DEVELOPMENTS IN SHORT-WAVE DIRECTIVE ANTENNAS 657 



(a) Receivers of very high gain characteristics are troubled with an 

 inherent noise adequately described as a "hissing" sound. This may 

 be due to several ^ causes such as shot-effect, etc. Much of this noise 

 can be minimized through proper design, the methods of which are 

 beyond the scope of this paper. Finally, however, an apparently ir- 

 reducible minimum of noise is encountered, commonly referred to as '^ 

 "Johnson" or circuit noise. This noise, under conditions of matched 

 impedances, is so related to the circuit signal efficiency that the ratio 

 of noise to signal cannot be appreciably altered except through some- 

 what impractical expedients such as lowering the absolute temperature 

 of the circuit. All this tends to show that the designer of receivers 

 must eventually rely upon his being able to increase the signal outputs 

 from antennas to override the residual receiver noise difficulties on low 

 field strength signals. 



(b) Unpublished work, by a member of our laboratories,^ has indi- 

 cated that on many occasions short-wave static is highly directional. 

 Interfering signals and electrical noises of human making are, of course, 

 directional. It is quite evident that where the desired signal direction 

 differs from that of the interference, receiving antenna directional 

 discrimination is of immense importance. 



(c) At times, remarkable reductions in short-wave fading have been 

 achieved through extremely sharp directional characteristics of the 

 receiving antenna. On the basis that certain types of fading are due 

 to phase interference between multiple path signals of varying path 

 length, it is reasonable to believe that where an angular difference exists 

 between these paths, fading can be reduced by directivity which ac- 

 cepts only one of the paths. This, of course, assumes that the ac- 

 cepted path is stable in its direction. When this is not true, the reduc- 

 tion of fading through directivity becomes difficult. 



The Relative Importance of the Various Circuit Limitations 



The most serious hindrance to reliable, long-distance, short-wave 

 communication is the great loss in signal fields which accompanies 

 magnetic storms. Maintaining service under such conditions, develops 

 into a battle against set noise and static. It is during these periods 

 that effective receiving antennas are the most appreciated. The re- 

 search worker on receiving antenna systems always welcomes such 

 periods for his experimental work, since he knows well that under con- 



ip B. Llewellyn, "A study of noise in vacuum tubes and attached circuits," 



Proc.'i. R. E., February, 1930. . ^ , . . . ^ •■ p, p 5-, 



2 J. B. Johnson, "Thermal agitation of electricity in conductors, Fliys. Kev., 32, 



97, 1928. 



3 K. G. Jansky, Bell Telephone Laboratories. 



