Analysis of cutoff frequency in trapezoid tunnel using edge element method

The effect of cross section dimension in trapezoid tunnel on cutoff frequency is analyzed with edge element method. The variation of cutoff frequency in trapezoid tunnel with top side, bottom side and height changes are found, respectively. It comes to a useful conclusion that the cutoff frequency of trapezoid tunnel is determined by the larger one of tunnel´s height and the average of tunnel´s top side and tunnel´s bottom side. A simple equivalent formula is given to calculate the dominant mode cutoff frequency in trapezoid waveguide. Computation results are presented to show the validity and the accuracy of the equivalent method.