DEVELOPMENTS IN SHORT-WAVE DIRECTIVE ANTENNAS 659 



The above arguments are intended to show that, at the shorter 

 wave-lengths, receiving antennas should be designed for a gain in signal 

 output. At the long wave-lengths, directive discrimination in recep- 

 tion is the major requirement. In contrast to this, a transmitting 

 antenna has no such wave-length eccentricities. Its purpose is always 

 to lay down at the receiving point as great a field as possible. We 

 must not forget, however, that the time is near when more attention 

 should be paid to marked directive discrimination in transmitting 

 antennas as a means of reducing interference between congested com- 

 munication channels. 



While set noise and static are at times important factors in limiting 

 successful short-wave communication, fading practically always pre- 

 sents varying degrees of annoyance. It is really surprising how much 

 fading can be tolerated without radically affecting speech intelligibility, 

 but for services such as high-grade program transmission where natur- 

 alness is also important vast improvements are required; consequently 

 much attention has been, and is being paid to this phase of the problem. 



Increasing the Signal Output of Receiving Antennas 

 Under conditions of optimum output impedances, the magnitude of 

 signal developed at the receiving antenna load is simply a function of 

 the ratio of the effective induced voltage to the effective antenna re- 

 sistance. The term effective induced voltage is used, as attention must 

 be directed toward proper phasing, where the antenna dimensions are 



•B 



Effects of antenna directivity. 



an appreciable part of a wave-length or more. Usually at short 

 waves, the effective resistance is almost entirely the resistance equiva- 

 lent of the reradiation losses. This resistance can be lowered through 

 directivity, a simple example of which can be illustrated with the aid 

 of Fig. 2. 



If we can conceive of a point source of radiation at A, equipotential 

 radiation surfaces would be spherical in shape and symmetrically dis- 

 posed around A. The field intensity at point B would be unaffected 

 if we had some means of avoiding radiation through the unshaded half 



