Page:Popular Science Monthly Volume 91.djvu/971

 Wireless Work In Wartime

V. — How to secure practice in working through interference By John L. Hogan, Jr.

��THE closing portion of the November article in this series was devoted to a brief discussion of the problem of reducing interference in radio telegraphy. Of the two general types of interference which exist, we may first consider that which arises from the overlapping of radio waves when several nearby stations are transmitting at the same time. This is of considerable importance, and, since its reduction depends largely upon the training of the operators, we will do well to study it at thjs point. The other sort of inter- ference, which is set up by natural or non- radio electrical disturbances, can be most effectively considered after the apparatus used in radio teleg- raphy has been stud- ied in further detail. For the present, then, let us take up the matter of "sta- tion interference," as it is called. It is not hard to see how such difficulties come up, if we remember that each radio transmit- ter sends its signal waves in all direc- tions with approximately equal strength, and that all normal radio receivers absorb signal waves with equal ease regardless of the direction from which they come. Sup- pose that two radio transmitters of equal power are located at Philadelphia and at Norfolk, and that both are sending at the same time. A receiving station at New York will have little difficulty in decipher- ing the signals from Philadelphia, since the distance is so much shorter than that be- tween Norfolk and New York that the mes- sages from Philadelphia will be much louder than those from Norfolk. By the mere in- creased strength or intensity of the signals from Philadelphia, it is easy to distinguish them from the Norfolk signals. But sup- pose that a receiving station at Washington wishes to copy the message which Phila- delphia is sending. Since the distance from Washington to Norfolk is about the

���Fig. 17. Each signal telegraph line is

��same as that from Washington to Phila- delphia, the two sets of signals will be heard simultaneously and with about the same degree of loudness. Clearly, since loudness alone is no longer sufficient to permit the receiving operator (at Washington) to dis- tinguish between the two sending stations, some other difference between them must be relied upon.

Wavelength and Tone Frequency

There are two characteristics, in addition to loudness, which are commonly used to separate desired from undesired signal waves. The first of these, wavelength or wave-frequency, will be considered in de- tail later in this series of articles. At pres- ent it will be suffi- cient to note that when several differ- ent transmitters use several different wavelengths, the ef- fect at a receiver is almost as though each sending station were operating over a separate wire. A rough idea of the effect of changing wavelength may be had by considering the approximately parallel condition in which several different wires connect four line telegraph stations A, B, C, and D. Stations A and B may com- municate with each other on one wire while stations C and D are also working together on a second connection, and there will be no mutual interference. The selection of the wire which is not "busy," as the telephone engineers say, must be made by trial; and the actual connection is made by "plugging in" to a terminal board to which the several wire lines lead. The analogous wireless or radio case provides several separate chan- nels of communication between the four stations, and each channel is termed a "wavelength." Stations A and C may intercommunicate on one wavelength while stations B and D signal each other on a second wavelength, without mutual inter-

��station of the buzzer connected as shown

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