Price: $69.95.
GAM antennas, I learned, are manufactured in Manchester, N.H. by GAM Electronics, Inc. who also specialize in the design and manufacture of high gain communications antennas. The FM antenna, which I tested in my lab, is marketed in the U.S. by Castle Marketing and in Canada by Mari-Tech Systems, Inc. There were no performance specifications supplied with the unusually configured antenna other than the claim that it "gives up to twice the power of conventional dipole antennas" and that it is "suitable for either indoor or outdoor use." The GAM Stereo One antenna consists of a stainless-steel whip which is mated through silver-plated, brass couplings to a precision base transformer which is hermetically sealed within a high-impact PVC tube. A heavy-duty aluminum base bracket allows for mast or windowsill mounting.
When fully assembled, the Stereo One measures 76 inches in height. It is extremely light, however, weighing a mere 12 ounces. Connection terminals are intended for 300-ohm twin-lead cable, but a 75/300-ohm balun transformer can be used at either end of the transmission line should you want to use a 75-ohm coaxial downlead for reduced noise pickup or because your tuner is only equipped with that type of input.
While I was very much impressed with the performance of the GAM Stereo One, I must warn prospective users of two things which may dictate against its use in certain circumstances. The antenna is omnidirectional. That is, it is equally sensitive to signals arriving from "all points" on the compass. The "whip," after all, is intended for vertical orientation, which means that it does not even have the familiar "figure-8" directionality of a simple half-wave dipole. Normally, I would be wary of such an omnidirectional FM antenna, particularly in mid city locations where multipath problems abound or in near-suburban areas where features of the local terrain or man-made structures also cause signal reflections. While my own lab's location fits the second of these descriptions, I was both surprised and gratified to find that the GAM Stereo One did not aggravate multipath distortion problems in my area.
The second precaution which should be observed results from the excellent gain characteristics of the antenna itself. Particularly when mounted outdoors and located fairly close to transmitters, the Stereo One is going to present an unusually strong signal to the antenna terminals of your tuner or receiver. As even the representatives of the company were quick to admit, low-quality tuners equipped with poorly designed front-ends are likely to be overloaded when presented with such powerful input signals.
No such problems should be encountered when using mid-priced to high end tuners or receivers which have adequate r.f. circuitry.
Measurements
Table I--Field strength readings of two antennas.
To be fair in testing the GAM Stereo One, I decided to treat it as though it were an indoor antenna. In that way, I was able to compare its gain with that of a standard half-wave dipole of the type normally supplied with most tuners and receivers, the simple Tee shaped from 300-ohm cable. From previous experiments involving antenna testing, I know that if I were to mount this (or any other) antenna on my roof, at an elevation of approximately 30 feet above ground level, signal strengths observed would increase by a factor of from 10-to-1 to 50-to-1.
(My lab is located approximately 20 airline miles from most of the FM broadcasting antennas in the New York metropolitan area.) For a meaningful comparison between a conventional dipole and the GAM Stereo One, I. positioned both antennas at the same spot in my lab (at ground level). With my assistant optimizing the orientation of the dipole for maximum signal pickup, I recorded signal strengths for three received signals, one at the low end of the dial, another at mid-band, and the third at the high-end of the dial. The experiment was repeated, using the GAM Stereo One and observing signal strengths for the same three stations.
In the case of the GAM Stereo One, no specific orientation was needed, since it was positioned vertically. Table I shows the results obtained. Readings are in microvolts, into a 75-ohm impedance. (A transformer was used between the antennas and the input terminals of my field strength meter.) From these measurements, it is clear that the GAM Stereo One does provide substantially more gain than an ordinary dipole-even one oriented for best reception. While I would still recommend a multi-element, directional outdoor FM antenna for best possible reception under all conditions, the GAM Stereo One would make an ideal window-mounted antenna in such instances where rooftop antennas are impractical or impossible. It would also make an excellent indoor antenna alternative to the less-than-ideal flexible wire dipoles normally supplied by FM hardware manufacturers. The unit was obviously designed and made by people who know a great deal about antennas and what makes them work well.
-Leonard Feldman
(Audio magazine, Jun. 1982)
Also see:
Terk Technologies Model Pi2 FM Antenna (Nov. 1990)
Magnum 95FM Sleuth FM Antenna Amplifier (Aug. 1982)
Antennas FM -- Try a Rhombic FM Antenna (Jan. 1982)
Kill FM Interference With Two Antennas (Jan. 1980)
FM Fidelity: Is The Promise Lost? (March 1985) and The Problem with FM (March 1985)
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