A MULTIPLE UNIT STEERABLE ANTENNA 391 



It has been tacitly assumed in the foregoing analysis of Fig. 34 that 

 the audio outputs of the several branches are delay equalized to add 

 and that there is no diversity action (all of the waves are assumed to 

 remain equal). The influence of fading is difficult to predict and will 

 be discussed later in connection with experimental results. 



Some readers, not concerned with details, may omit reading the 

 following subsections and find it sufficient to read only the Summarizing 

 Discussion of this section. 



Test Method 



From a practical point of view the best way of testing a MUSA 

 system would seem to be to operate it on transatlantic telephone signals 

 and compare its output with that of the reference system. Speech 

 volume and noise could then be measured in the conventional manner. 

 So far as the signal-to-noise improvement is concerned it would be a 

 laborious and lengthy task to get satisfactory data because so often, 

 during the test period, ^^ static and receiver noise is masked by trans- 

 mitted noise, interfering signals, and other man-made noise. To test 

 the experimental MUSA, therefore, a difi"erent method was selected 

 which gave significant data in a shorter time. 



Since the success of the MUSA is related so fundamentally to the 

 nature of the arriving signal the important thing to be determined by 

 the measurements is how well the MUSA is able to cope with the 

 various conditions of wave arrival. For instance, in the case of a single 

 bundle of arriving waves how close does the actual signal-to-noise 

 improvement come to the 10 log N decibel calculated for Case I (Fig. 

 34) in which a single non-fading wave was assumed? Likewise, for 

 the case of two-wave bundles do the calculations of Case III agree 

 with measurements? 



For these purposes the signal-to-noise measurements would have to 

 be free from directional static, interference, and transmitted noise; 

 otherwise the measured improvements would be distorted. To insure 

 uniform and desirable noise conditions it was decided to use thermal 

 noise originating in the receiver input circuits ^^ instead of whatever 

 noise might be present on the radio channel. This was accomplished 

 by inserting resistance pads in the antenna transmission lines to reduce 

 the signal (and external noise) to a level where thermal noise greatly 

 exceeded other noise. Signal-to-noise ratios in the range between 

 fifteen and forty decibels were obtained in this manner, free of inter- 

 ference and directional static, and of transmitted noise. 



2* Transmission conditions during 1935 were comparatively undisturbed. 

 ''^ A portion of the noise originates in the plate circuit of the first detector. For 

 the present purposes this is equivalent to first circuit noise. 



