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 



955 



