296 BELL SYSTEM TECHNICAL JOURNAL 



A test of superimposed phase modulation on FS signals was made over a 

 radio circuit approximately 200 miles in length. A frequency shift of 850 

 cycles, and one radian of 200-cycle phase modulation, was used. A 60- 

 word-per minute test sentence was transmitted and received without space 

 diversity. It was found over a period of several hours that the phase modu- 

 lation, on the average, gave a decrease in printed errors of about 50% when 

 the error rate was in the proximity of 1 to 2%. For short intervals the re- 

 duction in errors was often considerably greater. The use of 200-cycle 

 phase modulation when space diversity is used provides little or no improve- 

 ment and is therefore undesirable. 



A more effective way of employing phase modulation with FS signals 

 would be to use a phase swing of db 1.4 radians at a frequency of 2 to 3 times 

 the frequency shift and to demodulate separately the three major com- 

 ponents of the signal, thus obtaining in effect a triple-frequency diversity 

 system. This of course involves quite a wide transmitted band, but it 

 might be of use in cases where space diversity is impossible, such as on 

 board ships. When a space diversity arrangement is feasible it is much to 

 be preferred. 



Diversity Operation 



To obtain reliable operation in the H.F. range it is common practice to 

 employ space diversity reception. The use of frequency diversity, with the 

 increase of transmitted power and greater frequency space required, is sel- 

 dom justified if space diversity reception can be arranged. For AM radio 

 telegraph, double or triple-space diversity receiving arrangements are 

 frequently used. Since an FS signal generally covers more frequency 

 space, it is even more likely to be mutilated by selective fading than an 

 AM signal. It has been found, however, that a double-space diversity sys- 

 tem for FS signals usually gives sufficient diversity action provided it is of a 

 type that p>ermits switching between channels at signaling speed without 

 causing appreciable distortion. This is necessary since it is a frequent 

 occurrence that the mark of one channel may fade, leaving a good space, 

 while the opposite may occur on the second channel. Since an FS system 

 can accept rapid level changes, the main purpose of diversity methods is to 

 insure that both the mark and space portions of the signal will be received 

 above the noise level. In the case of AM telegraph, since it cannot accept 

 rapid level changes, diversity operation is important not only in keeping the 

 signal above the noise but also in averaging out some of the rapid level 

 changes. For this reason AM systems usually show considerable improve- 

 ment in going from double to triple diversity. It would be expected that a 

 like change would show much less improvement in an FS system. 



