Reflection of ${\rm TE}_{0{n}}$ Modes at Open-Ended Oversized Circular Waveguide

Reflection of circular symmetric TE₀n modes ( n= 1-6) at an oversized, open-ended circular waveguide (C76-waveguide, inner diameter=27.79 mm, 70 GHz) radiating into free space has been investigated theoretically employing two scattering matrix codes (SMCs), the finite-difference time-domain code EMPIRE and the uniform geometrical theory of diffraction (UTD) as well as the first time experimentally. The measurements utilized mode converters for generation of pure TE₀n modes and a wavenumber spectrometer for mode analysis in the oversized waveguide. The total power reflection computed by EMPIRE is 4.1 to 13.4 dB lower than calculated from free-space wave and waveguide mode impedances. In all cases, most of the reflected power is carried by the backward traveling TE₀₆ mode, which is the mode closest to cutoff. Experiments are in very good agreement with theoretical results.