A MULTIPLE UNIT STEERABLE ANTENNA 415 



= 60°). A loss of three decibels is about what one would estimate 

 upon inspecting the directional pattern for = 60 and 120, say, on 

 Fig. 20. The procedure in which two branches are steered at one 

 bundle as in the above is frequently employed and is an important 

 factor in the operation of a MUSA. 



The case of two wave bundles tabulated in Table V also yields an 

 improvement of 7.5 to 8 decibels. Of this, one decibel and three 

 decibels are due to primary and secondary diversity action, respec- 

 tively, as in the case of one bundle. This leaves 3.5 to 4 decibels for 

 the signal-to-noise improvement in each branch referred to a unit 

 antenna. But the unit antenna has the advantage of two bundles, 

 whereas the MUSA branch excludes one of them, a three-decibel 

 difference. In comparison with a unit antenna receiving only one 

 bundle, the improvement to be ascribed to one branch thus is increased 

 to 6.5 or 7 decibels. This result compares favorably with the seven 

 decibels yielded by one branch steered at one bundle. It is in this 

 case of two bundles that square-law detectors are most important. 

 Their advantage, amounting to an estimated one decibel, has already 

 been included in Table V, it will be remembered. 



The measurements which have permitted the above analysis of the 

 MUSA signal-to-noise improvement were of course supplemented by 

 aural observations made over the course of a year and a half. The 

 listening tests corroborate the analytical results as well as can be ex- 

 pected of such observations. Not infrequently they showed somewhat 

 less than the full eight-decibel improvement. The indications are, 

 however, that a larger MUSA with three (or possibly four) branches 

 would have yielded more nearly its full gain of 10 log N decibels. For, 

 a MUSA receiving system does not perform its functions properly 

 unless it is sharp enough to separate the waves sufficiently to permit 

 effective delay equalization; also, to obtain the full gain, enough 

 branches must be provided to utilize all of the important wave bundles. 

 The Holmdel experimental MUSA is really a conservative approach to 

 the field of steerable directivity. There is, of course, an upper limit 

 to the size of a MUSA, beyond which (1) technical difficulties in 

 phasing, etc., will occur, (2) the cost of the improvement may be less 

 if introduced at the transmitter, and (3) the directional sharpness 

 becomes too great to permit practical operation with waves of the 

 stability encountered in transatlantic transmission. At present, a 

 system about three times the length of the experimental MUSA com- 

 prising eighteen antennas and equipped with three branches seems 

 practical. It should yield an improvement of 10 log 18 ^ 12.5 



