A MULTIPLE UNIT STEERABLE ANTENNA 393 



carrier of branch B alone appears on the signal meter and noise from 

 branch B appears alone on the noise meter. At the same time the 

 diversity connection is broken and all except one of the six-phase 

 shifter amplifiers in branch B are biased to cutoff; i.e., only one unit 

 antenna is used. The pad "L" is adjusted to give the same audio 

 gain from rectifier B to the noise meter for connection 1 as for con- 

 nection 2. 



By manipulating the keys which control the cutoff biases on the 

 phase shifter tubes the "1" to "2" switchover may also be used to 

 compare one antenna (one receiver) with two antennas in ordinary 

 space diversity or one antenna with all six in a single branch. 



The use of receiver noise as a noise source depends upon (1) having 

 the noise equal in all six circuits and (2) upon having it originate ahead 

 of the point where the gain is varied. In well-designed receivers the 

 noise should approach the thermal noise limit of the first circuit. It 

 was found possible to have the signal-to-noise ratio, for a given signal 

 level, of all six high-frequency input circuits equal to within ± 0.5 

 decibel and within a few decibels of the thermal limit. 



The first tests were made with a local oscillator supplying the signal. 

 They really constituted tests of the measuring set up. All six input 

 circuits were fed simultaneously through 80-ohm pads giving equiphase 

 and equiamplitude signals on each detector grid. This corresponds to 

 receiving a single steady wave, and one branch was "steered" as if to 

 receive such a wave. When the multiple switch was manipulated as 

 to compare one antenna with the steered branch the indicated signal- 

 to-noise improvement was usually between seven and eight decibels, 

 compared with the theoretical value of 7.8 decibels (10 log 6). 



Such a local test using the switchover with all associated equipment 

 was made before and after every transatlantic test. Corrections based 

 upon 7.8 decibels were made to the data in cases where the local tests 

 showed a slightly dififerent improvement factor. In all of the work the 

 gains of the phase shifters were adjusted to equalize the difiference in 

 line loss. The effect of this is, however, trivial. 



In measuring on transatlantic waves with automatic gain control the 

 noise variation, corresponding nearly to the reciprocal of fading, 

 rendered visual noise readings too rough to be suitable. A Weston 

 high-speed db meter (copper-oxide bridge type) having a calibrated 

 range of 16 decibels was used as a noise meter. To this instrument was 

 added a fiuxmeter (Fig. 35) of low restoring torque which automatically 

 averaged the variations of the meter pointer over the 15-second periods 

 of observation used in these tests. The fact that the noise meter recti- 

 fier is linear means that the noise current is averaged arithmetically 



