The propagation constant of waveguide partially loaded with a semiconductor in the H plane is evaluated using a three-mode approximation analysis. As the waveguide is progressively filled, a large peak occurs in the attenuation coefficient due to higher order mode propagation. In the presence of a transverse magnetic field, propagation becomes nonreciprocal and this nonreciprocal effect is shown to be significantly increased in the region of the peak. The theoretical results are verified using n-type germanium samples in 26.5-40 -GHz waveguide.