392 BELL SYSTEM TECHNICAL JOURNAL 



Substituting thermal noise for external static may at first seem far- 

 fetched. Except for the fact that static is sometimes sufficiently 

 directional to be received with different intensity as the MUSA is 

 steered differently, the substitution is sound. In general, the static 

 output does not vary with steering, as the measurements described 

 later indicate but to avoid the distortion of results which would occur 

 when this is not so, it was desired specifically to substitute non- 

 directional noise. Studies of the characteristics of static and thermal 

 noise have shown that both are alike so far as the effect of band width 

 upon average and effective values is concerned, and have indicated that 

 both consist of extremely short, randomly distributed pulses which 

 overlap when received and detected by receivers of ordinary band 

 widths. In a given band width, the envelope of the currents produced 

 by static sources is highly irregular in comparison with that produced 

 by thermal agitation. It appears, however, that the character of either 

 envelope is not sensibly affected by the number of antennas combined 

 nor by the manner in which the branch outputs are combined, so that 

 both give the same improvement figures using any arbitrary noise 

 measuring method. 



There were several possibilities with respect to the signal to be 

 employed in these tests. A single tone, a large number of tones dis- 

 tributed throughout the audio band and other special signals were 

 considered. A simple method requiring no modulation was finally 

 adopted. It consisted in alternately connecting the output of the an- 

 tenna to be tested and that of the reference antenna to the same 

 receiver. Assuming that the automatic gain control of this receiver 

 would maintain a constant audio output level the signal-to-noise 

 advantage is the ratio of the noise levels. The automatic gain control 

 of the MUSA receiver did not, of course, hold the output level abso- 

 lutely constant but a correction was easily made for the small varia- 

 tions in level. 



The circuits of the measuring equipment are shown in Fig. 35. The 

 rectified carrier appearing in the linear speech rectifier is taken to be 

 proportional to signal and is measured simultaneously with the noise 

 demodulated in the rectifier. When the keys are thrown to position 2 

 (by a gang arrangement) the signal meter shows the sum of the two 

 rectified carriers and the noise meter reads the combined noise in the 

 output of the diversity mixing amplifiers. Using the sum of the two 

 rectifier currents to represent the signal implies that actual audio out- 

 puts from the two branches could be delay equalized to add arith- 

 metically. As applied to a MUSA system this assumption is justified, 

 in general. When the keys are switched to position 1, the rectified 



