ANTENNAS
An antenna’s primary purpose is to radiate the FM broadcast signal from the transmitter to surrounding FM radio receivers. In order to do this several conditions must be met. First, the antenna must be tuned to the frequency being transmitted. Secondly, it must be sited and oriented properly.
At FM frequencies the radio waves travel in a straight line until an obstacle is met. This is known as line of sight transmission. If the receiving antenna and transmitting antenna can “see” each other and the path distance is not too great to attenuate the signal, then the broadcast signal can be received. Radio signal strength is based on the inverse square law. Double the distance and the signal strength will be one-quarter of what it was.
Since FM broadcast transmissions are line of sight, the height of the antenna is very important. Increasing the height is more effective than doubling or tripling the power. Due to the curvature of the earth, the higher the antenna, the greater the distance to the horizon. Increased height will place the antenna above obstructions which otherwise would block the signal. Your antenna should be at least 40-50 feet above the ground. Count yourself lucky if you can site the antenna on a hill or a ten story building.
An antenna is rough tuned by adjusting the length of the radiating element(s). Many antenna designs are based on or derived from what is called a dipole, two radiating elements whose length is roughly equivalent to one-quarter of the wavelength of the desired frequency of transmission. Wavelength in inches is determined by dividing 11811 by the frequency in megahertz. The result is either divided by four or multiplied by .25 to yield the one-quarter wavelength. A correction factor of .9 to .95, depending on the diameter of the element, is multiplied times the one-quarter wavelength resulting in the approximate length of each element. A table of element lengths is provided in the appendix of this primer.
Fine tuning the antenna requires the use of an SWR power meter. SWR is an abbreviation for standing wave ratio, the ratio between power going into the antenna and the power being reflected back by the antenna. A properly tuned antenna is going to reflect very little power back. Correct use of an SWR meter is described a bit further down in this section. If you can afford $100, get a dual needle meter. It shows both reflected and forward power at the same time. A good brand is Daiwa.
A dipole with tuning stubs is one of the easiest antennas to make and tune. Two dipoles can be combined on a 10 foot mast if they are spaced three-quarter of a wavelength from center to center with the elements vertical and fed with a phasing harness. A phasing harness consists of two 1.25 wavelength pieces of 75 ohm coaxial cable (RG11) cut to a length that is the product of the 1.25 wavelength times the velocity factor (supplied by the manufacturer) of the cable. A PL259 plug is attached to the end of each cable. These are connected to a 259 T adapter with the center socket being the connection for the feed cable coming from the transmitter. The other ends go respectively to each dipole. Such an arrangement will increase the power going into the antenna by a factor of 2.
Besides the dipole, a number of other antenna designs are employed in micropower broadcasting. Each one has a characteristic pattern of coverage. Antennas can be broken down into two basic types – omni-directional and directional. Under most circumstances the omni is the antenna of choice for micropower broadcasting. Polarization is another aspect to consider but does not play that big of a role in most cases. Antennas can be vertical, horizontal or circular in polarization. Most micro broadcast antennas are vertically polarized. In theory a vertically oriented receiving antenna will receive better if the transmitting antenna is vertically oriented as well. Obstructions in the receiving environment will have a tendency to bounce the signal around so that the signal will be not be exactly vertically polarized when it hits the receiving antenna, particularly in a car that is moving. Commercial broadcasters employ circular polarization, yielding both vertical and horizontal components to the signal. It is said that this is best for car radios. This may be true, given the dependence of commercial broadcasters on “drive time” as a peak listening period.
A single radiating element vertically oriented will have a rather high angle of radiation where a good portion of the signal is going up to the sky at angle of around 35 degrees or more. When you combine two vertical elements such as two dipoles you reduce the angle of radiation to a point where the signal is more concentrated in the horizontal plane. This is what accounts for the apparent doubling of radiated power when you use two dipoles phased together. Power output from the antenna or antenna array is known as effective radiated power (ERP) and is usually equal to or greater than the input power.
Several vertical element antenna designs have a lower angle of radiation even though they only use one element. These are the J-Pole and the Slim Jim designs. Having a signal pattern that is more compressed into the horizontal plane makes the Slim Jim ideal for urban environments. Both can be easily constructed from 1/2” copper pipe and fittings. Plans are available from FRB directly or the FRB web site: www.freeradio.org.
Another class of antennas are the 1/4 and 5/8 wave ground plane antennas. A commercially manufactured 5/8 ground plane for FM broadcast purposes is available for around $100. It is an ideal antenna for those want an easy to tune and assemble antenna. Set up time is less than 15 minutes.
Directional antennas are not usually required for micropower broadcasting. If the area you wish to cover lies in one particular direction, you might consider the use of such an antenna. An easy way to do this is to put a reflecting screen 1/4 of a wavelength behind a vertical dipole. The screen will need to be bit taller than the total length of the elements and about 3 feet wide. This will yield a nice directional pattern with a fair amount of power gain. Your pattern will be about 90 degrees wide. Another type of directional antenna is the yagi, a basic dipole as the radiating element but additional elements as reflectors and directors. A yagi can be a bit difficult to build for those not well versed in antenna design and construction. Your best choice is a dipole with a reflector.
For those who wish for a practical design that can be built and put to use, the following is a basic dipole antenna which can be constructed from common hardware store items. It uses 1/2 inch copper water pipe and fittings along with aluminum tubing. A half-inch plastic threaded T is used with a copper 1/2 inch threaded to 1/2 inch slip adapters at all three points. An aluminum tube 9/16 of inch or so in diameter will fit into this slip adapter and is attached with two #6 self tapping sheet metal screws. This tubing is 20 inches long. Another piece of aluminum tubing 15 inches long with a diameter small enough to slip inside the other tubing is used as the adjustable tuning element. Four slots 90 degrees apart and 1 1/2 inches long are cut into in one end of the larger tubing. A small diameter hose clamp is slipped over that end. With the smaller tubing inserted inside the hose clamp is tightened to hold it in place. This is repeated for the second element. A copper half inch thread to slip adapter is soldered to one end of a 36 inch piece of 1/2 inchcopper tubing which is the support arm for the dipole. A copper T is soldered to the other end. Then, two 3 inch pieces of 1/2 inch copper tubing are soldered to the T fitting. This allows easy clamping to a mast. A solder lug is attached to each element using one of the self tapping screws holding the elements to the slip fittings. Your coaxial cable will be attached to these solder lugs. Center conductor to one, braid or shield to the other. You can get a little fancier and make an aluminum bracket to hold an SO239 socket and attach this to the T connector.
Once you have it all put together as shown in the diagram, it is time to tune it. Adjust the element lengths to the 1/4 wave length you arrived at with the above formula. Tighten the clamps so the tuning stubs can barely slide back and forth. Mark each stub where it enters the larger tubing. Using either hose clamps or U clamps attach the antenna to the end of a mast piece 10 feet long. The element to which the braid or shield of the coax is attached must be pointing down Support the mast so that it stands straight up with the antenna at the top. It is best to do this outside.
Set up your transmitter and connect an SWR/Power meter between the transmitter and the antenna. Adjust your meter to read SWR according to the directions that came with it. SWR is the ratio of power coming from the transmitter and the power reflected back from the antenna. A properly tuned antenna will reflect very little power back, resulting in a very low SWR ratio. Too much reflected power can damage the transmitter.
Turn on the transmitter and observe the SWR or amount of reflected power. Shut the transmitter off if the level is very high and check your connections. Rough tuning the antenna by measurements should have brought the readings down to a fairly low level. Turn off the transmitter and adjust each tubing stub up or down about 1/4 of an inch. Turn the transmitter back on and note the readings. If the reflected power and SWR ratio went lower you went the right direction in either increasing or decreasing the length of the stubs. Turn off the transmitter and continue another 1/4 inch in the same direction or the opposite direction if the SWR ratio and reflected power increased. Turn the transmitter on again. If the reading is lower, continue to go in the same direction in 1/4 inch increments, being sure to turn off the transmitter to make the adjustments. Continue to do this cycle until you have reached the lowest possible reading. At some point the readings will start to increase again. Stop there.
You can do this with two dipoles as mentioned earlier in this section. Each dipole is tuned by itself and then both are connected with a phasing harness when mounted to the mast section.
A dipole antenna has a power gain of 1. There are several designs which will provide a power gain of approximately 2-3. The first design is 5/8 Ground Plane antenna.
 The next design is a J Pole antenna. It is one of the easiest to construct provided you know how to solder copper pipe with a propane torch.