Page:Popular Science Monthly Volume 92.djvu/173

 Wireless Work in Wartime

��VI. — Atmospheric or static interference, and how to secure practice in operating through it

By John L. Hogan, Jr.

��WHEN the student has learned to send and receive correctly, and has had sufficient practice in reading wireless messages through artificial "station" interference, he is ready to take up the most important (and perhaps most difficult) problem that confronts the radio operator. This is the copying of received messages in spite of the interfering sounds produced by natural electrical disturbances, and called "static," "atmos- pherics," "X's" or "strays."

The December article described simple ways to practice reading messages when interfering sounds corresponding to un- desired signals from some external radio station are heard. The four earlier in- stalments of this series outlined the work necessary to learn the Morse code and the sending and receiving of messages. This article takes up the study of atmospheric interference, its effects, and the reduction of harmful results from it.

In the first place, we must examine the differences between strays and signals. These differences are, fortunately, usually well defined. First we shall consider the very practical distinction depending upon the fact that strays or static in general produce irregular noises at the receiver, while radio signals may be made to produce musical tones.

In the December article it was pointed out that practice enabled the receiving operator to distinguish between the sig- nals heard from two stations, so that messages from one of them could be written out even though both were send- ing at the same time. The more skilful the receiving operator, the more closely he is able to concentrate, and the nearer alike the two signal sounds may be without producing interference. The two distinc- tions usually relied upon are pitch and intensity; if the signal tones are equally strong, there must usually be a con- siderable difference in their pitch or frequency if one is to be read "through" the other. If the interfering signal is

��much weaker than that from the com- municating station, not so great a differ- ence in pitch is necessary in order that the receiving operator may concentrate upon the desired dots and dashes.

Static Noises and Signal Tones

As has been indicated, since strays usually set up irregular noises rather than tones at the receiver, the operator there is usually able to concentrate upon the messages he wants to receive and to ignore the^ interference because of the difference in sound. Static sounds are of various

����Simple method of producing a static and a more elaborate model for obtaining same resiilts

kinds; they have been described as hissy, scratchy, or rattly, and as resembling frjdng or bubbling noises. They vary in character from time to time, and are often much louder than the signals it is desired to receive. Static sounds are almost never musical in character, in even the smallest degree. Since the signals from radio stations may be made either musical or non-musical (as has been known for some years), and of almost any pitch, it follows that by choosing clear musical tones the difficulties of reading messages through static noises are largely over-

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