvature of the earth. The AM broadcast stations use ground wave propagation during the day and sky wave propagation at night. Because lower frequency radio waves conduct better through the ground, an AM broadcast transmitter on 540 kHz is many dB stronger than a transmitter on 1600 kHz when both are operated at the same power. This fact is important to understand why floor mounted verticals don’t perform as well at high frequencies as they do in the broadcast band. 3. Direct wave or line-of-sight propagation antennas located on high structures can “peek” over the horizon and “see” the receiving antennas. Because refraction is involved, direct waves travel 20% further than light waves due to the scattering of radio waves by the environment. Trees and other foliage are invisible to RF radio waves. Direct wave propagation is possible at all frequencies, but this mode of propagation is rarely used on our high frequency bands, but it is the common mode of propagation used by repeaters and others on VHF and UHF. If you watch TV on an outdoor antenna or on a “rabbit ear antenna”, you receive the signal by direct wave propagation. 4. Propagation by Refraction Refraction occurs when the bottom of a wave propagates more slowly than the top of the wave because the wave travels through two media. These media can be two layers of air at different temperatures, or air and a solid. One form of refraction is caused by a radio wave traveling over a hill or ridge bending as it passes the obstacle. This is known as “knife edge cracking”. Another form of refraction occurs when layers of air with different temperatures bend the radio waves around the horizon. This is called tropospheric ducting. This mode of propagation enables long-distance contacts at VHF frequencies. Tropospheric channeling occurs at frequencies 10 meters and lower and is noticeable when other forms of propagation are absent. On the high frequency bands, many radio amateurs mistakenly call tropospheric lines and direct waves “ground wave”. 5. Skywave Propagation Skywave propagation occurs when radio waves are reflected off the ionosphere. Virtually all HF communication is via skywave. In the ionosphere, the waves are actually refracted twice and only appear to be reflected. The reflections are frequency sensitive, meaning each ham radio band reflects differently than the others. Low frequencies like 80 meters are mostly reflected from the lower levels of the ionosphere and the reflected signal comes back down almost straight. This causes 80 meters to spread to points from locally to more than a few hundred miles during the day. At night, when the D layer and E layer are absent, signals hitting the ionosphere at lower angles can spread many thousands of kilometers at 80 meters. On the 20 to 10 meter bands, high angle signals pass straight through the ionosphere and are not reflected back to nearby stations. The small-angle signals on these higher bands reflect off the ionosphere near the horizon and return to Earth a few miles away. The region in between cannot hear the transmitted signals, nor can you hear signals coming from that region. The intermediate area is called the “jump zone”. Only when the ionosphere is weakly ionized do you have a jump zone at 80 meters. Another interesting type of skywave propagation seen on the higher HF bands is called chordal hop propagation, which is commonly seen in transequatorial (TE) propagation, which is propagation across the equator . When this happens, signals entering the ionosphere are trapped in the F2 layer and eventually bounced back across the equator to Earth thousands of kilometers away. There is no propagation between the signal entry point and the exit point. This is skipping at its extreme. On many occasions we have worked far away stations beyond the equator in the southern part of South America and stations in between have not been heard. We have frequently worked VQ9LA in the Chagos Archipelago in the Indian Ocean. The route to the Chagos archipelago crosses Europe and the Middle East and eventually crosses the equator to its Indian Ocean location. Once, when he was working simultaneously in Europe and North America, we couldn’t listen to the European stations, because our way to him was through the spread of chord jumps. Another way to describe chord jump propagation is to call it ionospheric channeling. The propagation of sky waves sometimes creates an effect called “backscatter”. What happens is the radio waves hitting the ionosphere, instead of just reflecting the father away from the transmitting station, some of the signal is reflected back to the transmitting station. Stations that are too close to hear each other through direct waves can communicate through the backward reflected waves. Both stations communicating through backscatter must point their directional beam antennas in the same direction, although their direction towards each other may have a different azimuth. Backscatter will confuse front-to-back measurements from directional antennas. This is because if you point the back of the antenna at the station you are listening to, you may be able to hear it from a direction opposite to it via backscatter. You will hear it from the ionized atmospheric cloud in the opposite direction. During intense solar magnetic storms, when aurora occurs at high latitudes, the stations can communicate through backscatter on VHF and UHF, with both stations aiming their beams at the aurora. This is due north for stations in the northern hemisphere and due south for stations in the southern hemisphere. Audio from Aurora Backscatter has a “wipe” sound. 6. Greyline propagation Greyline propagation occurs when the sun is low in the sky near dawn or dusk, although we have seen a greyline propagation two hours before sunset or as late as two hours after sunrise. It is often used to edit stations on the other side of the world on 160 and 80 meters. For example, at certain times of the year, when the sun is about to set here in the US, the sun has just risen in Asia or Australia and vice versa. At this time, radio waves propagate along the semi-dark path circling the Earth, called the gray line. Both locations must be in the gray line to make 2-way contacts. The tilt of the earth causes the position of the gray line to change with the seasons. Greyline propagation occurs between any two locations for a short period of a few weeks. After that, various locations fall into the greyline. An interesting example of greyline propagation occurs in the southeastern part of the USA for several weeks in the fall of the year. On 3915 kHz, the BBC outlet in Singapore can be heard through greyline propagation about an hour before sunset. Stations to the east hear it ahead of us. Stations farther west can hear the fading signal after it fades out here because the gray line moves as the earth rotates. For those who hear it, the signal fades in, it peaks, and it slowly fades out. 7. Long Path Propagation Long path propagation occurs when signals travel the long way around the world. It can appear on any band. It usually occurs from stations on the opposite side of the world from you. We worked South Africa a long way, beaming to 20 meters northwest early in the morning. When this happens we work it a long way through the night side of the earth. Da zu jeder Zeit auf der halben Erde Tag und auf der halben Erde Nacht ist, wird die Langwegausbreitung dadurch bestimmt, ob das Signal über den Nachtweg oder den Tageslichtweg ausgebreitet wird. Manchmal bringt der Tageslichtpfad Stationen durch Langpfadausbreitung herein, und zu anderen Zeiten stellt der Dunkelpfad eine Langpfadausbreitung bereit. Eines Nachts hörten wir auf 20 Metern eine Station in Indien, die gleichzeitig auf kurzem Weg und langem Weg kam, aber der kurze Weg war stärker. Zur gleichen Zeit bearbeitete Kalifornien Indien auf langem Weg und sie konnten ihn auf kurzem Weg nicht hören. Sie arbeiteten ihn durch den Tageslichtpfad, und hier an der Ostküste war er über den Nachtpfad stärker. 8. 160-Meter (1,8-2,0 MHz) Ausbreitung Jedes Amateurband breitet Signale unterschiedlich aus. Das 160-Meter-Band ist unser einziges MW-Band und verhält sich ähnlich wie das Rundfunkband. Es ist in erster Linie ein Nacht- und Winterband, da es unter hoher sommerlicher statischer Aufladung (QRN) leidet. Die meisten Funkamateure, die dieses Band für Nahkontakte verwenden, verwenden horizontale Dipole oder umgekehrte V-Antennen. Einige Funkamateure verwenden vertikale Antennen auf diesem Band, um entfernte Stationen (DX) zu bearbeiten. Diese DX-Kontakte werden im Herbst und Winter nachts über F-Layer- oder Greyline-Ausbreitung hergestellt, wenn die statischen Pegel niedrig sind. Dipole und invertierte V-Antennen funktionieren auf diesem Band nicht gut für DX. 9. Achtzig-Meter-Ausbreitung (3,5–4,0 MHz) Der CW-Teil dieses Bands wird als 80-Meter-Band bezeichnet, und der Sprachteil des Bands ist als 75-Meter-Band bekannt. Wie 160 Meter leiden 80 Meter im Sommer unter der gleichen QRN. DX auf diesem Band zu arbeiten ist eine beliebte Beschäftigung im Herbst und Winter. 80 Meter werden jedoch hauptsächlich für Arbeitsnetze und Ragchewing verwendet. Eighty Meter ist in erster Linie ein Nachtband. Dieses Band kann in Jahren mit wenigen Sonnenflecken den größten Teil des Tages geöffnet sein und in Jahren mit vielen Sonnenflecken während der Tagesmitte geschlossen sein. Viele DX-Kontakte wurden mit Dipolen und umgekehrten V-Antennen hergestellt, aber eine Vertikale mit vielen Bodenradialen ist besser. 10. Vierzig-Meter-Ausbreitung (7,0-7,3 MHz) Das Vierzig-Meter-Band hat eine Ausbreitung, die entweder wie 80 Meter oder 20 Meter wirken kann. Es hängt nur vom Stadium des Sonnenfleckenzyklus ab. In den Jahren mit hoher Sonnenfleckenzahl sind ganztägig Nahkontakte möglich. Nachts verlängert sich der Sprung und ermöglicht Kontakte zu jenen Teilen der Welt, wo es noch dunkel ist. Das Arbeiten mit DX auf 40 Metern ist ein Nacht- oder Greyline-Ereignis. Wenn die Sonnenflecken niedrig sind, können vierzig Meter tagsüber lange Sprünge haben, und nahe Kontakte können unmöglich oder sehr schwach sein. In der Zeit, in der wir unter geringer Sonnenfleckenzahl leiden, werden viele DX-Kontakte am frühen Morgen, am späten Nachmittag und in der Nacht hergestellt. Wenn Ihr Hauptinteresse auf vierzig Metern SSB ist, ist unser 40-Meter-Sprachband ein Rundfunkband in den Regionen 1 und 3. Region 1 ist Europa, Nordasien und Afrika und Region 3 ist der Pazifik, Südasien und Australien. Der obere Teil von 40 Metern ist ein Sprachband in Region 2, das ist Nord- und Südamerika. Um nachts auf vierzig Metern in SSB zu arbeiten, müssen Sie eine Frequenz zwischen den Sendestationen finden. Starke Sender, die nachts zu hören sind, beginnen langsam auszublenden, wenn die Morgensonne aufgeht und sich höher am Himmel bewegt. Wenn der Sonnenwinkel am Nachmittag abnimmt, beginnen die Sender, das Rauschen zu durchbrechen, das mit dem Sonnenuntergang stärker wird. Nur um die Mittagszeit sind auf vierzig Metern keine Sender zu hören. Da DX-Stationen in Region 1 und den meisten Regionen 3 nur unterhalb von 7100 kHz senden können, ist das Arbeiten mit DX auf 40-Meter-SSB weiterhin möglich. Stationen in diesen Regionen müssen unter 7100 kHz senden. (Australische und neuseeländische Amateure können bis zu 7200 kHz arbeiten.) Sie rufen CQ und geben bekannt, wo sie in unserem Sprachband über 7150 kHz hören. Dies wird als „Arbeitsteilung“ bezeichnet. 11. Thirty-Meter (10.1-10.15) Propagation This band has such a narrow frequency that the only modes allowed here are CW and digital modes. That means no SSB. Propagation here is much like 40 and 20 meters. Unlike 20 meters, this band stays open longer at night during years with low sunspot numbers. During the daylight hours, it has much shorter skip than 20 meters. In the United States, we are allowed only 250 Watts. 12. Twenty-Meter (14.0-14.35 MHz) Propagation The twenty-meter band is the best DX band because it is open for long-skip for more hours than any other band and it does not suffer from QRN as the lower bands. In years of high sunspot numbers, short-skip and long-distance DX can be worked at the same time during daylight hours. Although DX is there most of the time, most of the DX worked is at sunrise, sunset, and all night during peak sunspot years. During the years of low sunspots, it is common to work into Europe and Africa during the day and into Asia and the South Pacific during the evening hours and early at night. Low sunspot numbers cause 20 meters to go dead for east to west contacts at night an hour or so after sunset, but there is some TE propagation. During periods of moderate sunspot numbers, the propagation on this band is a blend of propagation of low and high sunspot years. 13. Seventeen-Meter (18.067-18.167 MHz) Propagation The 17-meter band propagation acts much like 20 meters except it is affected more by low sunspot numbers than 20 meters. In periods of low sunspot numbers, this band does not stay open as late as 20 meters, fading out as the sun begins to set. Yet, the 17-meter band does stay open all night when the sunspot numbers are high. The propagation on this band is like a blend of 20 meters and 15 meters, but it is closer to 20 meters. Most users of this band use dipoles and other simple antennas since triband beam antennas wont work here. 14. Fifteen-Meter (21.0-21.45 MHz) Propagation Fifteen meters is a fantastic DX band during the high sunspot years. This band may be open for 24 hours, and it is common to work more than 100 countries during a contest weekend on this band. Many have worked more than 300 different countries on 15 meters. In years of low sunspot numbers, 15 meters may be completely dead for several days in a row. When it opens during those years, you may hear only the Caribbean, South America, and on rare occasions the extreme southern part of Africa via TE propagation. 15. Twelve-Meter (24.89-24.99 MHz) Propagation The 12-meter band is much like 15 meters, but it is affected more by sunspot numbers. Because this band is little used, many hours can pass without hearing any amateur signals. Occasionally you will hear South American Citizen Band “pirates” on lower sideband. It is mostly a daytime band but openings to Asia and the South Pacific are common early at night during peak sunspot years. The reason this band is little used is that triband beam antennas dont cover this band. 16. Ten-Meter (28.0-29.7 MHz) Propagation The band that is most affected by the sunspot numbers is 10 meters. You may have noticed in this discussion, the higher the frequency, the more it is affected by sunspots. During peak sunspot years, 10 meters can be open some days for 24 hours. Mostly it is a daytime band. When they are at the peak, the sunspots enable you to work worldwide with power as low as 5 Watts. A 10-meter confirmed country total of over 250 is common. In the low sunspot years, the band can be closed for days. Ten meters can open for very short skip by sporadic E propagation during the summer months. Very short skip means contacts as close as 200 miles out to 1000 miles. Sporadic E propagation can suddenly occur without regard to the sunspot numbers. VI. STANDING WAVE RATIO A standing wave ratio bridge is used to measure the standing wave ratio, or SWR. SWR is an indication of how well the radiating part of an antenna is matched to its feed-line or how well the tuner is matching the antenna system. Most amateurs pay far too much attention to SWR. An SWR reading below 2:1 is acceptable, because the mismatch is so small that the feed-line loss can be ignored. If you are using a modern transceiver, its power may fold back to a lower power output above this SWR level. When you have mismatch between the feed-line and the antenna, part of the power feeding the antenna system reflects back toward the tuner and the transmitter. The part of the power going toward the radiating part of the antenna system is called forward power. The part reflected back down the feed-line is called reflected power. The larger the mismatch the larger the reflected power will be. If the feed-line and antenna are not matched, waves traveling toward the radiating part of the antenna system meet the waves being reflected back down the feed-line. The waves interfere with each other, and at certain points along the feed-line, the amplitudes of both waves combine. This will result in a current maximum to be found at that point; and at that point, the current will appear to be standing still. The length of feed-line and the frequency will determine where this point occurs. At another point, the forward and reflected waves interfere, and they subtract from each other. At that point, there will be a current minimum. If you could visualize this phenomenon, you would see a series of current maximums and minimums standing still along the feed-line. This is why we refer to them as standing waves. At different points along the feed-line, where you have high current, you will have low voltage, and where you have low current, you will have high voltage. At any point along the feed-line, multiplying the voltage times the current will equal the power in Watts. When the feed-line is matched to the antenna, current and voltage remain the same all along the feed-line because there is no reflected current to interfere with the forward current. As happens with the current, the voltage will also appear to be standing still. The voltage maximums and voltage minimums will not be at the same locations as the current maximums and minimums. SWR is the ratio of the maximum voltage to the minimum voltage on the line. It is called “Voltage Standing Wave Ratio” or VSWR, but we shorten it to just SWR. There is also a current SWR or ISWR, and it is the same value as the VSWR. For example, if the standing wave voltage maximum is 200 volts and the minimum voltage is 100 volts, the VSWR will be 2:1. If the voltage maximum and voltage minimum are equal, the SWR will be 1:1. If the voltage minimum is zero, the SWR is infinite. In measuring SWR at the transmitter, you need to realize that feed-line losses affect the SWR readings. If the feed-line losses are high, much of the power reflecting back from the antenna will be lost, and the SWR reading on the meter will indicate it is lower than it actually is. If a feed-line is so lossy that it consumes all forward and reflected power, it will measure an SWR of 1:1. When measuring SWR on an antenna having a small amount of reflected power, the length of the feed-line between the bridge and the antenna may affect your SWR reading. An example of this is a 70-ohm antenna being fed with 50-ohm coax. Different lengths of feed-line will give you small differences in SWR readings because at certain lengths, the mismatched feed-line starts to act like a series matching section. In the case of a 70-ohm antenna fed with 50-ohm coax, if the feed-line is a half wave long, the SWR will measure 1.4:1. At some particular length of feed-line and on one frequency, the SWR will measure 1:1 because that length of that feed-line transforms the impedance to make a match. Some hams have adjusted their feed line length to get a perfect match. This is called “tuning your antenna by tuning your feed-line.” With other feed-line lengths, you will measure something different. Suppose the impedance of the feed-line and the antenna are perfectly matched. Then there is no reflected power. You will get a 1:1 reading on the SWR-bridge with any length of feed-line. There is a myth that reflected power is burned up as heat in the transmitter . The reflected power coming back down the feed-line sees an impedance mismatch at the transmitter or tuner and it reflects back up again. The reflected power does not get back into the transmitter. Because the reflected power reflects back and forth, the radiating part of the antenna system absorbs most of the power being reflected back up each time. All of it eventually is radiated except for the power lost in the feed-line. The losses in a real feed-line will burn up some of the power on each pass. This is why the feed-line loss increases with SWR. Built-in tuners are found in most modern transceivers. If yours doesnt have one, then you can use an outboard tuner to give the transceiver a proper load. The place you want a 1:1 SWR is between the output of a transceiver and antenna or between the transceiver and the input of a tuner in order for the transmitter to deliver its maximum power. Because built-in tuners are in most modern transceivers, many hams use them to match antenna systems having high loss. VII. REAL ANTENNA SYSTEMS In this book, we will be talking about the losses that rob an antenna of its maximum performance. The ideal antenna system will radiate 100% of your transmitter power on all bands without a tuner and in the direction you want to work. Such an antenna system does not exist. Many new hams succumb to antenna advertisements making claims that are exaggerated. No antenna will have low SWR, work all bands without a tuner, and radiate efficiently at the same time. A dummy load has a low SWR and will load up on all bands, but it will not radiate a signal. A resonant coax-fed dipole antenna will have a low SWR and will radiate efficiently on the band for which it is resonant, but it will not work well on all bands. For example, if the tuning range of your tuner has a sufficient range, you will be able to load up any antenna with it, but it will not necessarily radiate a signal efficiently. It may have high tuner and feed-line losses. When you choose an antenna, you must decide how much loss you can accept. DXers and hams that work weak signals at VHF frequencies try to eliminate as much loss as possible. If your contacts are going to be made under good band conditions and without much interference, you can get by with high losses. In that case, coax-fed antennas used on bands where they are not resonant will allow you to make contacts. You can be greatly surprised by how little radiated power can be used to make contacts under ideal conditions. If you want to make contacts regularly under changing band conditions, you will want to eliminate as much loss as possible and use antennas with gain. Lower loss will enable you to hear weaker signals. Nothing will take the place of resonant half-wave dipoles, not because they radiate more efficiently, but because they dont require lossy tuners and dont have high coax losses. Remember that all antenna systems have compromises VIII. HALF-WAVE RESONANT DIPOLE ANTENNAS 1. The Half-Wave Flat-Top Dipole Most dipoles consist of two pieces of wire of equal lengths with one of the two ends connected together through an insulator. The far ends of the wires are also connected to insulators. The two conductors of a feed-line are separated and connected across the gap at the center insulator. The antenna is held up by rope that connects the insulated ends of the antenna to two supports. It is a “balanced” antenna, because equal currents flow on both halves of the antenna. Coax is an unbalanced feed-line. (The possible effect of using an unbalanced feed-line on a balanced antenna like a dipole will be discussed later.) The dipole that is stretched between two high supports is called a flattop dipole, distinguishing it from other configurations. The simplest antenna system of all is the half-wave resonant dipole fed with coax and no tuner. The only reason for using a half-wave resonant dipole antenna is to eliminate the need for a matching device such as a tuner. The feed-point impedance will be near 50 ohms at ordinary heights and they can be fed directly with 50-ohm coax from the output of todays modern radios. The two halves of a dipole are fed 180 degrees out of phase, meaning when one side is fed positively, the other side is fed negatively. That is why a feed-line has two conductors. Of course, the sides swap polarity on each half cycle. If you could visualize the current flowing on the half-wave dipole, the current will appear to be standing still. The maximum current will be seen at the center of the wire and no current will be at the ends. This occurs because the electrons flowing out to the ends reflect back toward the center where they meet the next wave and the current is reinforced there. The minimum voltage occurs at the center and the maximum voltage occurs at the ends of the half-wave resonant dipole. If you were to measure the voltage and the current at any point on the dipole wire, the voltage times the current will equal the power in Watts. Figure 1. Flat Top Dipole 2. Inverted-V Dipole Another configuration for the half wave resonant dipole is one having one support in the center and the ends stretched down toward the ground. The single support can be a tree, mast, or tower. The ends of a dipole have high RF voltages on them, and need to be at least 10 feet above ground for safety. This antenna is called an “inverted-V,” because the shape of the dipole looks like a “V” turned upside down. Most dipoles illustrated in this book can be put up in the inverted-V configuration. This configuration works well because the current is concentrated on the middle two-thirds of the antenna at the apex. The current in an antenna is what is responsible for the radiation. The ends of the antenna have very little current in them and it doesnt matter if the ends are close to the ground. The middle of the antenna is up high where the radiation is taking place and that is the place you want the radiation to be. An inverted-V has an advantage that the horizontal space required for it is less than what is needed for a flattop dipole. The angle between the wires on an inverted-V needs to be greater than 90 degrees. The gain of an inverted -V is 0.2 dBd and it has a radiation pattern nearly omni-directional. Since it is easy to construct and works so well, the inverted-V is the most commonly used dipole. An explanation of the decibel will come later. Figure 2. The Inverted-V Dipole Figure 3. Radiation Pattern of Inverted-V for 80-Meters at 65 Feet In figure 3 above, the top graph shows how the radiation would appear to you, if you were situated above the dipole and you were looking down on it. The plane of the antenna runs from side to side on the top graph, and that graph demonstrates only a 5-dB null off the ends of the antenna. Therefore, it is essentially omnidirectional. The bottom graph shows how the radiation would appear if you were looking at the antenna from the end of the wire. As you can see, the pattern shows no radiation at the horizon and its maximum radiation is at about 40 degrees above the horizon, and the radiation straight up is only down 3 dB from its maximum. This antenna was modeled on 80 meters with the apex at 65 feet above ground and the ends at 35 feet. It is a myth that a horizontal antenna orientation makes a difference on 80 meters at heights used by most amateurs . I have heard many amateurs say on 80 meters, “The reason my signal is weak to you is because you are off the end of my dipole.” The radiation pattern from a dipole is essentially non-directional until the dipole is elevated more than a half wave, that is about 125 feet on 80 meters, and it is 65 feet on 40 meters. The main reason it makes no difference regarding orientation is because propagation for signals closer than 500 miles (the distance of most 80 meter contacts) is essentially by high angle radiation nearly straight up and down. Only signals radiated and received at low angles make a difference in antenna orientation even at low heights above ground. At low heights, there are nulls about 3 to 4 dB off the dipole ends. 3. Dipole Shape Variations The wire of a dipole doesnt have to be run in a straight line. A dipole does not have to be perfectly horizontal. Thats the way it is usually depicted in books and magazines, but you can bend the legs of the antenna up, down or sideways. Figure 4. Two Dipole Shape Variations If you make either wire one-half wavelength long and carefully prune it to resonance, you can use it without a tuner on and near its resonant frequency. Both antennas have the current part at the top where most of the radiation takes place. The vertical parts of these antennas radiate a weak vertically polarized wave. The only reason these dipoles are contorted this way is to make them full-sized and to fit in the available space. Other shapes are possible, and you can be creative at your location. There are many more dipoles than the ones just described. We will explore the other kinds of dipoles in section “X” of this book. 4. Calculating the Length of a Half-Wave Resonant Dipole The approximate length in feet of a half-wave resonant dipole is found by dividing 468 by the frequency in MHz. The actual length of it will be determined by several factors. Using larger diameter wire will make the dipole resonate lower in frequency. Therefore, to make it resonant at the higher desired frequency, It must be shortened. Raising a dipole higher above ground will make it resonate higher in frequency. An insulated wire will make the dipole resonate lower in frequency than a bare wire. Using the above formula, cut the antenna a little longer than the calculations say. If the SWR is best at a lower frequency than you desire, the antenna will have to be made shorter by pulling the excess wire through the end insulators, folding the ends of the extra wire back on itself. Then wrap the ends of the overlapped wire on itself so it wont come loose. This causes the excess wire to “short” itself to the rest of the antenna. If you are using insulated wire, you will need to cut off the excess wire. The reverse is true if the antenna resonates too high in frequency. The extra wire can be let out to make it resonate on a lower frequency. This is why you originally cut the wire a little longer. 5. The Decibel The decibel (dB) is a unit of measurement for comparisons of the ratio of power, current, and voltage and is the term we will use in comparing antennas in this book. At one time, antenna comparisons were made using a dipole as a standard, but today most comparisons use the isotropic radiator as a reference. An isotropic radiator is an imaginary antenna that radiates equally well in all directions. It has no gain. The terms “dBi” and “dBd” are used to label which reference is being used. In this book, we will use the dipole as a standard for the most part. How do you derive decibels from power ratios? The formula for power ratios is dB = 10 log P1/P2. For voltage and current, the values are doubled. Formulas of this type are beyond the scope of this book. Doubling the power will produce a 3 dB stronger signal. Double the power and double it again will equal a 4 times power increase and that gives 3 dB plus 3 dB or 6 dB. Double 4 and that is a power increase of 8 and that adds 3 more dB for a total of 9 dB. Increasing the power from 1 Watt to 10 watts or increasing it 10 times will give a 10-dB increase. Multiply 10-Watts times 10 give us 100 watts, which adds another 10 dB above 1 Watt for 20 dB. Therefore, increasing the power another 10 times to 1000 Watts will produce a signal 30 dB stronger than 1 Watt. Your receiver, if modern, will have a signal strength meter or “S Meter.” That meter is calibrated in “S-Units” from one to nine and decibels over S-9. S-9 is usually calibrated using 50 microvolts ( uV) from a signal generator. Each S-unit is approximately a difference of 5 or 6 dB. Therefore, a reading of S-9 is about 6 dB stronger than S-8. Therefore, from S-0 to S-9 is 54 dB. On some low cost transceivers, the S-units and dB above S-9 are only relative signal readings and actually have nothing to do with decibels. IX. ANTENNA BASICS 1. Resistances and Reactance Two factors measurable in antenna impedance are resistance and reactance. When we refer to antenna resistance, we are referring to its radiation resistance. It is neither a resistance like the electronic component called a “resistor,” nor is it the same as the resistance found in all conductors. Those types of resistances, called “loss resistances,” change electrical energy into heat energy. Heat energy disappears by radiating out into its surroundings and it dissipates away to infinity. When we feed RF into the antenna, the energy put into the radiation resistance disappears from the antenna by radiation of electromagnetic waves, and that makes an antenna appear to have a resistor in it. Loss resistance robs power from the radiation resistance and lowers the efficiency of an antenna system, but the loss resistance in dipoles is very low if the feed-line loss is low. The efficiency of any antenna system is found from a ratio of radiation resistance and loss resistance. We can either calculate the loss resistance by the loss in the feed-line from published tables and by estimating the loss in tuning units. Feed-line loss and tuning unit loss can be measured, but that is beyond the scope of this book. Antenna systems having reactance prevent the transmitter from delivering its full power and the reactance needs to be tuned out. There are two kinds of reactance: capacitive and inductive. Antennas have both. In antennas, reactance is a virtual reactance meaning the antenna acts as if there were a capacitor or an inductor in the antenna, but neither is there. You can only measure the sum of both reactances but not a value for either one. Using an antenna analyzer, you can determine whether the sum of the reactance is inductive or capacitive. Inductive reactance is a negative number and capacitive reactance is a positive number. The reactance of an antenna forms the “J” factor in antenna impedance measurements. The “J” factor is measured in ohms and the reactance is expressed as + or “J” ohms depending on whether it is capacitive or inductive reactance. Capacitive reactance is expressed as +J ohms and inductive reactance is expressed as -J ohms. Capacitive and inductive reactance are opposite factors and one can cancel the other. An antenna having 6 ohms capacitive reactance or + J 6 ohms and an inductive reactance of J 5 ohms will result in an antenna with a reactance of 1 ohm capacitive or + J 1. Since one term is positive and the other term is negative, you subtract smaller value from the larger. The answer has the sign of the larger one. In antennas, the reactance and resistance together determine the overall impedance of the antenna. The J factor is mentioned here only because you may see it in other books and on the extra class examination, but it will not be used further here. A resonant antenna has equal amounts of inductive and capacitive reactance, and the sum of the reactance equals zero. As an example, when the inductive reactance equals J 5 and the capacitive reactance equals +J 5, their sum equals zero. When the sum of the total reactance of an antenna is tuned to zero, its impedance is totally resistive. The use of an antenna analyzer will tell you if the antenna is too long or too short for resonance. The simplest way to tune out antenna reactance is to change its length. The sum of the reactance of a long antenna will be inductive, and the sum of the reactance of a short antenna will be capacitive. If an antenna is short because it wont fit your property, it can be tuned to resonance by putting an inductor (coil of wire) in each leg. These coils are called “loading coils.” An equal amount of inductive reactance will cancel the excessive amount of capacitive reactance. An antenna with loading coils is described in section “X.” When an antenna is too long, the sum of its reactance will be inductive, and a variable capacitor can be inserted in each leg to tune out the inductive reactance. This is seldom done because it is easier to shorten the antenna. A resonant antenna may still have SWR if its radiation resistance is not exactly 50 ohms. Not many resonant antennas have a radiation resistance of exactly 50 ohms, and most real antennas have a small amount of SWR. An antenna is resonant only at one frequency per band. It will also be resonant on its harmonic frequencies, where its radiation resistance will range from high to very high. Hams talk about using resonant antennas. What is meant by this is they use an antenna on its fundamental frequency close to resonance, the resistance is near 50 ohms, and the SWR without a tuner is near 1:1. To calculate the impedance of an antenna with both resistance and reactance requires a mathematical procedure called the Pythagorean Theorem. That type of math is beyond the scope of this book. However, you should know how to use the Pythagorean Theorem to solve impedance problems on the Extra-Class test. Otherwise, you will have to memorize the answers from the question pool. 2. Feeding Dipoles Efficiently For maximum power transfer from transmitter to the antenna, the antenna system must be resonant, and the resistance of the load (antenna system) has to be equal to the internal resistance of the source (transmitter). Notice we said an antenna system, not the antenna, must be resonant. As mentioned previously, an antenna system consists of the antenna, the feed-line, and any matching networks (tuners). A tuner at the input end of the feed-line can make a non-resonant antenna system resonant, and have a resistance of 50 ohms, and that matches the internal resistance of the transmitter. A tuner will not change the SWR between the tuner and the dipole part of an antenna system, and will not remove the reactance from the dipole. When the load of an antenna system does not match the source and the impedance is high, the load will not draw power from the source and high RF voltages will be present at the output of the final transistors. In this case, high RF voltages can damage the output transistors of the transmitter. When the impedance of the load is low, too much of the power may be dissipated across the internal resistance of the transmitter possibly destroying the output transistors. These are the two reasons why transceivers “fold back” their power when the SWR is high. It is a myth that the dipole part of an antenna has to be resonant to be efficient. When power reaches the radiating part of the antenna system, it obeys the “The Law of Conservation of Energy.” The Law of Conservation of Energy states, “Energy can neither be created nor destroyed. Only its form can be changed.” (What is important is to get the power to the dipole itself, because in some systems power is lost in the feed-line, especially when using coax with high SWR) The miniscule amount of power in the dipole that does not radiate is changed into heat, another form of energy. Because the dipole part of an antenna system is made of conductors with low loss resistance, 99% or more of the power reaching it will radiate regardless of its length if that length is reasonable. The loss resistance of the conductors of the radiating part of most antenna system is so low it can be ignored. (Short mobile HF antennas are an exception because they may be lossy because of the very high current flowing in them.) Not all the energy fed into an antenna system will reach the antenna itself. If the system has a tuner, part of the power is lost in the inductor of the tuner and part is lost in the feed-line. When properly tuned, tuners using T-networks lose about 10% of the power and L-network tuners lose about 5% of the power being fed to them. Notice we said properly tuned. However, improper tuning of the antenna tuner may cause you to believe the feed-line is matched, but when this happens there is a very high circulating current in the inductor causing it to get hot. This causes extremely high losses, and very little power reaches the radiating part of the antenna. In addition, so much heat is produced in the inductor that it can be damaged. We melted the plastic insulation that forms the inductor on one tuner this way. For this reason, some hams dont like tuners, preferring to use resonant antennas. Read the instructions for your tuner for proper tuning or you may wind up with a poor signal and a damaged tuner. The resistive losses in the conductors of the feed-line and the dielectric losses in the feed-line also rob power from the system. These are the reasons for you to use the best tuners and feed-lines possible. Another loss to be considered is feed-line radiation. Any energy that radiates from the feed-line does not reach the radiating part of the antenna, and it may be absorbed by near-by objects and may not radiate in the desired direction. When coax radiates, it is called common-mode radiation. If the feed line can radiate, it can also receive signals. This can be detrimental because the coax can then pick up noise from near-by power lines, etc. Feed-line radiation will also destroy the directional pattern of a beam antenna. The causes of feed-line radiation will be described in the next section. As we pointed out earlier, when you are using a half-wave resonant dipole fed with low-loss coax without using a tuner, almost all of the power coming out of the transmitter will radiate. On its resonant frequency, the dipole is one of the most efficient antenna systems a ham can use. However, a half-wave resonant dipole has a finite bandwidth. Why use a tuner with resonant antennas? On 160 and 80 meters the bands are wide compared to the percentage of frequency. The width of 80 meters is 500 kHz and its frequency is 3500 kHz. The width of 80 meters is 14% of the frequency. The 350 kHz of 40 meters is 5% of the frequency and most of the band can be covered without a tuner. The 350 kHz width of the 20-meter band is 350 divided by 14000 kHz, or 2.5 % of the frequency, etc. The percentage of frequency for a band will determine if a resonant dipole will work the whole band without a tuner. If you are planning to move around on 160 or 80 meter bands, it makes sense to have a tuner, because the bandwidth of resonant dipoles on those two bands is narrow. For example, the normal 2:1 SWR bandwidth of an 80-meter dipole is less than 200 kHz and the band is 500 kHz wide. However, if you have an antenna resonant for the voice portion of the band, you can still use a tuner to work the CW part of the band without inducing more than a dB of loss. Except for 40 and 10 meters, full-sized resonant dipoles on the rest of the HF bands will have enough bandwidth for them to cover the whole band. The best place to insert a tuner is up at the antenna feed-point. However, if it is placed there, you wont be able to reach the tuners controls. Therefore, it is more practical to place it between the transceiver and the shack-end of the antenna feed-line. A piece of 50-ohm coax connects the radio to the tuner. With the tuner located in the shack, adjustments can be made. Remote automatic antenna tuners can be placed at the antennas feed-point, but the disadvantage of them is that the ones available today will not handle high power. A coax-fed dipole and a tuner should not be used to feed an antenna on its even harmonically related bands. The even harmonics are 2, 4, 6, etc, times the fundamental resonant frequency. If an 80-meter antenna being fed with coax through a tuner is used on 40 meters, it will put out a weak signal because the SWR will be around a hundred to one. Coax has a tremendous loss with SWR this high. Only a few Watts from a hundred-Watt transmitter will reach the antenna. However, you will be able to make contacts with those few Watts. If you want to use any antenna having high SWR, ladder-line has much less loss than coax. If you feed an 80-meter dipole on 40 meters using ladder-line and a tuner, it will only be slightly less efficient than a half-wave 40-meter coax-fed resonant dipole. However, the SWR will still be high between the tuner and the antenna, but this doesnt matter since ladder-line has an insignificant loss. Since the feed-point impedance will be high, the SWR will only be about 9:1 in the ladder-line because ladder-line is a high impedance feed-line. Extremely short antennas may not work at all because of the above mentioned reasons. To reiterate, the extremely high capacitive reactance may make it impossible for its reactance to be tuned out and reactance prevents a transmitter from delivering power to the antenna. Even if you are able to tune out the capacitive reactance, tuning it out requires an inductor and most of the power will be lost in the inductor. Do not take the statement about the Conservation of Energy to mean you can put up any piece of wire and it will radiate your entire signal. 3. The Cause of Feed-Line Radiation Contrary to popular myth, SWR in a feed-line will not cause it to radiate. The cause of feed-line radiation is unequal current in the two conductors of the feed-line. What are the causes of unbalanced current in a feed-line? They are an unbalanced feed-line feeding a balanced antenna; the feed-line being brought away from and parallel to one leg of the antenna; the antenna not being fed in its center; and one leg of the antenna being close to metal objects. In coax, unbalance causes RF to travel on the outside surface of the coax shield, and the shield radiates. When everything is balanced, coax normally has current flowing on its center conductor and on the inside of its shield. The shield prevents it from radiating. Ladder-line will also radiate when it is fed from the output of a tuner not having a balun. Baluns are discussed in the next section. Since the output of a transceivers tuner is unbalanced and feeding ladder-line directly from your transceivers tuner, the currents in the ladder-line will not be balanced. When balanced, ladder-line has equal currents with a 180-degree phase difference, which produce waves that null each other out, and no radiation takes place. Hams mistakenly refuse to bring ladder-line into the shack because of a fear of feed-line radiation, but ladder-line does not radiate when balanced. The simple cure for feed-line radiation is to use a balun at the antenna feed-point for coax and a balun at the output of the tuner when using ladder-line. 4. Baluns The word “Balun” is a contraction of ” balanced to unbalanced.” It is pronounced “bal un” like “bal” in “balanced and like “un” in “unbalanced”. Many hams mistakenly pronounce an “M” at the end of the word making it “balum.” A balun transforms the unbalanced transmitter output to a balance feed-line such as ladder-line. It is also used to connect an unbalanced feed-line such as coax to a balanced dipole. In the latter case, the balun is located at the antenna feed-point and is constructed so the balun takes the place of the center insulator. There are two kinds of baluns: voltage baluns and current baluns. They both accomplish the same thing. The difference in baluns is in the way they are wound. A voltage balun produces equal voltage with opposite polarity at its output. As its name implies, a current balun provides equal currents with opposite polarity at its output. Running the coax through ferrite beads can make a 1 to 1 current balun. In addition, you can build a 1 to 1choke current balun by winding 8 to 10 turns of coax around a two-liter sod a bottle and placing the coiled coax at the antenna feed-point. Any balun is designed to “divorce” your antenna from the feed line. It is used to prevent common mode radiation of coax, which makes the coax to be part of your antenna. You want it to be able to deliver all your power to the radiator itself. A choke balun does this perfectly, without using any ferrite beads or toroids. In most cases common mode coax radiation does not occur when a balun is not used, but it is preferable to use one to be safe. Other baluns provide a step-up or step-down impedance transformation. A 4-to-1 balun steps up the impedance four times. It will transform a 50-ohm impedance to 200 ohms. This type of balun transformer is used at the output of tuners to increase the tuning range of a tuner 4 times. If a tuner without a balun can match 500 ohms, a 4-to-1 balun will increase the range of impedances it can match to 2000 ohms. Many hams think the 4-to-1 balun is used to match 50 ohms to 450-ohm ladder-line but it is not. It would take a 9-to-1 balun to match 50 ohms to 450 ohms, and it is not important to match the impedance to ladder-line. A balun should always be placed at the input end of ladder-line or open wire feeders to prevent feed-line radiation. When using ladder-line a step up balun is commonly used although a 1:1 balun will work. X. OTHER TYPES OF DIPOLES 1. A Shortened Dipole Using Loading Coils If you are unable to put up a full-sized dipole on your property, putting loading coils into the dipole could shorten the antenna. See section IX, part 1. A short antenna has capacitive reactance and the capacitive reactance can be tuned out with a coil. The overall length of the shortened antenna will be determined by the amount of inductance in the coil. Pre-tuned antennas of this type are available from at least one manufacturer. The main problem with loaded antennas is they are very narrow banded. If the loading coils are wound with small diameter wire, the coils may introduce unwanted loss into the antenna. Loading coils can also be found in shortened vertical antennas for high frequency (HF) mobile use. Figure 5. A Shortened Loaded Dipole 2. All Band Dipole In the figure below, a dipole is cut to a half wave on the lowest band you want to operate. Feeding it with ladder-line and a tuner makes it possible for you to work all the other higher bands. The only losses in this antenna system are the loss in the tuner and the very small loss in the ladder-line. This system is more than 90% efficient. As mentioned above the balun in the tuner will be used, or if your tuner doesnt have a balun, an external balun can be connected between the tuner and ladder-line with a short run of coax. Four-to-one baluns are the most commonly used ones for this arrangement. Figure 6. All Band Dipole 3. The Sloping Dipole A lower angle of radiation can be achieved by tying one end of a half-wave dipole to a high support and the other end near the ground. It is fed with or without a balun with 50-ohm coax. The sloping dipole will show some directivity and have low angle gain in the direction of the slope. More directivity can be gained if the dipole is strung from a tower, and the tower is acting as a passive reflector. The sloping dipole is mostly a vertically polarized radiator and it works well for DX. Since the sloping dipole is fed in its center, it does not need to be grounded to the earth as a quarter-wave vertical does. Make sure the bottom end of a sloping dipole is at least 10 feet above ground because like all dipoles there is high RF voltage on its ends. Figure 7. Half-Wave Resonant Sloping Dipole In the picture above, the field of maximum radiation is in the direction of the slope or toward the right side of the picture. The formula for the length of a sloping dipole is the same for any half-wave resonant dipole. 4. The Folded Dipole The B&W Company makes a folded dipole that claims to have a good match on all bands and it does. However, on the low bands much of the power is burned up in the resistor that connects the two ends together. The power going toward the ends encounter the resistor and is consumed as heat. All that power is lost and does not radiate, and no power is reflected back to the feed point making the antenna have low SWR. On the higher bands, a large part of the power radiates before it reaches the resistor and the antenna is moderately efficient on those bands. On 80 meters the 90 foot-long dipole model will produce a signal at least 10 dB lower than that from a resonant dipole. If you remember the single channel TV antennas used years ago, the driven element was a folded dipole. Folded dipoles are very broad-banded. That is the reason they were used for TV antennas since a TV channel is 4 MHz wide. When constructing a folded dipole, the formula for calculating the length of it is the same as for any dipole. The folded dipole consists of two parallel conductors with the ends tied together. The conductors can be spaced from less than an inch to more than two inches apart when made from TV ribbon or ladder-line. At the ends, strip the insulation back several inches, Twist the bare wires together, solder them, and run them through insulators. The feed-point is in the center of only one of the two parallel conductors. The feed-point impedance of a folded dipole at resonance is close to 300 ohms resistive and can be fed directly with 300-ohm TV twin-lead or a tuner with its balun. This antenna was very popular years ago when coax was expensive and 300-ohm TV twin-lead was relatively cheap. A length of 450-ohm can be substituted for the twin-lead. An alternate feed method is placing a 6:1 balun at the feed-point and then feeding it with 50-ohm coax. The folded dipole will not radiate its second harmonic, so it is not good for a multi-band tuner-fed antenna. Another folded dipole type is the three wire folded dipole. We have seen this dipole only in books and do not know anyone who uses one. The feed-point impedance is 600 ohms resistive and is fed with home-built 600 ohm open wire feeders. Figure 8. Folded Dipole 5. The Double Bazooka Dipole The double bazooka is claimed by its users to be broad-banded, a quality especially interesting for those hams operating on 75/80 meters. Tests done at the A.R.R.L. have shown the double bazooka is only slightly more broad-banded than a regular dipole, probably due to the use of a large conductor (coax) for the center part of the antenna. The double bazooka will not transmit its second harmonic, and its users say it does not need a balun. Other users say it is quieter than a regular dipole. The center of the antenna is made from RG-58 coax. To find the length of coax needed, divide 325 by the frequency in MHz. The coax forms the center part of the double bazooka and a piece of number 12 wire on each end completes the antenna. The length of each of the end wires is found by dividing 67.5 by the frequency in MHz. To increase the bandwidth some builders use shorted ladder-line in place of the number 12 wire, which makes the end pieces to be electrically larger. The feed-point of the double bazooka is unique. At the center of the coax dipole, remove about 3 inches of the plastic covering, exposing the shield. Cut the shield in the center and separate it into two parts. Do not cut the dielectric or the center conductor. Leave the center conductor with its insulation exposed. On the feed-line strip off about 3 inches of outer insulation, separate the shield from the center conductor, and strip about 1 inches of the insulation from the center conductor. To attach the feed-line, solder the two exposed feed-line conductors to the two pieces of the separated exposed shield of the dipole center. It goes without saying: seal the feed-point to prevent water from getting in. At each of the two ends of the coax forming the center of the antenna, the coax is stripped back and the center conductor and shield are shorted together and soldered. The end wires are soldered to the shorted coax ends, run to insulators at the end of the antenna, and the soldered joints are sealed against the weather. Figure 9. Double Bazooka Dipole 6. Broad-Banded Coax-Fed Fan Dipole A broad-banded dipole for 75/80 meters can be constructed by attaching two equal length dipoles to the center feed-point and spreading the ends about 3 feet apart using PVC water pipe to separate them. The completed dipole looks like a bow tie. This makes the antenna to appear electrically to have that of a large diameter conductor. Because of this, the overall length will need to be shorter than a single wire alone. When we used the antenna, we found a length of 110 feet would cover most of the 75/80-meter band without a tuner. It is fed with 50-ohm coax. The use of a balun is optional. The antennas for most of the higher bands have enough bandwidth so they do not need broad banding. Figure 10. Broad-Banded Fan Dipole for 80 Meters 7. Two-Element Collinear Dipole The two-element collinear dipole is an antenna that is a full-wavelength antenna having a two-dBd gain. It can be fed with ladder-line and a tuner and used as a multiband antenna, or it can be fed with a quarter-wave-matching stub with 50-ohm coax cable to make it a single band array. In the stub matching system, a quarter wavelength of ladder-line is connected across the center insulator, and the opposite end of the ladder-line is shorted. A shorted quarter-wave piece of feed-line acts like an open circuit. Going from the shorted end of the ladder-line toward the dipole, there will be a point where a piece of 50-ohm cable will find a perfect match. The 50-ohm feed-point will have to be found empirically (trial and error). Figure 11. Two Element Collinear Dipole 8. Four-Element Collinear Dipole The four-element collinear dipole array consists of four half-wave segments connected end-to-end with an insulator between each two adjoining segments. The feed-point is at the center of the array. The antenna is fed with ladder-line through a tuner. A quarter wave shorted ladder-line stub hangs down vertically from the insulators between the inside and the outside half-wave segments. This stub provides a 180-degree phase shift so that all half-wave segments are fed in phase. This antenna has a 6-dBd gain and it radiates bi-directionally at an angle perpendicular or broadside to the plane of the wires. This antenna is too long for most hams to use on 80 and 40 meters, and the stubs hanging vertically will be too close to the ground. For 20 meters, the four-element collinear array will be 97 feet long and the stubs will be 18 feet. To find the length of each half-wave segment, divide 468 by the frequency in MHz, and for the quarter-wave stubs, divide 246 by the frequency in MHz. MFJ has begun marketing the four-element collinear monoband array. They have them for 20, 17, and 15 meters. This antenna is so easy to build that you can do it yourself. All you need is 5 insulators, antenna wire, and some ladder-line. It will have no gain if you use it on bands for which it is not designed because the stubs are used as phasing lines. It is definitely not a multiband antenna. It is possible to add more half-wave segments to the ends of this array to make it have 6, 8, 10, etc half wave segments. Adding more segments will add more gain and make the lobes narrower. Figure 12. Four-Element Collinear Dipole 9. Coax-Fed Dipoles Operated on Odd Harmonic Frequencies Antennas fed with 50-ohm coax can be used on other bands for which they are not cut. An 80-meter dipole will have a relatively low SWR and will be resonant at a single frequency on 10 meters and not much power will be lost in the coax even if operated off resonance. A 40-meter dipole will work the same way on 15 meters. Using coax, a dipole will work on its fundamental frequency and on odd-harmonic frequencies and it is not necessary to use ladder-line. The fundamental frequency is the frequency for which the antenna is a half-wavelength long, and the odd harmonics are 3 times, 5 times, 7 times, etc. the fundamental resonant frequency. A frequency of 21 MHz is 3 times or the third harmonic of 7 MHz, and 28 MHz is the seventh harmonic of 4 MHz. Antennas operated on their odd harmonics will be resonant a little higher in frequency than exact multiples of their fundamental frequencies. Since the odd harmonic antennas input impedance is higher than it is on its fundamental frequency, many amateurs use a series quarter-wave matching section of 70-ohm coax to give it a better match. The 80 meter inverted-V dipole in use here has a 2:1 SWR on 10 meters indicating it has an impedance of around 100 ohms. However, modeling the antenna for 10 meters shows the resonance to be below 28 MHz, probably because the antennas fundamental resonant frequency is 3920 instead of 4000 kHz. A quarter wave 70-ohm matching section should bring the SWR down to a much lower level. As said earlier, if you try to use coax with a dipole on its even harmonic frequencies, the feed-point impedance will be very high, the SWR will be extremely high, and the coax will absorb most of the power. In addition, when operating a coax-fed antenna on its even harmonics, the tuner may not be able to provide a match. Operating any antenna on any of its harmonic frequencies, odd or even, will work better if it is fed with ladder-line and a tuner. Figure 13. Three Half-wave Dipole This antenna is matched by a quarter-wave 70-ohm series matching section. Three half waves will resonate higher than you would expect because the center half wave doesnt have to contend with end effects. To calculate the length of a three half-wave dipole, divide 1380.6 by the frequency in MHz. Five half waves is found by dividing 2316.6 by the frequency. To use a 3 half-wave antenna on 15 meters, the 70-ohm matching section needs to be 7 feet 7 inches and the antenna needs to be 64 feet long for a good match. It will be just a little long on 40 meters. When using a 40-meter dipole with a 15-meter quarter-wave matching section, it will still have acceptable SWR on 40 meters. Figure 14. Radiation Pattern of a 15- Meter Three Half-Wave Dipole at 65 Feet The pattern shows 6 lobes, 4 major lobes and 2 minor lobes. The vertical radiation pattern shows low angle radiation. 10. All Band Random Length Dipole A random length of wire cut into two pieces can be used as a dipole, and it will radiate efficiently. It has to be at least a half-wave length on the lowest band you want to work. It looks the same as the all-band dipole and is the same, except it is not resonant on any band. The random length dipole is being described here to emphasize that the radiating part of an antenna doesnt have to be resonant. Because it will have a feed-point impedance that is unusual, it must be fed with ladder-line a tuner, and a balun. Since you are using a tuner, it can be used on multiple bands. If you make it very long, it can have gain over a dipole. For example, if it is four wavelengths long, it will have 3-dBd gain. As you move to higher bands, the electrical wavelength of the antenna increases, and each higher band will have more gain. A half-wave antenna radiates perpendicularly to the plane of the wire. As you move to higher bands, this antenna begins to show some gain, and instead of two lobes of radiation, the two lobes split into four lobes and the pattern resembles a 4-leaf clover. As you make the antenna longer, the four lobes move nearer the to the ends, the gain increases, and there are minor lobes of radiation between the major lobes. These minor lobes make it possible to work in all directions. The longer the wire, the closer the antennas major lobes radiate bi-directionally toward its ends Figure 15. All Band Center-Fed Random Length Dipole The problem with using a random length of wire for this antenna is you may find that because of limitations of your tuner, you may not be able to tune a particular length of antenna on some bands. Certain lengths will tune all bands and one of those lengths is 135 feet. That particular length will be nearly resonant on all bands of 80-10 meters. Resonance only makes it easier to tune, but it has no effect on efficiency. A length of 260 feet will tune from 160-10 meters. Lengths of 260 and 135 feet have been used here successfully. Some hams use random lengths of wire without problems. Then some hams have had problems with other random lengths. The ones having the problems solved the tuner problems by changing the length of the dipole wire. If you plan to put up this antenna using a random length of wire, you will need to experiment with various lengths until you find a combination that works. Tests were performed here using two towers of equal height and spaced 100 feet apart. On one tower, was an 80-meter inverted-V 120 feet long fed directly with coax, and running parallel to it on the other tower was a 135-foot long inverted-V fed with ladder-line and a tuner. At the resonant point of the coax-fed dipole and having tuned the ladder-line fed antenna, it was possible to switch antennas instantly and many hams were asked to look at their “S-meters” while the antennas were switched. All hams that participated in the test said the signals from both antennas were equal. The signals were measured on analog S-meters, not on segmented LCD meters found on most of todays transceivers. 11. A Two-Band Fan Dipole A two-band dipole can be constructed by connecting together the feed point two dipoles for even harmonically related bands. It is fed with 50-ohm coax with or without a balun. The best example of this is 80 and 40-meter dipoles connected together. Both dipoles are cut for half-wave resonance on each of the two bands. They are fed together and the ends of the wires are spread apart. If the ends are close together, there will be interaction between the dipoles. In such an antenna system, both dipoles must be carefully pruned for lowest SWR one band at a time. The lower band will be tuned first since the shorter dipole will not interact with the longer one. Each dipole has a low antenna resistance on the band for which it is resonant. RF energy follows the path of least resistance, and it automatically selects which dipole will receive power. The remaining antenna will have a high impedance. High impedance will block RF. Such an antenna will have a narrower bandwidth than a single band dipole, but close to the resonant frequency of each dipole, a tuner will not be needed. To connect many dipoles for multiple bands is possible, but it is not recommended because multiple wires are prone to interact and it will be impossible to achieve a low SWR on some bands. However, on the two band model, the 40-meter dipole will resonate close to 15 meters, the 80-meter dipole will resonate close to 10 meters, and working four bands with this set-up is possible. Some hams are using this antenna successfully with a tuner on all bands, although the signal on 20 meters suffers somewhat because of high SWR. Figure 16. Two-Band Fan Dipole for 40 and 75 Meters 12. Trapped Dipole for 75 and 40 Meters A trap is constructed from a capacitor and an inductor connected in parallel. It acts as an open switch on the frequency for which it is resonant. A trap is placed on each side of the dipole. For a 75 and 40 meter trapped dipole, the traps must be resonant on 40 meters, and each trap should be placed a quarter wave from the center insulator. The center section between the traps is electrically isolated from the ends of the dipole by the traps on 40 meters, and the center section of the antenna becomes a full-sized half wave resonant dipole for that band. This antenna is fed with 50-ohm coax and an optional balun. Wires connected to the outside of the traps are run to the end insulators and are tuned so the entire antenna resonates on 75 meters. The 75 and 40 meter trapped dipole will be shorter than a 75-meter dipole because the inductor in the 40-meter trap acts as a loading coil on 75 meters. In addition, the ends of the antenna can be tuned to operate on the 80-meter CW band instead of the 75-meter voice band. Several sets of traps can be inserted at the correct points in the dipole to make a multi-band dipole. Multi-band trapped dipoles are being sold, but in many cases they will require the use of a tuner. If a good match is found at a frequency on some bands, the bandwidth without a tuner will be very narrow. Figure 17. Trapped 75 and 40-Meter Dipole The antenna is only 108 feet long instead of 120 feet because of the loading effect of the traps on 75 meters. These dimensions are for antennas using the traps made by W2AU. If you use other brands of traps, the length of the end wires will have to be adjusted. What you do in that case is make the wire long, measure its resonant frequency on 75 meters, and prune the ends to resonance at your favorite frequency. 13. The Extended Double Zepp Dipole An extended double zepp is a long dipole with 3-dBd gain. It is the longest dipole antenna, which will radiate at right angles to the plane of the antenna. To find the overall length of an extended double zepp, divide 1197 by the frequency in MHz. Each leg of the antenna is 0.64 wavelength long and the total length is 1.28 wavelengths. An extended double zepp for 75-meters at 3.8 MHz is 315 feet. Not many amateurs have space for that antenna. The extended double zepp is mostly fed with ladder-line. Another method of matching an extended double zepp is to use tuned lengths of 450-ohm ladder-line as a series matching transformer connected between the 50-ohm coax and the dipole. The length of the matching section of 450-ohm ladder-line can be found by dividing 135 by the frequency in MHz.
Figure 18. Extended Double Zepp Dipole
14. The G5RV Dipole An interesting antenna you can buy that will work somewhat on all high-frequency bands is the so-called G5RV antenna. It is named after the call letters of Louis Varney (SK) who designed it. It is a 102-foot long or three half-wavelength dipole antenna on 20 meters (14.150 MHz), and can be used with a tuner on other bands as well. In his original design, Varney calculated the length to be 102.57 feet, but chose to make it an even 102 feet since a tuner was going to be used with it anyway. It was originally fed through a 34-foot 500-ohm homebrew open wire matching section from a 70-ohm coax or parallel conductor feed-line. The 34-foot open wire line is a half wavelength on 20 meters and at the end of a half-wave feed-line, you will see the antennas impedance repeated regardless of the feed-line impedance. The ladder-line helps partly to match the antenna on the other bands. The G5RV antenna is around 20 feet short of being a half-wave on 80 meters, and on bands on 20 meters and up, it has theoretical gain. We believe that gain is negated by losses in the coax of the feed system, except for 20 meters. At the frequency of the best match, commercially made models of the G5RV are said to have a 1.8:1 SWR on 80 meters. Where the coax joins the open wire, Varney recommended using a choke made of 8 to 10 turns of coax. He advised against using a balun, because, as he says SWR of 2:1 or higher may cause the balun to heat and possibly burn out. The SWR will be moderately high or high on bands other than 20 meters. Varney recommends using the lowest loss coax available and as short a run as practical because of feed-line losses caused by high SWR. This recommendation is very important today, as it was when Varney designed it. Some G5RV antennas put out decent signals and some others have relatively weak signals. Without further investigating, the only way to explain this is that some are using lossy coax and baluns while others are not, and the height above ground may play a part in how well it works. The G5RV antennas being made today use small diameter 50-ohm coax, 450-ohm ladder-line, and a balun between the ladder-line and the coax, contrary to Varneys suggestions. There are several variations of the G5RV antenna being sold today because many believe they can improve the original design. If you use a G5RV antenna, a tuner will be required. The G5RV shown below is close to the original version of the antenna. This one pictured below is from an old article that K4EFW found somewhere. It is like the one he used. As you can see, it uses 300-ohm TV ribbon. The length of the parallel TV ribbon is 36 feet, but modern designs of this antenna use 34 feet of 450-ohm ladder-line. All these variations work equally well when they are used with a tuner. It is shown in the inverted-V configuration but it could be put up in the flattop configuration as is, with no modification. Figure 19. G5RV Dipole Jeff, AI8H, in Oxford, Georgia, had a pair of G5RV dipoles oriented in different directions. Recently he put up a 75-meter half-wave inverted-V. Being able to switch antennas, he ran A-B tests on 3902 kHz and the inverted-V was 10 dB stronger than the first G5RV and 15 dB stronger than the other one. Now if we are saying the stronger signal is 40 dB over S-9 and the weaker signal is 25-30 dB over S-9, no one will notice the difference. Only under marginal band conditions will the difference be important. In addition, the G5RV antenna will work better on the other bands. 15. Off-Center Fed Dipoles A long dipole consisting of multiples of equal half-wave segments is normally fed in the center using ladder-line. Dipoles do not necessarily have to be fed in the center. They can be fed in the center of any one of these half-wave segments, even fed off-center. A fair match will occur if coax is used.
Figure 20. One wavelength Off-Center Fed Dipole
The dipole shown above is a one-wavelength dipole. It is nothing but two half waves end to end. It is being fed in the center of one half-wave segment or a quarter wave from one end. It is possible to make it any number of half waves, and if it is fed a quarter wave from one end, it will have a fair match. The way it is shown above is an example of how to feed an antenna with even multiples of a half wave using coax. A 2:1 or 4:1 balun will improve the match on longer versions. The windom antenna is another example of an off-center fed antenna. The original windom was fed off center with a single wire. The other side of the transmitter was connected to ground. The feed-point impedance at the transmitter was reported to be 500 ohms on all bands. The antenna was designed by William L. Everett and J.F Byrne at Ohio State University. W8GZ, whose last name was Windom, described the antenna in the September 1929 issue of QST . A lot of research concerning the modern variations of the Windom antennas has been done, including the ones described by Fritzel, K4ABT, W4RNL, The Carolina Windom, and ON4BAA. The main differences in these variations are the slight differences in the position of the feed-point and the impedance of the baluns used for matching. The Windoms are sensitive to the height over ground, meaning the height above ground affects the SWR. The offset position of the feed-point will also determine the feed-point impedance. The one sold by K4ABT is a variation of the Fritzel antenna, and the one sold by Radio Works, The Carolina Widom , claims it has a vertical radiator. There are two variations of Windoms, both claiming they have vertical radiators, The Carolina Windom, and the one previously marketed by W4COX have two pieces of transmission line in series. The upper piece is connected to the dipole, and the lower piece is connected to the transmitter. The feed-point of the dipole is placed off center. In The Carolina Windom being marketed today, the upper transmission line is coax. The one made by W4COX had the upper piece made from ladder-line, but in either case, the principle is the same. The two pieces are connected together through a line isolator, a type of balun. The line isolator keeps the lower piece of transmission line from radiating. Because the antenna is fed off-center, the marketers of The Carolina Windom claims it causes an unbalance of current in the upper piece of transmission line. This is doubtful because there is a balun at the feed-point, which should prevent the feed-line attached there from radiating. The main difference between The Carolina Windom and the one sold by W4COX is that a 4:1 transformer is between the coax and the ladder-line, and a 1:1 line isolator is between the upper and lower coax cables. Both variations of this antenna show low SWR on several bands, but a tuner is used to match it. Figure 21. Carolina Windom Another unique variation of the Windom dipole is the Fritzel antenna, named after its inventor and manufacturer, Dr. Fritz Spillenger (SK), a German ham, call sign DJ2KY. Alpha Delta is now selling an almost exact duplicate of the original Fritzel antenna. Alpha Delta calls it an OCF antenna and it is made by Buckmaster Antennas. There are two models of the Alpha Delta antenna: one for low power and one for high power, the power rating of the balun being the limiting factor. The Fritzels short side is 0.18 wavelength long and its long side is 0.32 wavelength long. It is fed with coax and a 6:1 balun. Theoretically, the feed point impedance is 300 ohms, and the balun provides a 50 to 300 ohm impedance transformation. Modeling the antenna on its lowest resonant frequency at 35 feet, it shows about 120 ohms impedance. The original Fritzel antenna being used by K4LMS reportedly will work all bands with a tuner, but it will work 40, 20, 17, 12, and 10 meters without a tuner with an acceptable SWR. The Windom being sold by K4ABT uses a 4:1 balun and the feed-point is at a slightly different location. That one is shown below. Figure 22. Windom Dipole (Fritzel Type) The difference between the Windom antenna sold by K4ABT and the original Fritzel is the difference in the offset of the feed-point. Since the K4ABTversion uses a 4:1 balun, it appears his is fed at the 200-ohm point, and the original Fritzel is fed at the 300-ohm point. On any resonant dipole, the lowest feed-point impedance is found at the center. As you place the feed-point offset toward either end, the impedance gets higher. The highest feed-point impedance occurs at the end of the dipole. XI. END-FED ANTENNAS 1. End-Fed Zepp A half-wave resonant antenna can be fed from its end. When fed this way, it is also known as an end-fed zepp. An end-fed zepp will work on its fundamental frequency and on odd and even harmonic frequencies. The name “Zepp” goes back to the days of dirigibles or Zeppelins, which used trailing wire antennas that had to be fed at one end. The end of a half-wave antenna has very high impedance, and an antenna fed this way is said to be voltage fed. Feeding a half-wave resonant dipole in the center means it is current fed. The normal way of feeding the end-fed antenna is with ladder-line. One side of the ladder-line is connected to one end of the antenna and the other side of the ladder-line is connected to nothing. To secure the unconnected side of the ladder-line, it is connected to a short wire running between two insulators. Since the antenna is connected at its high impedance point, no current flows into an antenna, but there will be a large current in the center of this antenna. No current flows from the open side of the feed-line because it is at a zero current point. Figure 23. End-Fed Zepp The end-fed zepp can be matched by cutting the ladder-line to a quarter wavelength with the bottom end of the ladder-line shorted. A certain distance above the short is a 50-ohm feet-point and it can be fed directly with coax. MFJ is marketing antennas of this type made for single bands, and they are selling the parts separately so you can build your own. You will have to find the 50-ohm point by trial and error. This method of feed makes it a single band antenna. Figure 24. Alternate Method of Feeding an End-Fed Zepp 2. End-Fed Random Length Antenna Below is another end-fed antenna made from a random length of wire connected to the back of the tuner. The wire then exits the shack and goes to a high support where it then runs horizontally to another high support. The tuners groundside must be connected to a good RF ground, since a poor ground causes high losses. This antenna is commonly called a “long wire.” Since the end of the antenna comes in the shack, you will be exposed to high levels of RF. In addition, this type of installation may cause RF to be picked up in the microphone, noted by distortion. The feed-point of the long wire being connected directly at the output of the tuner can have an impedance of a few ohms to a thousand ohms depending on the antennas length. If the wire is cut to a multiple of a half wave at the lowest frequency, the system will be efficient since it is fed at a voltage point and very little current flows into the ground. This antenna is really a variation of an inverted-L fed directly without a feed-line from the tuner. Figure 25. End-Fed Random length or Long Wire Antenna XII THE HALF SLOPER 1. The
Understanding Antennas for the Non-Technical Ham · PDF fileUnderstanding Antennas for the Non-Technical Ham ii Preface One reason for writing this book is to educate you so you can
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Understanding antennae for the non-technical ham
A book by Jim Abercrombie, N4JA (Jim Abercrombie [email protected])
Illustrations by Frank Wamsley, K4EFW
Edited by Judy Haynes, KC4NOR
Copyright July 2005. Second Edition
This is a book-length web article made available FREE by the author to all radio amateurs.
Note: This is copyrighted material and property of Hamuniverse.com and/or the author of the article and may only be used for personal, non-profit educational use.
Hauniverse.com is powered by Ham Radio! 2000 – 2008 N4UJW Hauniverse.com or article author! – All rights reserved.
mailto:[email protected]
understand antennas
foreword
One reason for writing this book is to educate you so you can make an informed choice about the best antenna for you. Another reason is to dispel the many antenna myths circulating in the amateur community. The third reason is the desire to teach the average radio amateur basic antenna theory. Therefore, to achieve that goal, you should read this book cover to cover. It was written primarily for the newbie and non-technical old-timer.
This book covers common medium wave and radio frequency (shortwave) antennas, but the theory presented here applies to antennas of any frequency. It is in condensed form and antenna theory is explained in a way that most radio amateurs can understand. Since many radio amateurs are mathematically challenged, only simple mathematical procedures are used. If you can add, subtract, and divide with a calculator, you’ll have no trouble reading this book.
Some principles here are based on inferences from the laws of physics. Everything else in this book can be found scattered throughout The A.R.R.L. antenna book and nothing here contradicts what is written there.
for the non-technical ham ii
Understanding antennae for the non-technical ham
contents
1. Why all the fuss about antennas. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
2. How antennas work. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Polarization of electromagnetic waves. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
4. Frequency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
5. The ionosphere and modes of HF propagation. . . . . . . . . . . . . . . . . . . . . . . . . . . 7
5.1 The ionosphere . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75.2 Propagation of Ground Waves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85.3 Direct Wave or Line of Sight Propagation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85.4 Propagation by Refraction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95.5 Skywave Propagation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95.6 Greyline propagation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105.7 Long Path Propagation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105.8 160 Meter Spread . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115.9 80 Meter Spread . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115.10 40 meter spread . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115.11 30 meter spread . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 125.12 20 meter spread . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 125.13 17 meter spread . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 125.14 15 meter spread . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 125.15 12 meter spread . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 125.16 10 meter spread . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
6. Standing Wave Ratio. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
7. Real antenna systems. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
8. Half-wave resonant dipole antennas. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
8.1 The Half Wave Flat Top Dipole . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 178.2 Inverted V-Dipole . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 188.3 Variations on the Dipole Shape . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
Understanding antennas for non-technical amateur radio iv
Understand antennas for the non-technical ham content
8.4 Calculation of the length of a half-wave resonant dipole . . . . . . . . . . . . . . . . . . . . . 208.5 The Decibel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
9. Antenna Basics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
9.1 Resistances and reactance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 229.2 Feeding Dipoles Efficiently . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 239.3 The Cause of Feeder Line Radiation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 259.4 Baluns . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
10. Other types of dipoles. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
10.1 A shortened dipole with charging coils . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2710.2 all-band dipole . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2810.3 The Oblique Dipole . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2810.4 Half-wave Resonant Tilted Dipole. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2810.5 The Folded Dipole . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2910.6 The Double Bazooka Dipole . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3010.7 broadband coax-fed fan dipole . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3110.8 Collinear Two-Element Dipole . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3110.9 Collinear 4-Element Dipole . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3210.10 Coaxial-fed dipoles operating at odd harmonic frequencies. . . . . . . . . . . . . . . . . . 3310.11 All-Band Random-Length Dipole. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3510.12 Center-fed all-band dipole with any length. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3510.13 A 2-band fan dipole . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3510.14 Captured Dipole for 75 and 40 meters. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3610.15 The Extended Double Zepp Dipole. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3710.16 The G5RV Dipole . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3710.17 Off-center fed dipoles. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
11. End-fed antennas. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42
11.1 End-fed Zepp. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4211.2 End-fed antenna of any length. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42
12. The Half Sloper. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44
13. Vertical Antennas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46
13.1 Why branches are used . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4613.2 Disadvantages of Using Quarter Wave Verticals. . . . . . . . . . . . . . . . . . . . . . . . . . . 4713.3 Long and Short Verticals. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4813.4 Unscientific Observations of Verticals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49
Understanding antennae for the non-technical ham v
Understand antennas for the non-technical ham content
13.5 The inverted L vertical . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5013.6 Vertical Mobile Antennas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Understanding Antennas for The Non-Technical Ham · PDF file Oriented Single Loop for 40 and 80 Meters, Single-Element Vertical Delta Loop, Directional beam antennas, Monoband Yagi,
Page 1 of 78 Understanding antennae for the non-technical ham By Jim Abercrombie, N4JA (SK) Copyright 2005 Illustrations by Frank Wamsley, K4EFW Edited by Judy Haynes, KC4NOR Updated 2019 by Alex Auerbach, K6AUR This book-length article was provided by the author , Jim Abercrombie (SK), Free to all radio amateurs, for personal, non-profit educational use only. Here are some of the topics you will learn more about: antenna systems, antennas, simple antenna formulas, basic antenna theory, feed lines, matching units, how antennas work, polarization of electromagnetic waves, frequency, ionosphere and propagation modes, ground wave propagation, direct wave or line of sight propagation, propagation through Refraction, skywave propagation, greyline propagation, long-path propagation, amateur radio propagation, antenna myths, standing wave ratio, real antenna systems, flat-top dipole, inverted-V dipole, dipole shape variations, calculating the length of a half-wave resonant dipole, decibels, resistances, and reactance , Efficient feeding of dipoles, cause of feedline radiation, baluns, other types of dipoles, truncated loaded dipole, all-band dipoles, tilt dipole, folded dipole, double bazooka dipole, broadband coax-fed fan dipole, two-element collinear dipole, four-element collinear Dipole, Coax-fed dipoles operating at odd harmonic frequencies, Three-half-wave dipole, All-band random-length dipole, All-band center-fed random-length dipole, A two-band fanned dipole, Captured dipole for 75 and 40 meters, The extended double -Zepp Dipole, The G5RV Dipole, Off-Center Fed Dipoles, Off-Center Single Wavelength Fed Dipoles, Carolina Windom, Windom Dipoles (Fritzel Type), End-Feed Antennas, End-Feed Zepp, Alternative Method of Feeding an End-Fed Zepp, End-Feed Random Length Antenna, The Half -Sloper Antenna, Verticals, Ground Mount Captive Verticals, Disadvantages of Using Quarter Wave Verticals, Long and Short Verticals, Unscientific Observations of Verticals, The Inverted L -Vertical, vertical mobile antennas, comparisons of HF mobile antennas, single wavelength single loop antennas, horizontally oriented loop, vertically oriented single loop for 40 and 80 Me ter, single element vertical delta loop, directional beam antennas, monoband yagi, three element yagi, captive multiband yagis, SteppIR antenna, The Log-Periodic Array, directional cubic quad and delta loop antennas, single band cubic quad, field strength meter, The Quagi, Gain vs Front-to-Back Ratio, Feed Lines, Antenna Safety, Erecting Antennas on Masts, Tower Safety, Quarter Wave Matching Sections of 70 Ohm Coax Diagram…and much more!
Page 2 of 78 The book starts here – enjoy! FOREWORD One reason for writing this book is to inform you so you can make an informed choice about the best antenna for you. Another reason is to dispel the many antenna myths circulating in the amateur community. The third reason is the desire to teach the average radio amateur basic antenna theory. Therefore, to achieve this goal, you should read this book in its entirety. It was written primarily for the newbie and non-technical old-timer. This book covers common medium wave and radio frequency (shortwave) antennas, but the theory presented here applies to antennas of any frequency. It is in condensed form and antenna theory is explained in a way that most radio amateurs can understand. Only simple mathematical procedures are used. If you can add, subtract, and divide with a calculator, you’ll have no trouble reading this book. Some of the principles in this book are based on the laws of physics. Everything else can be found in The ARRL Antenna Book and nothing herein contradicts what is written there. WHY ALL THE FUCK ABOUT ANTENNAS Definition: An antenna is a piece of metal, an electrical conductor, that you connect a radio to. It broadcasts your signal and receives the signals you want to hear. Definition: An antenna system consists of the antenna, the feed line and a possible matching unit. Most antennas are made of copper or aluminum, while most cellular antennas are made of stainless steel. A feedline consists of two conductors that carry the signal to and from the radio and to and from the antenna. A matching unit can be an antenna tuner, a series matching section, or one of several different types of matching circuits at the feed point. Does the type of antenna make a big difference? Many years ago a friend and I tested two antennas at 15 meters. He received a signal report on his antenna from a station in England which was 30 dB better than we got from the same contact on our antenna. We’ll explain decibels or dB later, but 30 dB means that to the guy in England, my friend seemed to be putting out 1,000 times more transmit power than me. He and I didn’t live far apart, so the difference wasn’t reproductive. We ran at about the same power and both antennas were 50 feet in the air. He used a homemade G4 ZU mini-jet; My antenna was a 15 meter commercial made 2 element beam with short loaded elements – and obviously a lot of loss. The comparison has shown that a good antenna can make a very big difference.
Page 3 of 78 Experienced amateurs know the enormous effect of a good antenna. Sometimes a guy with a bad signal blames band conditions or the lack of a linear amplifier for that bad signal report. If he ignores the effect of his antenna, he just sticks his head in the sand. After your radio, the most important part of your station is the antenna. Many years ago a vintage car said, “For every dollar you spend on a radio, you should spend two dollars on your antenna.” This is still true today. With antennas, you can do more to improve your signal strength than you ever could by increasing your power. The ability to make contacts on a particular antenna does not mean it will work well! Each antenna makes contacts, but your signals will be stronger on some antennas than others. Also, some antennas “hear” (receive) better than others. HOW ANTENNAS WORK. For an antenna system to work properly, it must first be matched to the transmitter. All modern transmitters have an output impedance of 50 ohms. However, antenna systems have an impedance range from a few ohms to several thousand ohms. There are several ways to adapt a transmitter to an antenna: shortening the length of the antenna; use of an antenna tuner; matching the antenna to a transmission line length, referred to as a matching section; or using one of several matching systems at the antenna feed point. (The details of antenna matching are beyond the scope of this book, and if you are interested you should consult a more comprehensive antenna manual.) Simple half-wave dipoles eliminate the need for a matching system because a resonant half-wave dipole has an impedance close to 50 ohms. You need to understand electromagnetism to understand how antennas work. When you connect the two poles of a direct current (DC) voltage source to the two ends of a coil of wire, current flows through the coil of wire and it becomes magnetized. The magnetized coil is known as an electromagnet. Its magnetism extends to infinity, weakening with distance. Remove the voltage and the magnetic field breaks back into the coil. When alternating current (AC) is connected to the coil, the magnetism moves out and collapses in the coil in step with the frequency of the AC source. The north and south poles of the electromagnet reverse each half cycle of AC voltage. If voltage and current can cause a coil to become magnetized, it’s the other way around: a magnetic field can create a voltage and current in a coil. This is known as Faraday’s principle of magnetic induction. A voltage is created at the ends of the coil of wire when you move any permanent magnet close to and parallel to the coil. The difference in this case is that the magnet must be kept moving. Move the magnet in one direction and the current will flow in one direction. Reverse the direction the magnet moves and the current will flow in the opposite direction. Alternating current is created by moving the magnet back and forth. An alternator rotates a coil of wire between the two poles of a magnetic field. It doesn’t matter which one moves; the coil or magnet can move. Any moving magnetic field can induce current in another coil. It doesn’t have to be a piece
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