Company Address: RF Limited, P.O. Box 1124, Issaquah, Wash. 98027.
Seems as though there's a trend toward tall, pipe-shaped devices in the world of audio. The June 1989 issue contained an "Auricle" review of Phantom Acoustics' Shadow acoustic controller, a tube which stands 83 1/2 inches high and has a diameter of 9V2 inches. This device is intended to serve as the last component in the audio chain-a component that can alter room acoustics.
The AudioPrism Model 7500 indoor antenna might well become the first component in your audio chain if you have FM reception problems that stem from the use of an inadequate FM antenna. The 7500 is even taller than the Shadow, standing 89 1/2 inches high, with a tube diameter of only 4 inches. At its regular price of $149.95, it comes with a base 13 inches in diameter, but a 17 inch base is available for an additional $3. The tube is coated with a coarse woven fabric, in black or beige. Audio Prism will also custom apply your own fabric to the outer casing. The base material is solid wood.
Because of the odd physical dimensions of the 7500, it is shipped in two cartons: One for the long, pipe-like antenna tube, the other for the flat wooden base.
The electrical design of the 7500 is based on sound principles of physics. The most efficient FM antenna will present as much surface as possible to arriving signals. The antenna elements of the 7500 consist of a full half-wave section (7 feet, 2 inches) over a quarter-wave matching stub that is shunt fed for true 75-ohm operation. By utilizing a full half-wave design, without the use of loading coils to reduce antenna size, the AudioPrism lets the receiver or tuner operate at its maximum dynamic potential without introducing additional noise. Of course, there are designs around, some of which I have tested in the past, but the nice thing about this one is that it is completely passive-no transistors, FETs, or circuitry other than the antenna elements themselves. Therefore, the 7500 does not need to be connected to any power source, yet it has as much gain as most amplified antennas, or more, without any accompanying problems.
The antenna's voltage standing-wave ratio (VSWR) is rated as 1.9:1 or less across the entire FM frequency band, As a side note, the manufacturer advises that the 7500's antenna elements are heavy enough to permit the antenna actually to transmit approximately 150 watts if hooked up to an FM transmitter! Because of its configuration and length, the 7500 can receive low angle transmitter signals with less fading and flutter than most other indoor FM antennas. AudioPrism rates antenna gain at 5.1 dB, and its vertical orientation makes it omnidirectional. Despite this omni-directionality, however. I multipath rejection than most other smaller indoor antennas I have tested.
Assembly of the AudioPrism 7500 took only a few moments. The hardware needed to connect the wooden base to the long tubular section is provided. All you need is a 7/16-inch socket wrench with which to tighten two 1 1/2 inch-long machine screws that hold the two parts together. It's important not to overtighten the screws since the underside of the wooden base is coated with an aluminized "ground plane" that is part of the antenna configuration; overtightening the screws can destroy the integrity of this ground plane.
A push-on F-type connector is packaged with the antenna, as is a 75/300 ohm transformer, in case your tuner or receiver does not have a coaxial 75 ohm input connector. I used a high quality coaxial cable about 10 feet long to connect the antenna to my reference tuner and, during bench measurements, to my Blonder-Tongue field strength meter; AudioPrism now supplies such a cable, which they claim has only 0.26 dB of total loss.
As in previous antenna tests, I compared the signal strengths of various stations across the FM band, as picked up by the 7500 and by a wire dipole of the type normally supplied with tuners and receivers. Both antennas were at ground level. The simple wire T antenna was oriented for best reception of signals arriving from the World Trade Center and the Empire State Building in New York City, some 18 airline miles from my lab.
The bar graph of Fig. 1 shows how much greater signal strength was obtained with the AudioPrism 7500 than with the simple dipole antenna. In every instance, the 7500 outperformed the dipole in this regard-in some cases, by an increase in signal strength of 2:1 or more.
Fig. 1--Signal strength vs. frequency for AudioPrism 7500 and simple wire-dipole
T antenna.
The superior performance of the AudioPrism 7500 as an indoor FM antenna is not confined to signal strength alone. In listening tests conducted over several days, I noted that some signals I normally receive with a fair degree of multipath interference seemed cleaner and less subject to such problems. During my listening tests, I logged no fewer than 52 usable signals, of which 46 were received in stereo with satisfactory quieting.
Since my reference tuner, operated in the automatic stereo mode, mutes at signal levels below about 30 dBf, all 46 stereo signals were received at signal strengths exceeding this level. Noise levels suggested that most were well above that minimum, or muting-threshold, level. With my roof-mounted (30 feet above ground level) multi-element outdoor antenna and a rotator, I can receive only 56 usable signals. The T wire indoor antennas supplied with tuners and receivers are seldom able to receive more than 30 usable signals in my location.
Of course, even though the makers of this antenna have attempted to "clothe" the tall pipe-like structure in fabric, the 7500 is still an imposing piece of gear. It would take a very understanding spouse (especially if that spouse is not as dedicated to good FM reception as you are) to tolerate this structure as a permanent fixture in the living room or den. Fortunately, I have a separate lab where I reign as king, and no one dictates how I furnish it.
In this respect, the people at RF Limited, of which AudioPrism is a division, have advised me that they have a couple of somewhat smaller models on the drawing board. Hopefully, these will be available soon for those households in which installing the superior Model 7500 is absolutely out of the question.
Until those other models are available, or if you want a no-compromise indoor FM antenna to bring out the best in your tuner or receiver for a not unreasonable $149.95, my advice is to install the 7500--even if you have to hide it in a closet!
--Leonard Feldman
Also see:
Dennesen Polaris Indoor FM Antenna (Dec. 1984)
Terk 9600 Pi FM Antenna (Feb. 1989)
FM Specifications Revisited by Leonard Feldman (April 1978)
FM Fidelity: Is The Promise Lost? (March 1985)
The Problem with FM (March 1985)
= = = =