I. Introduction
Rectangular waveguides have the advantages of larger power capacity and lower attenuation when transmitting signals. Waveguides and their derived devices are widely used in modern high-power-microwave communications and high-frequency-microwave communications. However, the metal flange surfaces are never completely flat due to the limitation of fabrication, and the pressure applied on the surfaces could be very low after assembling [1], which causes the contact imperfect. The imperfect contact in waveguide junctions is of an important influence on the transmission performance as the power and frequency becoming higher. A standard on the structural tolerances of the waveguide interface for frequencies above 110 GHz has been reported [2].