DEVELOPMENTS IN SHORT-WAVE DIRECTIVE ANTENNAS 663 



over-all static output is the same. For uniform distribution, the static 

 output level is independent of the degree of directivity, provided that 

 impedance matching between the load and the antenna is always main- 

 tained. \\& see, therefore, that the improvement in signal-to-static 

 ratio in this case is the same as the signal improvement alone. 



If static were always uniformly distributed about an antenna, the 

 problems of signal gain and improvement in the signal-to-static ratio 

 would be synonymous. The fact that short-wave static is usually 

 highly directional puts an entirely different aspect on the problem. If, 

 in Fig. 2, the static came from a direction included in the unshaded 

 portion of the characteristic, the improvement in the signal-to-static 

 ratio would be infinite. In a receiving antenna, therefore, emphasis 

 must be placed on the deep suppression of response in other than the 

 favored direction. 



Fig. 5 — A comparison of directive diagrams. 



Fig. 5 is intended to illustrate the case described. The antenna char- 

 acteristic 5-A, having a signal gain of 20 decibels over a nondirectional 

 antenna, does not accomplish deep rejection in other directions. It 

 follows, therefore, that the better discriminating characteristic 5-B 

 would give a vastly better signal-to-static ratio, in spite of a smaller 

 signal gain. 



Fading Reduction 



Many schemes for counteracting fading are in use and have been 

 suggested. These include compensation for fading through automatic 

 control of the receiver gain, the automatic selection of the best of sev- 

 eral antennas, single side band with an unvarying locally supplied car- 

 rier, etc. All of these systems have merit, but are not a complete cure 

 for the very prevalent selective type of fading, where several depres- 

 sions may exist within a frequency band width of speech magnitude. 



