298 BELL SYSTFAf TECHNICAL JOVRXAL 



and little or nothiiif,^ of the |)resent-day experimental technique had yet 

 appeared. At this time, the position of this new art was perhaps com{)arahle 

 with that of radio prior to the time of Marconi. 



The history of waveguides changed abruptly about 1933 when it was 

 shown that they could be put to practical use. Several patent applications 

 were filed/ and numerous scientific papers were published. More recently a 

 great many papers have appeared, too many in fact for detailed consideration 

 at this time. Three of the earlier papers are mentioned in the footnote 

 below.'' Others will be referred to in the text that follows. 



The writer's interest in guided waves stems from some experiments done 

 in 1920 when such waves were encountered as a troublesome spurious effect 

 while working with Lecher wires in a trough of water. In one case there were 

 found, superimposed on the waves that might normally travel along two 

 parallel conductors, other waves having a velocity that somehow depended 

 on the dimensions of the trough. These may now be identified as being the 

 so-called dominant type. In another case, the depth of water was apparently 

 at or near "cut-off," and conditions were such that water waves in the 

 trough gave rise to depths that were momentarily above cut-off, followed a 

 moment later by depths that were below cut-off. This led not only to varia- 

 tions in power at the receiving end of the trough but also to variations in 

 the plate current of the oscillator supplying the wavepower. Indeed these 

 effects could be noted even when the wires were removed from the trough. 

 These waves were recognized as being roughly like those described the same 

 year by Schriever.^ 



Several years later this work was resumed and since that time a con- 

 tinued effort has been made to develop from fundamental principles of 

 waveguide transmission a useful technique for dealing with microwaves. 

 The earliest of these experiments consisted of transmitting electromagnetic 

 waves through tall cylinders of water. Because of the high dielectric con- 

 stant of water, waves which were a meter long in air were only eleven centi- 

 meters long in water. Thus it became possible to set up in the relatively 

 small space of one of these cylinders many of the wave configurations pre- 

 dicted by theory. In addition it was possible, by |)roducing standing waves, 

 to measure their apparent wavelength and thereby calculate their phase 

 velocity. Also by investigating the surface of the water by means of a probe, 



' Reference is huuIl' particuhirlv lo U.S. Palenls 2,120.711 (lik-d 3/16/33, 2.12'),712 

 (filed 12/9/33), 2,206,923 (filed 9/12/34) and 2,106,768 (filed 9/2.S/34). 



-Carson, Mead and Schelkunnff, "Hv])cr-I're(iucncv \Vavef!;uicles — Mathenialical 

 Theory," B.S.TJ., Vol. 15, pp 310-333, .\i)nl 1936. G. C. Southwortii, "Hyper-frequency 

 Wave (niides — Oeneral C'onsideralions and l'".\i)eri menial Results," /^..S'.7\/., Vol. 15, pp 

 284-309, April 1936. .Mso "Some I-'undamenlal ICxi)eriments with \\'ave<];uides," Proc. 

 r.R.R.,\o\. 25, i)p 807 822, Jul\- 1937. \\ . L. Harrow, "Transmission of Mleclromagnelic 

 Waves in Hollow 'I'uhes of Metal," Proc. I .R.E., Vol. 24, pp 1298 1398, October 1936. 



■■' The waves actually observer! are now known as TEm waves in a reclanRular guide, 

 wliile those described by Schriever are now recognized as TM.ji waves in a circular guide. 



