Radio Broadcast (Nov 1923-Apr 1924)

The Factor That Limits Long-Distance Reception 285 generally come from cities), but even in what seemingly are ideal receiving locations, in a country community, where signals are not always received with good volume. And perhaps some fairly close station that should be received well comes in weakly or is not heard at all. In the latter case, however, provided the radio-frequency noises are not bad, a sensitive receiver will bring up these near-by stations, in spite of "dead spots." On the other hand, the ratio of radio noise — or static, as we are calling it — to signal strength is increased in such a locality so the possibilities for doing good long-distance work are decreased in the same proportion. These dead spots can be attributed to numerous causes. In a city, it often happens that the receiving location is surrounded by a good many steel structures. It may be that some of these steel structures have a natural frequency corresponding to some particular wavelength. If this is so, any station of this wavelength is difficult to receive, because the main energy of the wave is absorbed before it reaches the receiving stations. In a way, this situation is similar to that brought about by stepping behind a big tree while out in a strong wind. The tree absorbs the force of the wind while you, behind it, receive little or none of its energy. Again, a receiving installation will sometimes receive short wavelengths admirably, but cannot receive long wavelengths satisfactorily, or vice versa. This trouble is also invariably blamed upon the dealer or the type of set used. But here again the cause is usually due to one range of wavelengths being absorbed more than another, a situation brought about by surrounding conditions. RECEPTION UNDER VARIOUS CONDITIONS OF STATIC AND SIGNAL STRENGTH HOWEVER, the large part played by the ratio of signal to static, in our distance reception, is the main point I wish to emphasize. Let us suppose again the case of a set with unlimited powers of radio-frequency amplification. Suppose this set is located in New York City, and while in the course of operation on a certain night, a Pacific Coast station is picked up and brought in beautifully. No noise, no interference, nothing but clean-cut speech and music. On the following night, perhaps, the same set is tuned-in on the same station when they are on their usual schedule; but with the same adjustments and same amount of amplification being used, nothing is heard. Then the operator starts increasing his radio-frequency amplification. He continues this until the set is amplifying to the point where the static level on that particular night is brought up to audibility. He begins to hear clicks, buzzes and bangs in his re 0 FIG. 2 Here, the, static level is the same as in Fig. 1. But the broadcast signal is weakened in its passage eastward, due to conditions between the Pacific (A), and the Atlantic (C) Coasts, with the result that it sinks below the static level before reaching New York (C) and no receiver now known car receive it ceivers — and still no signal. Why? The signal was there the night before and could be brought out clearly with less amplification than he is now using. And to-night, every additional step simply increases the roar in his phones. If the signal is being brought in, it is so weak in comparison to the noise that it cannot be heard. The answer is just this: for some reason, somewhere between Los Angeles and New York, certain prevailing conditions weaken the wave sent out by this particular station to a greater degree than the night before. Its energy has fallen off more quickly, and by the time it has reached this particular location in New York it has dropped below the level of radio-frequency noise and cannot be brought above it again by any ordinary methods. This can well be illustrated by the diagram, Fig. 1. Let the two points, A and C, represent the Pacific Coast and New York, respectively, with the intervening distance, indicated by the line between them. Let the height above the line AC, measured along the line AD, represent signal strength or audibility. The irregular line, EF represents the static level that existed on the first night of reception. The line XY represents the radio signal that is sent out from the Pacific Coast as it travels toward the East. It will be seen that the farther it goes in the easterly direction, the