184 BELL SYSTEM TECHNICAL JOURNAL 



length range. Also, as the wavelength decreases, it becomes possible to 

 construct antennas large in comparison with the wavelength so that high 

 antenna gains are obtained and the corresponding directivity further re- 

 duces the interference areas. 



Since about 1930, with the exception of the war years, we have conducted 

 fundamental studies in radio propagation, taking advantage of advances in 

 the art to extend the wavelength range from about four meters (ultra-short 

 wave region) in the beginning to 1.25 centimeters (microwave region) at 

 the present time. A considerable portion of the effort of those engaged in 

 propagation studies has, of necessity, been devoted to the development of 

 measurement techniques and reliable measuring apparatus. The present 

 discussion, however, will be concerned with the results of experiments rather 

 than with a description of the apparatus and methods. Most of these 

 results have been described in the Uterature; the following is a review in- 

 tended to show the development of the background leading to the present 

 field trial of a microwave repeater circuit. 



The object in making propagation studies has been to evaluate and to 

 understand the effects of the terrain and of the lower atmosphere upon the 

 transmission of ultra-short-wave and microwave signals. The evaluation 

 is usually obtained by amassing sufficient data on a particular transmission 

 experiment so that a statistical analysis can be made. Efforts to under- 

 stand the transmission phenomena usually take the form of experiments 

 involving specially designed apparatus. These experiments are varied 

 from time to time as information is obtained or as it becomes desirable to 

 check the validity of such theories as may be devised. The hope is always 

 present that an understanding of the phenomena may suggest a means for 

 reducing the transmission difficulties. 



The absence of ionospheric reflections at these frequencies suggested at the 

 start that propagation studies would probably be concerned mainly with 

 phenomena familiar in optics, namely: reflection, refraction and diffraction. 

 Two of the early papers^* ^ treated ultra-short-wave propagation from this 

 viewpoint. It was soon observed that diffracted signals tended to be un- 

 stable in the shadow region ; furthermore, as the wavelength is decreased the 

 shadows cast by obstacles such as hills or the bulge of the earth itself become 

 more sharply defined. For these reasons, a considerable part of our ex- 

 perimental work has been done on paths for which a Une-of-sight exists be- 

 tween transmitter and receiver. The chief interest, therefore, has been in 

 ground reflections and the effect of the atmosphere. 



1 J. C. Schelleng, C. R. Burrows and E. B. Ferrell, "Ultra-Short-Wave Propagation", 

 Proc, L R. E., vol. 21, pp 427^63; March 1933. 



« C. R. Englund, A. B. Crawford and W. W. Mumford, "Some Results of a Study of 

 Ultra-Short Wave Transmission Phenomena", Proc. L R. E., vol. 21, pp 464r492; March 

 1933. 



