Reflections in open-ended elliptical waveguide with an infinite flange

Experience with smooth-wall horn radiators of rectangular and circular cross-section has shown that significant reflection can occur from apertures of moderate size, resulting in mismatch at the horn input. The existing literature gives no indication of the level of reflection in open-ended elliptical waveguide, although W. C. Chew et al. (1986) recently formulated the problem. The purpose of this study is to outline another solution to the problem and to present preliminary theoretical results. To demonstrate the effect of the open end, the computed reflection coefficient of the TE_c11 mode is plotted as a function of normalized frequency ka with waveguide ellipticity as a parameter. In these calculations, coupling to other modes in the aperture was neglected, an approximation known to work well provided the high-order modes are below cutoff. Reflection in elliptical waveguide increases as a/b increases in much the same way as reflection of the TE₁₀ mode in rectangular waveguide. Well above cutoff, the level reflection of the TE_c11 mode was found to be approximately the same as for the TE₁₀ mode in open-ended rectangular waveguide with the same value of a/b.