Circular electric wave transmission in a dielectric-coated waveguide

The mode conversion from the TE01 wave to the TM11 wave which normally occurs in a curved round waveguide may be reduced by applying a uniform dielectric coat a few mils thick to the inner wall of the waveguide. Such a dielectric coat changes the phase constant of the TM11 wave without affecting appreciably the phase and attenuation constants of the TE01 wave. Thus, relatively sharp bends can be negotiated to change the direction of the line or deviations from straightness can be tolerated. For each of these two cases a different optimum thickness of the dielectric coat keeps the TE01 loss at a minimum. At 5.4-mm wavelength a bending radius of 8 ft in a ⅞-inch pipe or 50 ft in a 2-inch pipe can be introduced with 0.2 db mode conversion loss. Deviations from straightness corresponding to an average bending radius of 300 ft can be tolerated in a 2-inch pipe at 5.4-mm wavelength with an increase in TE01 attenuation of 5 per cent. Serpentine bends caused by equally spaced supports of the pipe may, however, increase the mode conversion and cause appreciable loss at certain discrete frequencies. Compared with the plain waveguide the dielectric-coated guide behaves more critically in such serpentine bends. The mode conversion from TE01 to the TE02, TE03, … waves at transitions from a plain waveguide to the dielectric-coated guide is usually very small.