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December 14, 2004 - New content coming VERY soon! Sorry about the delay.


Antenna Theory and Design

LPAM antennas are more difficult to construct than their LPFM cousins. Firstly, they have to be longer. Secondly, they have to be vertical to work well (which becomes an issue if the antenna is longer). Thirdly, they need to have rather long ground radials for the same reason. I will get into why shortly, but just keep in mind that, despite your best efforts, practicality and the constraints of your property will severely limit the efficiency of the antenna you can construct. If you plan on building a Part-15 station, which limits the antenna size to 3 meters, your efficiency will be especially low. Efficiency for Part-15 antennas is measured in the single digits, and sometimes even less than that.

But don't read too far into this. Practical LPAM antennas, while seemingly awful on paper, work quite well in the real world. Lower frequency signals propagate better than higher frequency, which almost makes up for antenna inefficiency. Also, there are tricks to make shortened antennas perform better.


Antenna Theory

First, it is important to understand that frequency can be measured in terms of physical length. To find the length of a given frequency, use this formula:

Where W is the wavelength ( or "wave") in feet, and kHz is the broadcasting frequency. This is important because we can use the wavelength to get an idea of how long the antenna should be. For example, to find out how long a wavelength for 1600kHz would be:

The vast majority of antennas are not made the full size of the wavelength, however. They are usually made in fractional sizes of the full wavelength. A 1/4-wave antenna, for example, would be 1/4 the length of the full wavelength. Lets say you want to make a 1/4-wave antenna. Simply multiply the fraction into the previous equation:

So if I wanted to make a 1/4-wave antenna for 1600kHz, I would make it 154 feet long. Similarly, we can find the wavelength of an antenna for a given frequency if we know its height:

Where:
A = Antenna wavelength
H = Height of antenna (in feet)
W = Wavelength of frequency (984,000/kHz)

The (overall) optimum height for a LPAM vertical antenna is 1/4-wave. If you can make a 1/4-wave antenna, that's great. But unfortunately, a 1/4-wave antenna for the AM Broadcast Band is very long and impractical for most people. From the above formula:

As you can see, these are very long antennas! For reasons of practicality (and for Part-15 broadcasters legality) most people will have to settle for much shorter antennas.


Shortened Vertical Antennas: An Overview

Shortened vertical antennas actually consist of five different elements. Each is a separate component that comes together to create the antenna.

The main components are as listed:

A) Vertical Radiator
B) Loading Coil
C) Ground Plane
D) Capacitance Hat
E) Insulator


Vertical Radiator

The vertical radiator is the element that actually radiates the signal. It can be a thin wire, a copper pipe, irrigation piping, or any other sort of metal conductive material (even an existing radio tower).

Irrigation pipe is the popular choice of commercial broadcasters. Putting up anything that bulky, however, requires guy wires to keep it from falling, and it can be dangerous if it is not done right. This explains the popularity of wire antennas or smaller elements for amateur broadcasters. One popular material for making amateur antennas is TV mast. Make sure to have plenty of guy wires to keep the radiator from falling, perhaps a set of three guy wires at the top and (if needed) another set halfway down the mast. It is suggested that you not try to make any large-diameter antennas too tall without having more advanced knowledge of antenna construction.

If you do use guy wires, use non-conductive ones, such as thick nylon wire. Using conductive guy wires can cause a lot of problems. The diameter of the vertical has to be large (and sturdy) enough to keep it from bending. If you are just making a 10 foot Part-15 compliant antenna, a piece of 1/2-1 inch diameter copper piping should suffice, and it shouldn't be necessary to have guy wires.

If you have a long tree in the broadcast area, one idea is to string a wire antenna up using the tree as the support. To do this, take a trout sinker and tie it to light fishing line. Using a slingshot, shoot the sinker over a tree or other support until you can get it to come down through the tree (this could take a few attempts). Once that is accomplished, tie some nylon string to the fishing line and pull the fishing line back towards where it was shot until the string has replaced the fishing line. Attach a wire antenna to one end of the string, with the other part of the wire antenna attached to an insulator or some sort of heavy weight. Pull the other end of the string until the wire antenna is firmly in the air, and then tie it to a nearby tree. To maximize the performance of your radiator, make the vertical as long as you can (up to 1/4-wave). The longer the vertical is up to that point, the better the antenna will perform.


Loading Coil

Impedance is a complex number, related to the resistance and reactance in a signal. It fails simple description, but for the purposes of this guide consider it a "type" of signal, and that in order for your antenna to work well with the transmitter and coax cable, it must be the same impedance. If the output impedance of a transmitter and coax cable is 50-ohms (), the antenna impedance must also be 50-ohms. A shortened antenna exhibits a high level of capacitive reactance; in essence, it behaves like a lossy capacitor. In order to make the antenna 50-ohms, the excess capacitive reactance needs to be cancelled out with its electrical opposite, inductive reactance, which is done with a loading coil.

A loading coil is, in all respects, a large variable inductor that tunes the antenna's impedance to that of the coax and/or transmitter, which is in most cases 50-ohms. Loading coils are designed to be variable, because for amateur broadcasters it is impossible to determine the exact impedance value of a homemade antenna.

Loading coils are made by taking insulated wire and wrapping it around a coil form, such as a piece of large diameter PVC pipe. Every 10 turns or so, there is a tap that comes out from the coil, giving you something to attach the transmitter signal to. You then tune the coil by selecting the tap that provides the best match for your antenna.

This method, however, only provides a coarse tuning. In order to get the maximum amount of power out of your antenna, you also need to be able to fine-tune the coil to the exact point of impedance match (called resonance). To do so, either a variable capacitor is inserted, or a variometer is constructed. Loading Coil design is detailed later in this guide.


Ground Plane (Counterpoise)

The counterpoise is one of the more confusing elements of an antenna. In a nutshell, it does two things. First, it's what the vertical portion of the antenna "pushes" off of, an electromagnetic behavior that is required to make the antenna actually work. The second practical function of the counterpoise is to provide a low-resistance return for electromagnetic current near the antenna to make up for earth losses.

A ground plane is a type of counterpoise that consists of radials (wires) extending horizontally from the base of a vertical antenna in a spokes pattern. All vertical antennas have some sort of ground plane, as all antennas require one to work well. You need a ground plane for your antenna to work well. The major difference with LPAM ground planes is that they are much longer than ones for higher frequency antennas. From the above formulas, a 1/4-wave radial for 1600kHz is 153.75 feet long.

Note that a ground plane is not the same thing as an electrical ground (though they are often electrically connected to ground via the return of the coax). That is to say, you cannot just bury a grounding rod and then use that as your "ground plane". The behavior and purpose of a ground plane is quite different than a simple low-resistance path to earth, though the use of the word "ground" for both elements often makes them sound like they are related. They are not.

If you place the radials on (or bury them in) the ground, you will have to use a lot of radials to make up for earth losses. Commercial broadcasters use as many as 120 1/4-wave radials for their stations, which is considered the optimal configuration. But this is a potentially time consuming and expensive task (at 1600kHz, you would need 18,450 feet of wire to do this!) Like with the vertical radiator it is often impractical to achieve an optimal ground plane, but try to make it as good as you can. A lousy ground plane will work far better than no ground plane at all.

One way to reduce earth loss is to place the antenna on top of an elevated roof. If the roof happens to be metal, attach the electrical ground to it and the roof itself will serve as the ground plane. If the roof is not metal, placing an antenna on it will still most likely improve range. Another benefit is that considerably fewer radials will be needed. Elevated antennas only typically need 4-8 radials to achieve optimal efficiency, far less than are needed to reach the same level with an earth-based antenna.

Here are some general rules for constructing an effective ground plane:

  • Have all radials equally spaced in a circle around the antenna. Attach each radial to a central location at the base of the antenna, such as a pie pan or a circular loop of thick copper wire. Make an electrical connection, such as an alligator clip, or solder the radials to the centerpiece.

  • The radials should be as long as possible, keeping property size in mind. Optimal length for radials varies with the length of the vertical radiator, but generally the longer the better. Don't use radials longer than 1/4-wave unless you know what you are doing.

  • The more radials the better, but the gain becomes progressively smaller. Increasing the number of radials from 4 to 16 significantly increases antenna efficiency, but after that the gains become less radical. Increasing the number of radials from 16 to 60 would provide less of a gain. If you are using an elevated ground plane, you shouldn't need more than 4-8 radials.

  • More is better than longer. 8 radials of 1/8-wave are better than 4 radials of 1/4-wave.

  • Insulated or bare wire can be used. Copper is preferred, but aluminum can be used if the acid level of the soil is low. Sizes can be anywhere from 5-20AWG. Go with what's cheapest.

  • The radials don't need to be in a perfectly straight line from the antenna, nor do they have to be perfectly evenly distributed. They can be bent, risen, or lowered slightly to fit the property.

  • If it's not possible to make a radial system, there are a few long-shot alternatives. One alternative is to bury a few ground rods or several copper pipes near the base of the antenna (eight feet into the ground), wire them together and then use that as your ground. You could also attach the ground to underground water pipes or a nearby faucet. It might be better than no ground system at all, but don't expect miracles.


    Capacitance Hat

    A capacitance hat is a component that helps to even the current flow through an antenna. Without one, more of the signal is propagated at the bottom of the vertical radiator than at the top. If more of the signal is propagating at the top of the antenna, the signal does a better job of getting above trees and nearby objects. Capacitance hats also help to increase the bandwidth of the vertical radiator, which can lead to improved audio fidelity if the "Q" factor of the antenna is too high.

    A capacitance hat can be a large metal disk, a pie plate with spokes of stiff copper wire extending from it, a pyramid of wires extending a few feet from the top and attaching to the guy wires, or simply anything metal coming out horizontally from the top of the antenna such as several wires stringed to insulators (anything non-conductive) on nearby trees. Capacitance hat construction is not critical, feel free to experiment with whatever works best for you.


    Insulator

    This is the base of the antenna. It has to be non-conductive and sturdy enough to hold the antenna in place. Avoid using concrete and wood to directly insulate the antenna, because they can absorb water. Rubber and plastic insulators work best. If you are using a wire antenna strung from a tree, nylon string tied to something heavy on the ground serves as a good support to tighten and hold the antenna in place. If your vertical radiator is thicker than a wire, the insulator will probably have to be hammered/concreted into the ground for stability. Don't forget the guy wires.


    Building a Loading Coil

    Four things are considered in the development of a loading coil. First, they need to be able to coarsely vary in inductance, which is implemented in the form of tap points in the coil at certain intervals. Secondly, they must be able to fine-tune to the exact level of inductance needed. Thirdly, they need to be sturdy, which means the wires must be firmly on the coil and the coil must be able to handle unfavorable weather conditions. And finally, they need to have enough inductance to properly tune the antenna. Variable loading coils can take a little time to make (a day or two), but once you build one you never have to do it again, even if you change frequencies or the antenna.

    There is no standard for what wire size to use, but it's a good idea to use 20AWG or larger wire to avoid loss due to wire resistance. The wire must be insulated when wrapped onto a form. Plastic insulated, litz, and enamel-coated wires work just fine. Enamel coated wire might look like bare copper at first sight but it isn't, it tends to have a darker, redder color than copper. Enamel coating can be removed with sandpaper or burned off with a lighter. The coil form most people use is a large (4 to 12 inch) diameter PVC pipe. PVC piping can be purchased at most local hardware stores by the foot; 2-3 feet should be all that is needed. Basically anything that is round and doesn't cause significant RF loss will work well, such as a plastic drinks container or an oatmeal cylinder. If you do get PVC pipe, get the white kind. The other kinds of PVC have different "dielectric" properties and can be lossy at RF (though this is a bigger concern at higher frequencies than it is at mediumwave).

    How much inductance (how many turns of wire) should your coil have? This depends on antenna length, antenna diameter, and operating frequency. We can approximate the amount of inductance required to tune an antenna, which is good enough for designing a loading coil. Make sure you design the coil so that it is somewhat larger than needed. If the coil is too small, you won't be able to tune it without adding more inductance, so good engineering practice calls for the design of a loading coil that has more inductance than is needed for the specific application.

    Loading Coil Inductance Values

    Frequency (kHz)
    Ant. Height (ft)
    Diameter (in)
    Inductance Req. (uH)
    1600
    10
    1/16
    134
    1600
    30
    1/16
    86
    1600
    50
    1/16
    62
    1600
    10
    1/2
    119
    1600
    30
    2
    62
    1300
    10
    1/16
    204
    1300
    30
    1/16
    133
    1300
    50
    1/16
    97
    1300
    10
    1/2
    181
    1300
    30
    2
    95
    1000
    10
    1/16
    345
    1000
    30
    1/16
    256
    1000
    50
    1/16
    168
    1000
    10
    1/2
    305
    1000
    30
    2
    162
    700
    10
    1/16
    703
    700
    30
    1/16
    463
    700
    50
    1/16
    349
    700
    10
    1/2
    623
    700
    30
    2
    333
    500
    10
    1/16
    1379
    500
    30
    1/16
    910
    500
    50
    1/16
    689
    500
    10
    1/2
    1222
    500
    30
    2
    654

    Once the required inductance is known, the following formulas can be used to form the coil:

    or

    Where:
    L = Inductance of coil in microhenrys (ÁH)
    N = Number of turns
    D = Diameter of coil in inches (twice the radius)
    H = Length of coil in inches



    Tapping the Coil

    One of the "gotchas" of loading coils is that you cannot wrap together a large coil, sand off an up-down section of the insulation and tap the wires as-is. It simply doesn't work, because alligator clips cannot attach to the wire. Unless you are designing a roller-inductor (which is usually more difficult), you have to build taps into the coil while you are looping the wire. The best way I have found to make taps is to use this method:

    First you drill three holes into the form every 1/2 inch or so, moving up until the coil has several sets of holes. Then, start wrapping the wire around the loading coil firmly, and when you come to one of the holes, push the wire into it, and then pull it out tightly through the center hole. Place the wire for the next segment in the other hole and through the center, strip off the insulation with sandpaper or a wire stripper, and then twist and solder the two wires together. Repeat for every hole until you have the desired maximum inductance. The inductance can be found by applying the inductance formula as you go along.

    Another quick and dirty way that works is to simply twist the wire at each tap point as you are wrapping the wire to the form. After you are done, sand off the insulation from the twisted elements and you have coil taps. This could be difficult, because the loops might come unturned when tension is applied to make the wire fit the form tightly. One way to resolve this is to apply superglue or solder the twisted wire together as with the above method. When you are looping the wire to the form, keep applying tension to the wire, otherwise it will loosen and the coil will deform. I have found that applying a small amount of superglue helps to keep the wires in place. Every tap point or so, dab some super glue near the taps and in a few places across the looped wires. Let it dry for a few minutes, and then continue.


    Fine Tuning Loading Coils

    The coil tap method only gives a coarse tuning range. In order to get good performance out of your antenna, you have to be able to fine-tune to the exact point of resonance. That is, we want to have the ability to both coarsely-tune and then finely-tune the loading coil. The coarse tuning part (the taps) will change the amount of inductance greatly, whereas the fine-tuning part will allow us to change the inductance a smaller but more precise amount. This involves either inserting a variable capacitor between the loading coil and ground, or making a variometer. Inserting a variable capacitor between the coil and the ground allows for a fine tune, because it tunes more smoothly and precisely. Variable capacitors can only tune a small amount because of their low capacitance values, so coarse tuning is still required. With a variable capacitor, one sets the inductance to be slightly higher than is required for resonance, and then the capacitor is turned until the excess inductance is cancelled out, which creates exact resonance.

    The biggest drawback with a variable capacitor is that it can be hard to find them (your best bet is at HAM fests and old electronic parts stores). Also, after about 10 Watts they have to be high voltage or else they won't work (the RF electricity will arc across the capacitor's metal plates). If you run into this problem or can't find a variable capacitor, an alternative is to use a variable-inductance method of fine-tuning, such as a variometer.


    The Pseudo-Variometer

    A pseudo-variometer is basically a large outer loading coil, with a smaller inner coil inside of it. The inner coil can rotate, and is connected electrically in series with the outer coil. When the inner coil is turned, it slightly changes the overall inductance of the loading coil, allowing for a fine-tune.

    Variometers have become a rare item since the advent of high frequency communication. In the early days of radio they were used for tuning receivers, but were quickly phased out in favor of the more selective variable capacitors. Despite being out of use for years, they have seen a recent revival in amateur broadcast antenna tuning. In my own work, I have found that variometers work very well for tuning shortened broadcast antennas. They can be built from scratch out of readily available parts, and aren't prone to shortouts at high voltages. Variometers will be slightly more lossy than variable capacitors, but not by a large amount. Real variometers have a large amount of inductance on the outer coil and an equally large amount of inductance in the inner coil. This makes their tuning range very wide, which is why they were popular with early radio receivers. A pseudo-variometer is very similar to a variometer, except that the inner coil contains significantly less inductance. The idea is to make a coarse tune with the taps on the outer coil, and then to use the inner coil to change the inductance slightly.

    Good engineering practice would call for the inner coil to be able to change the inductance more than the total amount of inductance between each tap point. Because this is a fairly complex analysis, I have opted to keep it in the Advanced section of the LPAM Handbook. The most important thing is to make the inner coil have more inductance than is needed.

    The primary (outer) loading coil for a variometer can be made exactly the same way that the loading coils above are made, except that there is a space in the middle of the outer coil for a handle to turn the inner coil with. The construction details of the inner coil are not critical, 15-45 turns sloppily wrapped around a form 2-6" in diameter and perhaps 4-8" long (depending on the outer coil size) will work just fine. 40-80uH of inductance should be more than enough to do the tuning.




    Tuning the Antenna

    Once the loading coil is ready, you have to tune it to resonance. Tuning a mediumwave antenna is different than most other types of antennas. Because of the lower frequency, most commercially retailed wattage/SWR meters do not work. Because of this, a little bit of ingenuity is needed to actually know when you have tuned the antenna.

    When the loading coil is properly tuned to resonance, the power level going to the antenna and the signal strength coming out will be maximized. Knowing this, we can determine the point of resonance by finding the peak rating on meters that determine either of these things. The first way to do this is to put an RF ammeter between the loading coil and the antenna. This is the best method, but it requires an ammeter that works at mediumwave frequencies.

    If you don't have an RF ammeter, the next best way is to use a field strength meter. If you don't have a field strength meter, you can make a crude one very easily. Field strength meter schematics are available here.

    Homemade field strength meters won't give a calibrated reading, but they are good enough for determining the resonance point of an antenna. When using one, make sure to keep it within a few feet of the antenna, otherwise the transmitted signal will be too weak for the field strength meter to see. Then, simply tune the antenna until the highest point on the meter is reached.

    Tuning the antenna is simple. First, attach the antenna clip to the uppermost tap and attach the transmitter clip to the lowermost.* Turn the transmitter on (if power is variable, set to lowest power). Don't put any audio into the transmitter while tuning. Then, while looking at the meter, take the transmitter clip and keep moving it up, one tap at a time, until about the middle of the coil. Whichever tap gives the highest reading on the meter is the tap point you will use (if there's no noticeable difference, set it to the first tap after the ground). The point of this is to find the best impedance match for the transmitter clip.

    The next step is to tune the antenna. Take the antenna clip and move it to the same tap as the transmitter. Slowly turn the variometer or the variable capacitor, and if the antenna doesn't resonate, move to the next highest tap and try again. You will notice that when the antenna is resonating the meter reading will go up significantly. Once the antenna is tuned and ready, set the transmitter to full power and you're ready to broadcast.

    *The image above intends to show the electrical connection, connecting the male end of a coax cable directly to the tap would be impractical. Mount a female SO-239/BNC connector onto the unused space of the loading coil, solder wires to it, run them through a drilled hole to the outer coil, place alligator clips on the ends, and then tap the coil that way.


    Safety Factors

    Static Charge

    In dry windy conditions, antennas can generate a static charge that can damage the transmitter. It can also shock you, which isn't dangerous, but could cause an accident if the antenna is on top of an elevated roof and the shock causes you to fall. Because the static charge is DC, it is relatively easy to separate and remove it from the antenna. Infact, if there is a connection between the vertical radiator and the ground on the loading coil, the static charge should drain off naturally. If not, you can install a high-inductance RF Choke between the hot wire and ground. The RF won't pass through, but the static charge (which is DC) will. Installing a 500k-1MEG ohm resistor between the antenna and the ground will also bleed off the charge, but will take a small amount of power out of the antenna.


    RF Burns

    An RF burn is the result of touching something carrying RF power (such as the antenna wire). It usually just creates a tingling sensation at the touch point, but at higher levels (greater than 10 watts) it can create a burn, similar to if a hot surface or coals were touched. RF power usually conducts through the outer skin and therefore isn't typically dangerous to humans in terms of heart failure or shock. However, this "skin effect" is no guarantee: it is still possible to electrocute, shock, or even kill yourself with RF power! It's a very good idea to take precautions to avoid exposure to RF power. If you are running higher power levels, it is advisable to put up a fence or a warning sign so that others do not touch the wire. When tuning the antenna, putting on rubber gloves of the gardening or chemical type will keep you from being burned.


    Lightning

    Lightning is a daunting problem for low power broadcasters. If you live in an area that is prone to thunderstorms, some form of lightning protection will be needed. Don't think it can't happen to you! I have heard many stories of lightning hitting unprotected low power broadcast antennas, and the results are very destructive (though thankfully, I have yet to hear of a fatality).

    When lightning hits an unprotected antenna, the electricity will enter the transmitter and destroy anything connected to it (audio amps, computers, ect). If a DJ happened to be on the microphone, he could also be the victim of a lightning strike. A defensive mechanism needs to be put into place to keep equipment and people safe in the broadcast room.

    The common practice for radio broadcasters is to bury a ground rod (or copper pipe) under the antenna, and attach to it a thick copper wire (4AWG) with its end very close to the bottom of the vertical radiator without physically touching it. Lightning will spark across the small air gap to the copper wire, and then run into the ground rod. The ground rod must be at least 8 feet long to be effective. If you're paranoid, there are lightning suppressors that go in series with a 50 ohm coax cable, which also require a grounded wire. A combination of both methods could be used.

    If you are extremely paranoid or are in a high risk area, additional steps can be taken with the transmitter. It wouldn't be a bad idea to put the transmitter in a different area than the broadcast room (remember that for Part-15, the transmitter needs to be at the antenna anyways). The bottom prong on three-prong AC outlets is electrically connected to a ground rod in most homes. If the transmitter case is metal make sure it is grounded, this provides another route for lightning. Another option is to separate the physical connection between the transmitter and the studio by using a wireless link, such as a WiFi Internet system or a Studio to Transmitter Link (STL).


    Summary

    LPAM antenna design is fairly complex, and if you're new at this it might take a few reads through this chapter before the concepts and procedures make sense.

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