Normal Mode Bends for Circular Electric Waves

In dielectric-coated round waveguide the degeneracy or equality of phase constants of TEoi and TMn waves is removed. In such a non-degenerate waveguide, mode conversion in bends can be reduced by changing the curvature gradually instead of abruptly. With curvature tapers, which are of the order of, or longer than, the largest beat wavelength between TE i and any of the coupled waves, propagation of only one normal mode is maintained throughout the bend. Linear curvature tapers can easily be made by bending the pipe within the limit of elastic deformation. Changes in the direction of a waveguide line can thus be made by inserting a dielectric-coated guide section which is elastically bent over a fixed center point. A thirty degree change of direction of a 2-inch I.D. pipe with 30 ft of a dielectric-coated guide yields a total bend loss at 5J mm wavelength that is twice the TEoi loss in 30 ft of straight pipe. An optimum bend geometry is found which minimizes the total bend loss. The normal mode bend is a broadband device. 0 r A major problem in circular electric wave transmission is the question of negotiating bends. In curved sections of a round waveguide the TE mode couples to the TEn , TE12, T E j · · · modes and to the TMn mode. 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. TE i power introduced at the beginning of the bend will be almost completely transferred to the TMn mode at odd multiples of a certain critical bending angle.