WAVEGUIDE AS A t'OMMUXICATlON MEDIUM 1211 



iVeMluencios above 12,000 mv. V^c of the sj)e('tium a])()ve tlie 10,000- 

 20,000 me region secMiis to reiniire a slielhMvd transniission medium. 



Circular electric waxc transmission may also liiul application in slioi't 

 coimecting links, such as hetweiMi subscribers re(iuii'ing \-ery broad band 

 circuits, between two central oflic(\s as a multi-channel carrier link, or 

 between a radio relay antenna site and a somewhat remote transmitter- 

 receiver location chosen for accessibility. 



In each of these cases, the broad bands a\ailal)le in the microwave 

 l)orti()n of the spectrum, the complete shielding afforded by waveguides 

 generally, combined with the low-loss properties of the circular ele(;tric 

 wa\'e would seem to pro\'ide an ideal transmission medium. We there- 

 fore seek knowledge of the precision required in the w^aveguide and some 

 indication of general system complexity to facilitate a judgment as to 

 whether the cost will be competitive. 



ORDINARY VERSUS CIRCULAR ELECTRIC WAVES 



Let US approach a discussion of circular electric waves by considering 

 their relation to the waveguides which are now used in oui" I'adio relay 

 systems and which found widespread use in the radars of World War II. 

 The vast majority of waveguides in commercial use now are rectangular 

 in cross section and have dimensions large enough so that one and only 

 one wave-tj^pe, usually called the ''dominant mode", can i)ropagate. To 

 simplify this discussion, such waveguides will be called ordinary wave- 

 guides. Ordinary ^vaveguides are analogous to coaxial or parallel-wire 

 lines in manj^ respects. Because only one mode can propagate, departures, 

 from an absolutely straight tube of constant cross section show up as 

 reactance effects only. A dent in the side wall of the guide or of the co- 

 axial, an abrupt change in cross section, or a twist or bend of the line all 

 appear as non-dissipative reflection effects which may be cancelled at 

 one frequency (or in one band of frequencies) by the addition of another 

 compensating reactance at a point suitably located. A great many of the 

 components used in ordinary waveguides, including the frequency 

 selective filters, depend on such reactance cancellation effects in order to 

 achieve satisfactory operation. 



Since the techniques for employing ordinary waveguides have been 

 thoroughly explored, it is natural to inquire as to whether we can use 

 them for communication purposes. We do use ordinary waveguides in 

 lengths of the order of 100 feet and more to connect the antennas and 

 repeaters in the 4,000 mc (TD-2) radio relay syst(^m. The attemiation is 

 excessive, however, for long-distance applications. The particular type 



