Circular Electric Wave Transmission in a Dielectric-Coated Waveguide

In a curved section of cylindrical waveguide the circular electric wave couples to the T E n , T E J 2 , TEi 3 · · · modes and to the TMn mode.1 The coupling to the TMn mode presents the most serious problem since the TEoi and TMn modes are degenerate, in that they have equal phase velocities in a perfectly conducting straight guide. In a bend all TE0x power introduced at the beginning will be converted to the TMn power at odd multiples of a certain critical bending angle. One can reduce this complete power transfer by removing the degeneracy between TEoi and 1253 1254 T H E B E L L SYSTEM T E C H N I C A L J O U R N A L , S E P T E M B E R 1957 T M n modes. The finite conductivity of the walls introduces a slight difference in the propagation constants and in a 2-inch pipe at 5.4 mm wavelength a bending radius of a few miles can be tolerated with about double the attenuation constant of the TEoi wave. To get more difference in the phase constants of the TMn and TE 0 i modes, one might consider a dielectric layer next to the wall of the waveguide. Since the electric field intensity of the TE 01 mode goes to zero at the wall but the electric field intensity of the T M n mode has a large value there, one might expect a larger effect of the dielectric layer on the propagation characteristics of the T M n wave than on the TE 0 i wave. In doing this, however, one has to be aware of the influence the diplectric layer will have on the propagation characteristics of the TEi m modes which also couple to the TEoi wave in curved sections.