A MULTIPLE UNIT STEERABLE ANTENNA 411 



and (9) give the same result, but for unequal amplitudes there is an 

 advantage in using square-law detectors. 



This analysis shows that square-law detection introduces just the 

 correct amount of emphasis upon the stronger waves and that any 

 additional expansion or contraction of the differences among the 

 waves is detrimental. This means that the gains in all branches should 

 be equal. It also indicates that any arrangement in which the stronger 

 of the several waves is automatically switched in and the remaining 

 ones switched out is inferior. 



The experimental MUSA receiver is equipped with both linear and 

 square-law detectors, and some signal-to-noise ratio comparisons were 

 made using locally generated signals. Figure 43 shows schematically 

 the essential parts of the test circuit. The noise generators represent 

 the thermal noise originating in the receiver input circuits. The input 

 signal Cs was modulated with a tone. The calculated curves shown in 

 the figure are obtained from (8) and (9) which reduce to 



(10) 



u — 2 

 and to 



(11) 



The equation for the square-law detector is sound and was verified by 

 the measurements. The equation for the linear detector should apply 

 only over a certain range of signal and noise levels. The measurements 

 indicate this. 



Automatic gain control was not used in these tests since the two gain 

 controls could not be relied upon to "track" sufficiently well. To 

 make the measurements significant manual gain control was used to 

 maintain the receiver gains equal and the output normal. It may be 

 pointed out here that in receiving actual radio signals with linear de- 

 tectors accurate equality of gains is not required. Moderate differ- 

 ences in gain (of a few decibels) can be depended upon to be beneficial 

 as often as detrimental. With square-law detectors no departure from 

 equality can be beneficial. 



The curves of Fig. 43 show that 10- and 20-decibel differences in 

 signal level give the square-law detector an advantage of one and two 

 decibels, respectively. In receiving two bundles of waves the branch 

 outputs commonly fade in and out with the result that their average 

 ratio is of the order of 10 decibels. Such were the conditions, as well 



