WAVEGUIDE AS A COMMUXIPATIOX MIIDIUM 1213 



The fact that it is necessary to use a waveguide wliose dimensions are 

 large enough to permit the existence of a numhcr of mcxles has far- 

 reaching influence on the research being discussed lici'c. Practically, the 

 iii(le])(Mulenc(> hetwecMi the \-arious modes of ])ro])agation is limited by 

 tolerances of various kinds. In the multimode \va\-eguide, changes in 

 cross section or bends or twists I'ciiuii'e design attention with regai'd to 

 mode purity as well as with regartl to impedance match, and it is not 

 jxnmissible to insert arbitrarily shaped probes or irises for impedance 

 matching purposes as is the common practice in ordinary waveguides. 

 This means that a complete new technifjue is required for tlu^ old com- 

 ponents, such as frctiuency-selective filters, hybrids, and attenuators, as 

 well as for a new series of components such as pure mode generators and 

 mode filters. 



TIIEOHETICAL CHAKACTEHISTirS OF THE CIHCULAU ELECTKIC WAVE 



Sinc(> it has been found necessary to use a waveguide in the multimode 

 i-egion in order to get the tlesired losses in a reasonable size waveguide, 

 we may inciuire as to which of the modes is best suited to our problem. 

 At a gi\'en fre([uency the loss for any one of the modes may be reduced 

 as much as is desired by making \hv cross sectional area of the guide 

 large enough, but there is a mode for which the loss decreases with in- 

 creasing guide size much more rapidly than for any other mode. This is 

 the circular electric (TEoi) mode in straight round pipe. It turns out that 

 no current flows in the direction of propagation in the metallic walls of 

 a straight round pipe carrying the circular electric mode. It is the ab- 

 sence of current in the direction of propagation which p(»rmits the circu- 

 lar-electric-wave attenuation to decrease indefinite^ as the frequency 

 increases, and this difference between ordinary transmission lines and 

 the circular-electric wave is further illustrated in Fig. 2. In the familiar 

 parallel-wire line the electric field extends directly from one conductor 

 to the other, resulting in charge accumulations at half-wave intervals 

 along the axis of propagation and associated conduction cuirents in the 

 copper wires. These conduction currents in the direction of propagation 

 do not diminish as the frecjuency of operation increases, since they are 

 associated with the energy transmitted to the end of the transmission 

 line. With the circular electric wave the electi'ic field lines close upon 

 themselves, are always tangential to the conducting wall, and do not 

 n^sult in a charge accumulation on the walls due to the main energy flow. 

 The wall currents which do fl<jw are mei'ely sufhcient to "pre\-ent the 

 ])ropagating energy from spreading out as it would if the metallic walls 



