Reviews  

ST-2 Antenna and The Signal Sleuth FM Antenna Amplifier 205

Reprinted with permission from The Inner Ear Report.

Before we get into the merits of antennas of any sort, allow us to address the subject of FM signals. Signal interference problems generally increase as the distance between the transmitting and receiving antennas increases. Along with noise mixed inseparably with the signal, there can be fluttering and fading of the signal caused by aircraft flying within the area. As well, there is "tropospheric scatter" which causes part of a distant signal to be bent, or "refracted" back down to earth some distance away. This is often evident when a temperature inversion is somewhere overhead in the signal's path. Such a signal reflection can create reception where there was none before, or can disrupt reception of a local station already on that frequency. The obstructions to an FM signal range from small hills and buildings to office towers and mountains. Depending on the signal's strength when meeting such an obstruction, the effect could be partial absorption or reflection. As a result, you might end up with the main signal, plus any number of reflected signals; each of which is now out of step with the main signal. We refer to these extraneous signals as "multi-path interference". It is one or more mult-path signals arriving at your antenna out of time/phase synch with the main signal that usually disrupts the quality of the signal, rendering it "fuzzy", and very often, unlistenable.

Aside from noise generated within the locale of the antenna, most other types of interference are easily dealt with by the characteristics inherent in an competent FM tuner design. The problem is that interference can make itself known at different points within the tuner's circuitry.

The circuit's ability to deal with interference from other station signals lies mainly in its "selectivity". Some tuners may not have this quality in abundance as it could detract from the sensitivity of lesser-capable "front-end" designs. Without the necessary selectivity, the RF stages in such a design can "saturate" with signal when an antenna with more gain than dipole is used. The symptoms of such a problem present their unwelcome "images" at more than one point on the dial. Finally, a good sensitivity figure puts the tuner in a position to pull in most stations so that they can be heard in a mostly noiseless state, and in their intended stereo. This is expressed in terms of dBf or microvolts (uv). While there are three different specifications used to express sensitivity: "IHF", "Usable", and "50dB quieting", the latter, expressed in terms of "mono" and "stereo", is the most significant. It is around the 50dB quieting level that the signal should be quiet enough to provide audio worth listening to.

As we can see, receiving a clean FM signal can be as uncertain as the weather, which influences the signal. There are days when signals abound and the quality is good. And then there are days when stations, normally received well, come in so poorly that we would rather not listen to radio at all. The ST-2 FM antenna may be the solution to at least some of our problems. The whip-like device can be installed on the wall or the ceiling. It can be used in a vertical or horizontal position, where multipath interference is particularly annoying. In the horizontal position, the ST-2 becomes directional, or more precisely bi-directional, when it is placed at right angles to the transmitter site. In an upright, vertical position, the reception may not be as good, but the choice of stations increases. The best possible installation of the ST-2 is near a window or outside the house, as high up as practicable. Magnum Dynalab has designed-circuitry in the base loading coil which connects to the whip and to the tuner via a 75 ohm cable. A good length of the cable is supplied with the ST-2 -- as are succinct instructions to get the most out of the antenna. The ST-2 offers an alternative to elaborate rotor antennas which will improve signals and cut down noises under the worst condition. Those of you who have been connected to cable are well advised to try this antenna which, when in the proper position, allows the tuner to deliver a much cleaner signal. And for those who live in fringe reception areas, there is a gadget called ...

The Signal Sleuth - Model 205

This remarkable device does exactly what the name implies; it investigates FM signals and tracks them. Weak signals are amplified, thereby offering the possibility of listening to some of the fringe FM stations that are impossible to receive under ordinary conditions. The Sleuth works on adjacent channel interference by reducing the strength of the dominant channel so that the weaker signal can be handled by the tuner. In other words, the 205 adds gain to the FM signal so that your tuner can use it. Note that a good antenna (such as the ST-2, as well as a good tuner should be used to assure acceptable FM reception. If the signal is simply weak, the 205 will do its task very well indeed. When a signal is combined noise at the receiving end, the 205 will amplify this information as well which may result in amplified hullabaloo. The Sleuth cannot purify a bad signal -- this is up to the tuner's capture ratio as well as the antenna's capability to distinguish noise from signal. However, the 205 can raise the FM signal above the noise peaks by defining the pass-band and eliminating spurious interference. Weak signals from distant stations can be amplified by the Sleuth and reception becomes astoundingly clear if there is no interference close by. If noise happens to be present around the reception area, the only cure is to find the source of the noise (it may be a car with dirty sparkplugs, your AC voltage, or your power line outside). The 205 is not a cure-all device. It simply adds three more RF stages to a tuner's front end and offers an RF gain control to regulate potential overloading. The available gain is close to thirty times the input value. A tuning control additionally specifies the frequency being selected, hence appropriating attenuation to a narrow band and not to all frequencies equally. This unique method provides more selectivity. The sound of the Sleuth is as good as the back-up system, which is to say that it has no sonic signature of its own. If anything, it defines the common signal, adds texture and depth to the sound and lets one focus on the program material. Magnum Dynalab's manual is succinct, enlightening, and provides answers to questions that linger in every radio listener's mind. To sum up, the 205 is one those devices that contributes to any critical music system.