Manufacturer's Specifications:
Amplifier Gain: Adjustable, 0 to 18 dB.
Selectivity: 0-dB gain at 50 MHz and at 150 MHz.
Twin-Tone Intermodulation Distortion: Less than 0.1% at 100 mV.
Noise Figure: 3 dB.
Output Matching Accuracy at 75 Ohms: Standing Wave Ratio less than 1.2:1.
Pickup Pattern: Omnidirectional or directional.
Price: $75.
Company Address: 740 Broadway, New York, N.Y. 10003.
Every time I write about the need for a good outdoor directional FM antenna, I am bombarded with correspondence from irate readers who tell me they aren't permitted to (or can't) mount a roof antenna. Once in a while I am asked if there is an alternative to a high-gain, directional, multi-element, outdoor FM antenna. The last one I came across was the Beam Box, a clever little component marketed by BIC/Avnet. Alas, that antenna, and the company that developed and sold it, are no more.
But don't despair, you apartment dwellers who, for whatever reason, can't mount antennas atop your homes. Now there is another excellent indoor antenna available, the Terk 8403. This little antenna, which comes from Italy, is an active device; that is, it contains an r.f. booster amplifier circuit and must therefore be connected to an a.c. outlet. Normally this is no problem, since you can connect the antenna's line cord to a switched a.c. receptacle on your amplifier or receiver.
The configuration of the Terk 8403 is unusual, to say the least. It stands about 16 inches tall and resembles a tapered obelisk. A visitor to my lab described it as a miniature model of the Washington Monument--except for its black color. Emanating from its base is a coaxial cable, which terminates in a standard F-type connector, and the power line cord. If your particular tuner or receiver is not equipped with a standard, 75-ohm coaxial connector, you can attach the 75/300-ohm transformer supplied with the antenna.
The 300-ohm end of this transformer is terminated with a short length of standard twin-lead cable and a pair of spade lugs that fit nicely under most antenna-terminal screws. The owner's manual tells you in detail how to connect the Terk 8403 to a wide variety of antenna terminals-both 75and 300ohm-that are-found on typical FM tuners and receivers.
The only user control on the 8403 is a gain adjustment whose small shaft is accessible from the bottom of the antenna base. Since this control covers a very wide range (from no gain to around 24 dB of gain at the middle of the FM band), I found its location a bit disconcerting. Gain is supposed to be adjusted for optimum reception, but to do so you must upend the entire antenna, which often changes the strength of the received signal that you are trying to optimize. In my opinion, the gain control should have been brought up to the front, near the tiny red pilot light.
That would allow you to adjust gain without changing the orientation of the antenna itself.
Terk makes a big point of the fact that the antenna, omnidirectional when it is normally oriented (standing on its base), can be made directional when it is oriented horizontally. Indeed, I found this to be the case, and as a result I decided to measure the antenna's reception capabilities for both its omnidirectional and directional orientations.
As in previous tests of this kind, my reference antenna was a carefully measured, folded dipole of the type normally supplied with most tuners and receivers. As we all know, very few users take the trouble to tack up these T-shaped, 300-ohm wire antennas properly, with the upper part of the T stretched out horizontally to its full length and oriented perpendicular to the direction of the incoming signal.
But for the purpose of these measurements I did so, mounting mine on a wall that faced New York City's Empire State Building and World Trade Center, where most of my local FM signals originate.
The Terk antenna was placed on the test bench a few feet away from a Blonder-Tongue Model FSM-2 field strength meter. This meter covers the entire VHF and UHF TV-frequency bands, and, since it is continuously tunable, it covers the entire FM band as well. A headphone jack on the test instrument permits the user to monitor the incoming signal modulation. By comparing what I heard with what was received on my frequency-synthesized tuner, I was able to pinpoint the incoming signal frequency without having to wait for station identification announcements. Altitude at the test location is about 75 feet above sea level, and the distance to mid-Manhattan is around 18 miles.
Measurements
Because I wanted to compare this with the kind of performance I would get from the 300-ohm, twin-lead dipole antenna mounted on the wall, I considered whether I should re-orient the reference antenna optimally for each incoming signal. I decided against it because typical users are unlikely to reposition a wire dipole antenna every time they change stations. Accordingly, I picked up nine signals, spaced reasonably apart in frequency, using the simple reference dipole. Signal strengths for these nine different signals, measured in microvolts referred to 75-ohm antenna input impedance, are tabulated in the second column of Table I. With the dipole in its fixed position, signal strengths ranged from a low of 40 µV (the station at 105.9 MHz) to a high of 275 µV (at a frequency of 92.3 MHz). The third column in Table I represents signal-strength readings for those same stations when picked up using the Terk 8403, positioned vertically for omnidirectional reception. The two readings in this column, separated by a slash, are for minimum and maximum settings of the antenna's gain control. Notice that in every single case, the maximum-gain readings obtained with the Terk antenna in its omnidirectional mode were higher than those obtained with the ordinary dipole placed in a fixed position. The differences in signal strength, however, are not consistent from station to station.
Perhaps this is because some stations were more on the dipole's axis than others; perhaps it is because some stations transmit all their energy in horizontal polarization, while others use both vertical and horizontal.
Table I--Comparative strengths (in µV) of incoming FM signals, for standard
dipole and Terk 8403 antennas.
Notes:
1. Gain figure is the ratio, in dB, between the signal strength from the Terk 8403 when set for maximum gain and oriented directionally towards each station and that from a standard dipole. See text.
2. N.A. = Reading too low on field strength meter to accurately read; signal too weak to be usable.
The readings obtained at minimum gain settings of the Terk antenna are all lower than those obtained with the standard twin-lead dipole. This is to be expected since, among other reasons, the overall length of the Terk antenna is barely more than one-quarter of a wavelength at FM frequencies, whereas the standard twin-lead dipole is a half-wavelength long.
The next column contains pairs of readings (for minimum/maximum gain settings) with the Terk antenna positioned horizontally and oriented for highest signal readings. Generally speaking, I obtained higher readings for most of the nine stations I checked when in this directional mode than when the antenna was standing up in its omnidirectional mode. However, readings were the same for two stations, regardless of the Terk's orientation, one station consistently came in weaker with the antenna directionally oriented, and another station came in weaker in the directional mode than in the omni mode when the antenna's gain was set to minimum, but stronger when the gain was set to maximum.
Since maximum signal strength was obtained most often with the antenna in its directional mode, and oriented toward the station, I used these figures (for the Terk's maximum gain setting) to show its gain, in dB, relative to that of a standard dipole for each of the nine stations used in this test. For example, the ratio between the signal strength at 92.3 MHz with the Terk antenna set for maximum gain and oriented towards the station (650 µV) and with the dipole (275 µV) is 650/275, or 2.36. Converting this to decibels yields a result of 7.5 dB, using the formula:
dB = 20 log10 V1/N2.
Gain is not always the most important aspect of a directional antenna's performance. The ability to reject signals from some angles is also important, in terms of the antenna's ability to reject multipath signal reflections. Directional roof antennas usually reject signals arriving from the rear; the Terk, in its directional mode, rejects signals arriving from the side. Its pattern, in other words, is a figure-8, somewhat like that of the standard dipole.
But the Terk antenna's shape and variable gain make its directivity more useful than a dipole's. Its shape makes it physically easier to orient in any desired direction. And its gain can be varied to meet the task at hand: In the rare cases where aiming the 8403 at the station would bring in multipath along with the strong, direct signal, it can be aimed at a single, weak reflection and its gain raised to compensate.
Alternatively, gain can be lowered to drop weaker multipath reflections down to signal levels that the tuner will be able to ignore.
To measure the Terk's directional abilities, I rotated it, still in its directional (horizontal) mode, so as to obtain minimum readings for each of the nine stations checked. The larger the ratio between signal strengths when the antenna is oriented for maximum and minimum reception of a station, the more directional that antenna is. The ratio of rejection is usually given in dB, as I have done in the last column of Table I. For example, the ratio between the signal strength at 92.3 MHz with the Terk antenna at its maximum gain setting and oriented toward the station (650 µV) and away from it (75 µV) was 650/75, or 8.67:1. Converting this to decibels yields a result of 18.7 dB.
Use and Listening Tests
From earlier tests of active antennas, I have learned that signal-strength readings hardly describe a given antenna's merits. The mere fact that an active antenna provides gain doesn't mean that reception will be improved.
For example, if noise is amplified along with the desired signal, there will be little or no improvement in FM quieting.
I felt, therefore, that it would be important to experiment with the Terk under actual listening conditions. After doing just that for a week or so, I can report that this little antenna really does a great job. For the most part, I used the antenna with its gain control set nearly to its highest point and with the antenna standing straight up, for omnidirectional reception. Admittedly, my location does not suffer too much from multipath interference, so one night, while visiting someone in New York City, I convinced my host to disconnect his roof antenna (he is one of the lucky few who is allowed to have his own) and to substitute the 8403.
This test was much more severe than the listening tests I had conducted miles away from the city, for now we were plagued with all kinds of multi path and overload problems. Happily, the gain control on the antenna was able to solve the overload problems easily, and careful orientation of the antenna itself (now operated horizontally for greatest directional sensitivity) solved all but the most severe multi path problems. A trace of multipath distortion still could be heard on two incoming signals-but at least a dozen signals had suffered from severe multi path distortion when we tried the standard dipole in this location. Interestingly, the Terk even cleaned up some of the stations that had exhibited multi path distortion when their signals were received via my friend's carefully oriented outdoor antenna.
As an FM-radio devotee, I am always pleased to see new products that make it possible to hear FM more nearly as it was meant to be heard. The Terk 8403 is just such a product, and its cost will hardly be noticed compared to that of your FM tuner or receiver. It will help you get more out of your FM radio than you thought was possible-especially if you are a city dweller who had all but given up hope of receiving clean, noise-free, distortion-free signals.
-Leonard Feldman
(Audio magazine, Jan. 1986)
Also see:
Terk Technologies Model Pi2 FM Antenna (Nov. 1990)
Magnum 95FM Sleuth FM Antenna Amplifier (Aug. 1982)
GAM Electronics Stereo One FM Antenna (Jun. 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)
= = = =