Normal Modes in Overmoded Dielectric-Lined Circular Waveguide

Communications systems are currently under development by the Bell System and abroad which utilize as the transmission medium dielectric-lined circular waveguide (DLG) excited in the low-loss circular electric (TE 0 i) mode. The presence of a thin dielectric liner, bonded to the wall of the waveguide, eliminates the undesirable degeneracy which exists between the phase constants of the TEoi and TM ii modes in unlined metallic guide. It also significantly modifies the propagation constant (phase and attenuation) and field distribution of many other modes compared to their counterparts in unlined waveguide. 453 454 T H E B E L L SYSTEM T E C H N I C A L J O U R N A L , A P R I L 1973 In order to determine the TEoi transmission characteristics of lined waveguide, it is necessary to determine the characteristics of the normal modes of the structure. The general problem of determination of the modes in a hollow, perfectly conducting metallic cylinder with a lossless dielectric lining has been of much interest since the initial investigations of Bucholz1 and Wachowski and Beam.2 Unger3'4 discusses the basic propagation characteristics of a thinly lined DLG suitable for use as a wide-band carrier in the millimeter-wave region. He determines the phase velocities of the normal modes exactly, but uses a first-order perturbation theory, with lining thickness as the perturbation parameter, to obtain the heat loss of the normal modes of DLG. It can be shown that the actual heat loss characteristics of the normal modes of DLG differ significantly from those predicted by the simple perturbation theory for a thin lining, in that modes not of the circular electric class may have very low heat loss at high frequencies.