Radio broadcast .. (1922-30)

286 Radio Broadcast more it falls off and approaches the line AC, but as this is the decreasing progression that carries on to infinity, it will never actually reach the line AC, but at some point such as B, east of New York City, it is bound to run under the static level, EF. In this particular case any receiver in New York that is capable of giving sufficient amplification to reproduce a signal of intensity CG will be able to bring in this station. At this point the signal audibility is well above that of the static level and can be reproduced without interference from this source. Now let us turn to Fig. 2. Here the same conditions prevail, with one exception — the signal-static ratio is changed. It can be seen that we have two variables here, both of which lie between fairly wide limits — that is, the actual signal strength itself represented as the distance of the line XY above the line AC and the strength of the static level EF (which is shown at the same height above AC in both Fig. i and Fig. 2, indicating it to be the same strength on these two nights). However, in Fig. 2, the line XY is seen to pass under the line EF before it reaches New York. In other words, due to poor conditions prevailing between the points A and C, the radio signal is not carrying as well the second night as it did the first, falling off more rapidly. And after it has passed beyond the point B, FIG. 3 On this night, the signal which reaches New York is represented as of the same intensity as in Fig. I . The static is worse, and there is "nothing doing" on West Coast signals where it reaches the static level, it is lost, as far as radio reception, as we know it, is concerned. There is another case that might be considered : the signal strength on these two succeeding nights might have remained the same, or varied only slightly, while the static level itself changed. This condition is illustrated in Fig. 3, where the signal strength represented by XY is identical with that indicated in Fig. i, but the signal-static ratio or the line EF, is higher. If this were the case on the second night, the operator would never have attempted to increase his radio-frequency amplification, because when he turned his set on with the same adjustments as he had on the first night, i. e., would immediately get a terrific roaring in his phones that would force him to decrease his amplification; and even though the signal itself was present with as great intensity as on the preceding night, the static would be "in" with still greater volume. And when he decreased his amplification to the point where the noise had diminished suificiently to listen-in, there would not be enough amplification to bring in the signal. Now, if you follow this thought closely you may ask: What is the use of a sensitive receiving set? What, for instance, can be done with a good super-heterodyne that cannot be done with any other receiver? This can also be illustrated most simply with the diagram, Fig. 4. Supposing the same lines, AC and AD, to represent distance and signal strength respectively, and supposing the same signal to be sent from the Pacific Coast, let us see what happens with different types of receivers, and how it is possible to receive farther on one type than another. Suppose, further, that the signal submerges beneath the static level at point B, some indefinite point between the Pacific and Atlantic Coasts. Now let us consider three types of sets: first, the ordinary crystal set; second, the regenerative receiver, and lastly, a super-heterodyne with unlimited powers of amplification (the latter, for instance, being capable of bringing any radio-frequency noise that is above the line AC, into audibility). The amplification power of the regenerative set may be indicated by the dash line GH — that is, it is capable of receiving any radio-frequency impulse whose strength is represented by the distance AG above the line AC, and lastly, the crystal receiver whose power of reception is represented by the dash line IK, and which will receive any radio-frequency impulse whose strength is greater than the distance of the line I K above AC. In this diagram we are supposing a good average winter night with the static level, EF, very low. In fact, it is well below the limit of amplification of a good regenerative receiver as indicated by its distance below GH. In other words, no static disturbance or noise will be heard on either the crystal or the regenerative receiver. The signal starts from the Pacific Coast from