On the radiation from an open-ended corrugated pipe carrying the HE<inf>11</inf>mode

The fields radiated by anHE₁₁mode in an aperture of a corrugated pipe with quarter-wave teeth are derived by considering the pipe to be situated in an infinite perfectly conducting ground plane. Unlike the case for a smooth wall pipe, this model is reasonable since the corrugations force the edge taper in theEplane of the pipe's aperture to be significantly increased from its smooth wall value of 4 dB. This considerably reduces the usualE-plane edge diffraction effects (those in theHplane are even less significant, being essentially the same as with no corrugations since theH-plane taper is is infinite in either case). This model reveals that theE-plane pattern of the pipe approaches theH-plane pattern (with its wider main beam and lower sidelobes) and that the radiation is nearly linearly polarized only if the aperture is sufficiently large(C ≡ 2π a/λυ 2π, whereλυis the free-space wavelength and a is the radius of the aperture). This is in agreement with experimental results given. (Existing solutions of Minnet-Thomas and Clarricoats-Saha based on the Silver formulation predict beam symmetry and zero cross polarization despite aperture size.) The radiation characteristics: beam efficiency, aperture efficiency, gain, and cross-polarization ratio are computed for typical horn aperture sizes (i.e., horn diameters of about one to ten free-space wavelengths). Recognition of the fact that the main beam radiation can be described by the universal horn pattern of Kelleher facilitates reflector feed design procedure for arbitrary reflector edge illumination levels.