The A-B-C's of Radio Waves and Antennas By Greg S. Carpenter
GregsBasicElectronics.com
What is the most important thing in common with both the transmitter and receiver? It's the antenna and without a good antenna, communication over any appreciable distance is not possible. A strong, basic knowledge of antennas is essential in electronics today. In this ebook I will cover the fundamentals of radio wave propagation and important basic antenna characteristics. Most antenna books will resort to higher mathematics and difficult analytical examples, I have limited this discussion to only simple math but I will explain a myriad of facts about antenna behavior.
Notice of rights copyright 2008 All rights reserved. No part of this book may be reproduced or transmitted in any form or by any means, without prior permission of the publisher. To obtain permission for reprints or excerpts contact Greg Carpenter at gs_carpenter@yahoo.com
CONTENTS Chapter: 1 --Catch The Wave 2 -- What Makes It An Antenna 3 -- Feeding Your Antenna 4 -- Common Types of Antennas 5 -- Which Way Is It Going? 6 -- Putting It All Together & Making It Work
Chapter One Catch The Wave
The A-B-C's of Radio Waves and Antennas
A radio wave consists of two components. The electric field and the magnetic field. These two parts of the wave are always aligned 90 degrees to each other and are constantly changing and moving. The speed of the change is related to the frequency of the wave, the movement is equal to the speed of light.. Here's an important piece of information. When the electric field component is horizontal, the radio wave is said to be "horizontally polarized" and when the electric field is vertical the wave is "vertically polarized". Remember that concept as it will be very important later. Below is a drawing of the electromagnetic wave, notice how the two fields are aligned to each other at 90 degrees. The blue in this diagram is the electric field and is vertically polarized so this signal is said to be vertically polarized.
Have you ever driven past an AM radio station transmitter site? If so you will have seen the very tall tower that is the antenna for that transmitter. That antenna is known as a vertical and radiates a vertically polarized wave. The reason the antenna is so tall has to do with the operating frequency or the "dial position". The lower the frequency the longer or taller the antenna has to be. There are 7 main frequency bands AM broadcast radio operates in the "Medium Frequency" band. .
Here are the all the Frequency bands. Very Low Frequency (VLF) 10 to 30 Kilohertz. Radio waves in this band are very reliable for long distance communication over 1000's of miles. However, the antenna systems needed at this extreme low frequency are very large and expensive. This band is often used for military communications with submarines as these low frequencies will penetrate water to some degree. Low Frequency (LF) 30 to 300 Kilohertz. Coverage at this frequency is somewhat less than in the VLF band. Night and daytime effects begin to be noticeable with signals traveling further at night. Distances of a few hundred miles are common in the daytime. Medium Frequency (MF) 300 to 3000 Kilohertz. (3000 Kilohertz is equal to 3 Megahertz) This is the region of airport beacons, the AM broadcast band and the lower frequency amateur radio bands. Daytime coverage of around 100 miles is common with very good night coverage. The increase in distance covered at night is due to the wave bouncing or reflecting off the upper atmosphere after the sun goes down in that area. This is call "Sky-Wave" propagation. Listen for this effect yourself with an AM radio at night tuned to a weak station. You will hear it fade in and out as the electrically charged winds of the very upper reaches of the atmosphere ebb and flow. High Frequency (HF) 3 to 30 Megahertz. Daytime coverage here is very short being only 10 or 20 miles from the transmitter but night sky-wave coverage is grand! Especially up to 20 Megahertz. World wide propagation is possible over the dark side of the planet. . Very High Frequency (VHF) 30 to 300 Megahertz. This is where you find the VHF TV channels and the FM broadcast band. Antennas for these frequencies are now getting smaller and more practical. However, the "line of sight" effect is becoming more pronounced. This in effect means that the transmit antenna must be "visible" to the receive antenna. You will find that transmitter sites are now located on hills or on tall towers to get coverage. The day and night effects are almost completely gone. Signals are the same strength both day and night, but there is another effect that becomes noticeable now and that is "sporadic E layer" skip. This is sometimes noticed in the summer and fall months. During these times you may see a "new" TV station on your TV where none was before and as you watch you will find that this station is really 500 or more miles away. Hardly "line of sight"! Or the FM radio band may suddenly fill up with new stations you have never heard before only to be gone again in a few minutes or maybe an hour.
Ultra High Frequency (UHF) 300 to 3000 Megahertz. OK you guessed it. This is where the UHF TV channels are. It's also where your cell phone and "wy- fi" or other wireless enabled computers work. Your microwave oven is here as well. Distances covered at these frequencies depend on the height above ground of the transmitting and receiving antennas. Absorption of the signal by the atmosphere due to moisture also increases. Ducting effects, where a warm air weather front meets cool air, can sometimes propagate waves in this band for a long distance. Superhigh Frequency (SHF) 3000 to 30,000 Megahertz. This is the world of direct broadcast TV satellites and microwave communication. Atmospheric absorption is high for frequencies above 10,000 Megahertz or 10 Gigahertz. However, amateur radio communications are being attempted at frequencies as high as 142 Gigahertz. (142,000 Megahertz) Back at frequency band number 3 (MF) the medium frequency band, I mentioned an effect called “sky wave�. It only happens at night when the sun has set and is no longer exciting the electrons in the ionosphere with radiation. At this time the ionosphere begins to reflect radio waves in the MF band and these waves then arrive at distances many times their normal range.
As you can see from the diagram the angle of the wave as it arrives at the ionosphere determines if it will be reflected back to earth. At the VHF band this effect is no longer seen.
Chapter Two
A-B-C's of Radio Waves and Antennas
What Makes It An Antenna
Transmission lines. A transmission line is anything that is used to deliver radio frequency energy from a transmitter to the antenna or from an antenna to a receiver. You are most likely familiar with the small coax cable that connects your TV to the cable company or the roof top antenna. That is a transmission line. If you are old enough to remember the flat ribbon cable used with TV antennas, that also is a transmission line. An antenna can be considered as a special type of transmission line. In a normal coaxial cable the two wires are close to each other and the currents in the two conductors, the center conductor and the outer shield, are flowing in opposite directions and therefore cancel out radiation leaving the cable. Now, if you were to split the coax into two separate wires and arrange them at 180 degrees to each other, the currents would no longer cancel and a signal would be radiated or received. That type of two wire antenna is known as a dipole. Antennas are not normally made from coax, they are usually fed by coax and made of wire or metal tubing. The workhorse of antennas is the dipole and it can be seen in various forms everywhere. The most common form is the "half wave" dipole. The physical size of an antenna depends on the frequency it is designed for. A half wave dipole for example, is only a half wave at one frequency. That length is determined by the speed or velocity of electromagnetic energy in space. We all know the speed of lightis 186,000 miles per second and that is the speed of all electromagnetic energy. Think about this, If you had an alternating voltage that changed polarity one time per second, that is it went from zero volts to maximum positive then back to zero then to maximum negative and back again to zero in one second you would have a one cycle per second or one Hertz electrical signal. If you sent that through a wire, how far would it travel in one second at the speed of light? Answer, 186,000 miles! So the wavelength of that signal is 186,000 miles. By the time the voltage finished it's positive to negative cycle the first part had arrived at the distance of 186,000 miles. In electronics we measure antennas and wavelength in meters. 186,000 miles is very close to 300,000,000 meters so we would call that a 300,000,000 meter signal. If you wanted to design a half wave dipole for that signal how long would it be? Answer, it would 150,000,000 meters long. That would reach to the moon! Now, what would be the size of a half wave dipole antenna for an FM broadcast transmitter operating at 100 Megahertz?
That's 100 million times faster cycling between positive and negative that the 1 Hertz signal. The wavelength of a 100 Megahertz signal is 3 meters, so the half wave dipole now would only be 1 1/2 meters long. (To be exact, the speed of light in a pure vacuum is 299.793077 meters per second!) To find the wavelength divide 300,000,000 by the frequency in hertz's or cycles per second. If the frequency is in megahertz, as is normally the case, use 300 divided by the frequency. So in the FM broadcast example, the frequency is 100 Megahertz and the formula is 300/100 = 3 or 3 meters. Take a look at an AM broadcast antenna operating at 1 Megahertz. (1000 Kilohertz) That's 300 meters. So a half wave antenna would be 150 meters tall!! However, the half wave antenna usually consists of two quarter wave sections that together add up to a half wave. So what you see at the AM transmitter site is a quarter wave vertical 75 meters tall. The other quarter wave length of the antenna is in the ground wires that surround it. Think of that as producing a mirror image of the vertical. Take a pencil and hold it to a mirror while you view down at an angle. The reflection of the pencil is the other quarter wave section of the antenna and the mirror is the ground system. Chapter Three
A-B-C's of Radio Waves and Antennas
Feeding Antennas
Yes antennas love to be fed, but must be fed correctly. When you are fed, you normally eat through your mouth at the top part of your body, so we would call you “top fed�. Antennas can be fed at the top, bottom, or middle. The standard half wave dipole is two quarter wave sections fed at the center. When the antenna is operated at its design frequency (half wavelength) the characteristic impedance of that feed point is about 73 ohms. That is, the antenna will look just like a resistor with a value of 73 ohms but only at the design frequency. We say the antenna is resonant when the feed point impedance is around 73 ohms resistive, and this will only happen at one frequency where the current and voltage are exactly in phase, and that frequency will be the resonant frequency of this antenna. To couple the maximum amount of energy to or from this antenna it must be fed by a transmission line with a characteristic impedance that matches the antenna. 75 ohm coax is a good match for a half wave dipole in free space. That's the same as the coax that connects your TV to the wall. A dipole can be modified to be fed by 300 ohm flat twin lead a type of very low loss transmission line not seen much anymore. This type of antenna is known as the folded dipole and has a 300 ohm feed. The point here is that the size of the antenna determines the operating frequency and the shape determines the feed impedance at resonance.
Typical coaxial cables have characteristic impedance values of 30 to 100 ohms. This is determined by the ratio of outer diameter of the cable to the inner center conductor diameter. The coax characteristic impedance (Zo) is found by the formula Zo = 138 log D/d Any coax cable you may have around your house is most likely to be 75 ohm cable. If you at one time operated a CB radio, then the coax that fed your antenna was probably 50 ohm cable. These are by far and away the two most common values. The important thing here is that it is very necessary to correctly match the antenna to the transmission line. If the coaxial line is 75 ohms it must connect to a antenna that looks like 75 ohms in order to get maximum energy transfer. Here's some strange information you won't find in too many books, but the brown plastic covered power cord known as zip cord often used on lamps, can be used as transmission line. It has a characteristic impedance of about 100 ohms. The big problem with zip cord is that losses begin to mount up as frequency is increased. It is usable up to about 8 megahertz. You could use it to make a shortwave dipole antenna complete with feed line. Let's say you want to make a half wave dipole for the 40 meter ham band using that inexpensive zip cord. How big would it have to be? Since I still measure in feet and inches, I'll use this formula to figure that out. The actual formula in feet for a half wave antenna in free space is 492 / f (MHz) but in the "real" world the actual length will not be equal to a half wavelength in space, but depends on the thickness of the antenna wire compared to the wavelength. So to be accurate, use this formula for all frequencies up to 40 MHz. 468 / f (MHz) OK let's design this antenna for an operating frequency of 7150 KHz. 7150 KHz is 7.15 MHz so as they said in high school, "plug in the numbers" 468 / 7.15 = 65.45 feet or 65 feet 5 inches. Now we know that each "leg" of our dipole will be 32 feet 8 1/2 inches long. Together they add up to one half wavelength at 7.15 MHz. which is 65 feet 5 inches. To make the antenna un-roll about 33 feet, 8 and one half inches of zip cord wire, then make a small cut between the two wires and start "un-zipping" those two wires. When you are done unzipping, wrap some electrical tape around the cord at the point where the two wires begin to separate. This will prevent any further separation. Measure the length of each leg of the dipole to be sure it's correct and then attach it to a support pole. Now un-roll the zip cord until it reaches the receiver or transmitter. Well, while this antenna will most certainly work, it won't work quite as well as it would if it were fed with 75 ohm coax. However, in a pinch this would be a great emergency antenna.
Chapter Four
A-B-C's of Radio Waves and Antennas
Common Types of Antennas
The most common antenna is the old standby half wave dipole. In the ham radio world, you may find dipoles strung between trees, or poles for use at HF or high frequency band work. (3-30MHz) As the frequencies of interest get higher the dipole may look different but it's still there. Below is a picture of a UHF band TV antenna known as the “bow tie” antenna. You may even have one or at least seen one before. The dipole part of this antenna is the “bow tie”! The reason it's shaped like that is to increase the bandwidth of the dipole. Remember a basic wire or rod dipole is only resonant at one frequency and this antenna needs to receive many channels in the UHF TV band.
Here is another common antenna known as the “ground plane”. It's just a quarter wave vertical radiator with a built in ground plane below consisting of 4 or more wires or tubing arranged at a 90 degree or greater angle.
The ground plane antenna radiates a non directional vertically polarized signal and since it is non directional it has a slight advantage over the dipole. It's usually seen used at frequencies in the VHF and above bands where size becomes practical. Look for these antennas at your fire station or police station. Here's a very unusual wire antenna. It was designed to be used on the old zeppelin air ships. Today we see the blimp flying about but these Zeppelin's were giant blimps much bigger than todays and were really Air Ships.
This antenna does not require a ground plane and is end fed. Since it is fed at the end it is a very high impedance device and cannot be fed with low impedance coax. That's why you see flat open ladder feed line which also must be cut to proper length for this antenna to work. Another sort of uncommon antenna is the random or “long wire” antenna. This is usually an end fed HF (high frequency)band antenna and it is not resonant due to its random length. The basic rule here is the longer the better. A device known as an antenna tuning unit or match box is needed to match the complex impedance of this antenna to a coaxial line. At UHF frequencies and above, parabolic dish antennas become practical. You may even have a small dish antenna on your house to receive one of the direct broadcast satellite TV services. The diameter of the dish does not indicate the frequency for which it is designed but rather is a measure of how much signal it can gather. The bigger the dish the more signal it can supply to the receiver. This can be carried to an extreme as with the deep space radio telescopes with gargantuan diameter dishes designed to gather the tiniest deep space signal possible. Near the upper end of the UHF band and all of the SHF band, coax is no longer used as the losses now are far too great. A new type of very low loss feed line is employed at these frequencies called “waveguide”.Waveguide seems magical as it has no center conductor and is only a rectangular or oval hollow pipe. The sides of this “pipe” are related to the frequency of use and at SHF frequencies the size is small enough to make this type of transmission line practical. Waveguide would work at lower frequencies but would be as large as the antenna it was feeding! At 7 MHz that would be a 66 foot diameter pipe. I think a piece of RG 8 coax would be much better.
Chapter Five
A-B-C's of Radio Waves and Antennas
Which way is it going?
Lets say you live on the west coast of the United States and are ready to put up a 40 meter dipole to see if you might be able to hear ham radio stations on the east coast or perhaps in Europe, which way should you “point� the antenna. Here is a field strength plot of a half wave dipole antenna.
On this graph you are looking down on the antenna. The antenna wire is the horizontal line in the center of the chart. The actual center of this chart is the feed point. As you can see there are two lobes of radiation. Both lobes leaving from the broadside of the antenna in opposite directions. A minimum amount of signal is radiated off the ends. To receive signals from the east and west you would orient the antenna so the ends run north and south. So the dipole is a directional antenna. How about a vertical antenna? The vertical antenna radiates a circular vertically polarized signal in all directions. It is non directional by nature but it does require a good ground plane to operate and the dipole does not. Another advantage of the vertical is that it has a low angle of radiation which makes it slightly better for long distance sky wave work. The angle of radiation of the dipole depends on how high off the ground it is, with the higher the better being the rule. The feed point impedance of the dipole is also dependent on the height above ground. Becoming lower with lower height and reaching true 73 ohms a few wavelengths above the ground. The big problem with verticals is the required ground plane and their height at lower frequency bands. That's why you will see them most often in the VHF bands. However, you do recall the AM broadcast tower, now there is a low frequency vertical with a massive ground plane!
I know a lot about those big antennas, I spent many years with them as a broadcast engineer. How do you make a dipole non directional? You stand it on it's end.
The dipole now radiates a vertically polarized signal in all directions just like the vertical ground plane antenna, but does not need the ground plane. The formula L = 2832 / f MHz is used for finding the length of the elements in inches. It's handy for work in the VHF -UHF bands. This antenna will often be seen stacked one on top of the other with the dipole elements folded into an elongated “U� shape connected at the top and bottom.
Here's the classic Yagi Beam Antenna.
In the basic 3 element version it consists of a driven element (dipole) a reflector element and a director element. The driven element is cut to the operating frequency. The reflector is made longer and the director is made shorter than the driven element. The reflector is always located just behind the driven element. Design one of these for the VHF TV band, put it up on a roof top and you have the neighbors TV antenna. The Yagi beam is directional in one direction toward the director (the shortest element) and the more director elements you add the more directional it becomes. As it becomes more directional, it also picks up weaker signals better, so the long boom TV antennas, are sold in areas of weak signals and the short boom Yagi's are for city grade TV signals. This is a 7 element Yagi designed for the VHF 50 MHz 6 meter ham band.
Notice here that the reflector, the element on the extreme left, is longer than the D.E. (driven element), that's the first element to the right of the reflector, and the directors get shorter and then remain the same for the rest of the antenna. The direction of radiation of this antenna is toward the directors and the right hand side of you screen. This antenna is shown in the vertical polarized mode, to make it horizontal just lay it down flat.
The loop antenna is another popular type. There are various types of loops. At my web site GregsBasicElectronics.com you will see a picture of me holding a loop antenna designed for the LF ( low freq.) band. This loop antenna was designed to receive signals from low power airport non directional beacons. I can hear weak signals from many miles away with this loop even living in the city. One of the features of this type of loop is that it, like the dipole, has two broad lobes and two areas of minimum reception but those minimum area lobes are very sharp and so allow me to turn the loop to null out any interference. It's built on a wooden cross frame with several turns of wire and a variable capacitor to resonate it to the desired frequency. This loop has the advantage of extremely small size as compared to the wavelength of the received signal which is hundreds of meters long. However, being this small means it's not as efficient as a dipole, but I think the advantage of size makes up the difference The bigger you make this kind of loop the better it performs. This small loop is only for receiving. A similar type of antenna is found in any portable radio. This loop is wound for the AM band and has an iron core. That core makes it very small and convenient. Take the cover off your radio and look for it. The Quad Loop Antenna. Another type of loop that works for both transmit and receive is the quad or cubical quad as it is sometimes called due to it's cube shape.
The quad consists of two elements, the driven element and a reflector. In this picture the driven element is L1 and L2 is the reflector. This antenna is easy to build for the VHF band and is great for finding hidden transmitters or radio tagged animals.
To build one make L1 one wavelength long in total, so that each section of the loop is 1/4 wave. Make L2, 5 percent longer and space the two by 1/4 wavelength. Be sure to use wooden or plastic supports. This is a great antenna for the two meter ham band at 147 MHz as it size at that frequency is very manageable. As shown this antenna can be fed with 75 ohm coax. To use 50 ohm, move the reflector closer to the director. A VSWR bridge will be helpful when doing this. VSWR stands for voltage standing wave ratio. If any antenna is mismatched to it's feed line, a standing wave will appear on the transmission line. When the proper match is made, the standing wave will be at minimum and the VSWR bridge is a device that will measure that. If you are only using this antenna for receiving then find a good steady signal and adjust the antenna for maximum signal strength when aimed at the signal source. Here is a photograph of a two element quad built for the 50 MHz 6 meter ham band.
This picture gives you a good idea of the size of the 6 meter quad. In this picture the antenna was being used to locate a low power transmitter.
Chapter Six
A-B-C's of Radio Waves and Antennas
Putting It All Together & Making It Work
So far we have covered antennas in the various common forms you might see everyday. By now you have noticed that the actual working part of the antenna is a half wave long, and usually divided into two quarter wave segments. The resonate feed impedance of the free space dipole is 73 ohms resistive but can be changed by the shape of the antenna while the size determines the resonate frequency. In order to transfer energy from a transmitter to an antenna, or from an antenna to a receiver, a transmission line is used and the characteristic impedance of the transmission line must match that of the antenna for maximum transfer and minimum loss. There are three common types of transmission line they are: • Shielded coaxial line. (coax) • Unshielded 2 conductor flat line. • Waveguide. To build an antenna for a particular frequency use L = 2832 / f MHz to find the quarter wave length in inches and 468 / f (MHz) to find the half wave length in feet. Coax cable is used from the LF band up to the mid UHF band before losses become too great. Flat transmission line has lower loss at higher frequencies, so can be used at slightly higher frequencies that coax. Finally waveguide which is a hollow oval or rectangular pipe can be used from the upper UHF band all the way through the SHF band. You might be wondering about your satellite TV dish. Since it operates in the SHF band but seems to have a coax feed line to your TV. That is possible because right at the feed point of the dish is a down converter that converts the SHF signal down to the VHF band, and then sends that VHF signal through regular 75 ohm cable into your house. Horizontally polarized waves are best received by a horizontally polarized antenna, the same applies to vertically polarized signals. In the USA all television signals are transmitted with horizontal polarity. That's why you see all the Yagi beams on roof tops set horizontally. In Europe television is transmitted vertically polarized, so all the antennas on the roof tops there are set vertically. AM signals are of course vertically polarized which works out fine for the vertical whip antenna on the car. What about FM broadcast signals? In the beginning, they too were only horizontal like TV, but later in the 1960's a vertical component was allowed. At that time most stations modified their transmit antennas to radiate both horizontal and vertically polarized waves. The reason for this is that most desktop radios use a built in loop for AM but use the power cord of the radio for the FM antenna. The power cord might be in any random position not always vertical or horizontal. So by transmitting both polarizations, the radios were able to pick up the FM band stations much better. Also in the 1960's cars were being equipped with FM radios and cars used the vertical whip antenna to receive.
Today most every FM broadcast station uses circular polarization to maximize reception regardless of receive antenna orientation. I hope you enjoyed learning about radio waves and antennas. If you are interested in learning basic electronics and continuing your exploration, then you should read my book "An Introduction To Basic Electronics" available at my web site. "GregsBasicElectronics.com"
Greg Carpenter