Audioclinic (July 1977)

FM Antennas

Q. This is a request for your help in setting up the best antenna system for my location. Most of the stations I would like to receive are from Montreal, Canada, 120 miles up the Champlain Valley. Should I use a stacked or an unstacked antenna, how many in the stack? Tower, rooftop, or mast; how high? How about getting the last microvolt of signal down to the ground; are there commercial amplifiers as well as super cables and connections to do the job?

A. It is often quite possible to obtain satisfactory FM reception from stations which are more than 200 miles from the receiving site, but if the terrain is not favorable, no antenna system, no matter how great its gain, will ever help. If the antenna cannot be erected high enough to clear obstructions you stand little chance of obtaining reliable reception. You may, at times, hear these signals, but this will be a matter of weather conditions rather than solid groundwave reception.

Where obstructions are not present, you should experience no problem, especially with today's tuners which are far better than the best commercial units available 10 years ago.

The FM band covers a wide portion of the r.f. spectrum and in order for the antenna to encompass such a range, its Q must be rather low which results in a decrease in available gain.

However, if you are not concerned with tuning in the entire FM band, but want only to hear some specific station or cluster of stations within a certain section of the dial, an antenna could be cut to resonate at this area of the dial. The response over the rest of the band would not be optimum, but you would have gain where you need it.

By stacking antennas, gain can be increased, regardless of the Q of the antenna systems used. Adding a second antenna will add 3 dB gain. Adding two or more antennas will add another 3 dB gain, for a total gain of 6 dB.

While it is possible to buy readymade or custom cut commercial antennas, it is also possible to make your own if you know the formulas involved. This may be obtained from "The Antenna Handbook" by Krauss, a classic in the field. Also, "The VHF Handbook" by the American Radio Relay League in Newington, Conn. is another good source of information.

No matter what kind of antenna you use, the higher you erect it the better your reception will be. However, any antenna mounted in the open, high above surrounding objects, must be protected against lightning. A competent person must do this work. Also such an antenna must be protected against the force of high winds. The more complex the antenna system, the greater will be the effect of the wind upon it.

If the run of cable between the receiver and antenna is great you will best be served by what is known as an "open wire feeder" which resembles a chain ladder-two parallel conductors are separated by insulators spaced about 18 inches apart, running at right angles to the conductors. The distance between the conductors determines the impedance of the feed line.

Where the need exists for use of coaxial cable, there may be a need for using antenna booster amplifiers, which are located right at the antenna and make up for cable losses. However no significant improvement in the signal-to-noise ratio can be expected from the use of such amplifiers, even though there is more signal. If there are strong local signals in your area, the amplifier will boost them also and their added strength can possibly overload your tuner.

However, this condition can also occur by virtue of having a better antenna system than previously. In this event, band-pass or band -reject filters may have to be used to reject the effects of these strong signals.

(Source: Audio magazine, July 1977; Joseph Giovanelli)

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