264 BELL SYSTEM TECHNICAL JOURNAL 



If the path lengths differ by any considerable amount, for example, a 

 few hundred miles, the wave interference is of such a character as to 

 affect the frequencies across a band consecutively rather than simul- 

 taneously. 



With the presence of selective fading there comes into being the 

 necessity of guarding against rapid even though small variations in the 

 transmitted frequency, since if such variations are present a peculiar 

 kind of quality distortion of the telephone signal results. 



The varying load which speech modulation places on the transmitter 

 circuits tends to cause slight variations in the instantaneous equivalent 

 frequency which are known as "frequency modulation" or "phase 

 modulation" depending on their character. To prevent this effect 

 the control oscillator must be carefully guarded against reaction by 

 shielding and balancing of circuits and the design must be such as to 

 preclude variable phase shifts due to modulation in subsequent circuits 

 of the transmitter. 



It is apparent that if there are two paths of different lengths, two 

 components which arrive simultaneously at the receiver may have left 

 the transmitter several thousandths of a second apart. If the transmit- 

 ter frequency has changed materially during this brief interval trouble 

 may be expected. The trouble actually takes the form of a distortion 

 of the speech as demodulated by the receiving detector.^ 



Defects in short-wave transmission due to radio noise, minor varia- 

 tions in attenuation, fading, and distortion are nearly always present to 

 some extent and, when any or all are severe, cause a certain amount of 

 lost service time. These interruptions are of relatively short duration 

 and, furthermore, there is enough overlap in the normal times of useful- 

 ness of the several frequencies available, so that shifting to another 

 frequency may give relief. There is, in addition, a kind of interruption 

 which from the standpoint of continuity of service is more serious. At 

 times of disturbance of the earth's magnetic field, known as " magnetic 

 storms," short-wave radio transmission is generally subject to such 

 high attenuation that signals become too weak to use and sometimes 

 too weak to be distinguishable. These periods affect all the wave- 

 lengths in use and may last from a few hours to possibly as much as two 

 or three days in extreme cases. They are followed by a recovery period 

 of one to several days in which transmission may be subnormal. 



Severe static may cause interruption to both long- and short-wave 



services at the same time but the short waves are relatively less affected 



by it and are usually able to carry on under static conditions which 



3 For a discussion of this phenomenon see "Some Studies in Radio Broadcast 

 Transmission" by Bown, Martin, and Potter, 7. R. E. Proc, Vol. 14, No. 1, p. 57. 



