Analysis of conical quasi-TEM horn with a hard corrugated section

There is an interest in using electromagnetic bandgap (EBG) surfaces to realize quasi-TEM waveguides and horns. TEM horns will have a uniform aperture distribution and a corresponding high aperture efficiency and low cross polarization. In the present paper a conical quasi-TEM horn with a hard-walled section excited in the TE₁₁ mode from a regular waveguide is analyzed. The hard wall is formed by longitudinal dielectric-filled corrugations. The analysis is based on a stepwise cylindrical approximation of the conical surfaces with subsequent application of the mode-matching method in combination with the method of factorization. The modal field expansion of the longitudinally corrugated cylindrical sections is found by using the asymptotic boundary condition of the corrugated surface when the corrugation period approaches zero. The indicated approach refines the data for the limiting aperture efficiency corresponding to the open-ended waveguide, and reveals strong influence of the hard section length as well as the interaction between the aperture and throat on the horn performance. It is shown that in the preferable case when the corrugations occupy the whole inner horn surface, it is possible to provide the crosspolar level below -30 dB and aperture efficiency from 85% to 95% in some frequency band up to 20% depending on the horn parameters.