Observed 50 to 60 Gc/s Attenuation for the Circular Electric Wave in Dielectric-Coated Cylindrical Waveguide Bends

In a nonideal section of circular cylindrical waveguide, the circular electric wave TE 0 i couples to other modes of transmission which are permitted. 1 The strongest coupling for bends is to the T M n mode since it is degenerate in a perfectly conducting straight waveguide. The degeneracy of the phase factors of the TE 0 i and T M n modes in the uncoated cylindrical waveguide gives rise to the serious problem of an increase in loss upon bending. For example, in a 2-inch i.d. bare pipe at a 5.4-mm wavelength, the attenuation constant is doubled when the radius of curvature is a few miles.1 The power transfer between the two modes can be reduced by removing the phase degeneracy. One technique for doing this is to introduce a thin layer of dielectric next to the wall of the waveguide. The electric field intensity of the TEoi mode vanishes at the wall but that of the T M n mode has a large value there, hence, the effect on the propagation constant of the T M n mode is larger than on the TEoi. 723