FREQUENCY SHIFT TELEGRAPHY 293 



inversely with a change in bandwidth. At extreme level differentials the 

 bias may rise very rapidly due to the amplitude and phase characteristics 

 of the input passband; transients from the greater amplitude condition may 

 severely interfere with the lower amplitude condition. For the types of 

 bandpass filters used in the tests it appeared that the amount of level dif- 

 ference required to produce 20% bias did not change greatly with band- 

 width. Severe wave shaping of the signal at the transmitter was found to 

 be an aid in reducing the bias effect due to such transients but the charac- 

 teristic distortion became too great to give any practical improvement. 



The fading modulator used in obtaining the data for Fig. 33 caused no 

 change in phase. Selective fading over an actual radio circuit would involve 

 considerable phase shift and greater distortion might be expected. -The 

 data of Fig. 32 were obtained by use of the phase control associated with the 

 crystal filter of the radio receiver to vary the loss-versus-frequency charac- 

 teristics of the receiving pass band and thus cause unequal mark and space 

 amphtudes. This method gave an ampUtude and phase characteristic for 

 the transmission band more like that over an actual radio circuit. 



MuLTiPATH Propagation Effects 



The rapid fading conditions prevailing in the H.F. range are brought 

 about by multipath propagation. Under such conditions, the signal in- 

 duced in a receiving antenna by a distant transmitter may be the resultant 

 of two or three separate waves each propagated over a different path. If 

 two waves arrive over paths differing in length by an odd number of half 

 wavelengths the resulting 180° phase difference causes maximum cancella- 

 tion. On the other hand if the paths differ in length by an integral mul- 

 tiple of whole wavelengths the waves arrive in-phase and maximum rein- 

 forcement results. The difference in path lengths may at times be as great 

 as 500 to 1500 kilometers (delay times of 2 to 5 milliseconds) which in the 

 H.F. region corresponds to thousands of wavelengths. Under these maxi- 

 mum conditions waves at one frequency may arrive in phase while waves at a 

 frequency a few hundred cycles away may arrive in phase opposition. Since 

 the path lengths are constantly changing, the transmission at a given fre- 

 quency is subject to wide variations in amplitude and phase with time. 

 When the difference in path lengths is not great enough to cause frequencies 

 in one portion of a communication channel to fade differently from those in 

 another portion the term ''non-selective" or "flat" fading is applied. When 

 the difference in path lengths becomes great enough to cause considerable 

 amplitude or phase distortion over the transmission band the term "selec- 

 tive" fading is used. Since the propagation paths existing at a given 

 moment vary for different antenna sites, the fading patterns obtained from 

 two or three antennas separated by several wavelengths usually show a 



