Reflection and transmission at an abrupt change in cross-section of a rectangular waveguide in the E and H plane

For a symmetric cross-sectional enlargement of a rectangular waveguide, the reflection and transmission coefficients of the H10 mode are calculated. At low frequencies, they agree well with the values for the H11 mode at an axial-symmetric junction of two circular waveguides. An axial offset of the two rectangular waveguides has a small influence on these coefficients.