Examinations of higher order mode cutoff frequencies in symmetrical TEM cells

Previous studies on the higher order mode cutoff frequencies for symmetrical TEM cells include matrix solutions from a mode matching method, approximate expressions based on the assumption of small gaps between the inner conductor and the sidewalls, or numerical simulations based on meshing the cross-section. Good agreement among the methods has been achieved in most cases. However, some discrepancies among results have remained, and these differences are investigated in this paper. A computer code is made available to compute higher order mode cutoff frequencies of any symmetrical TEM cells. In addition, a simple curve-fit algebraic expression for the first higher order mode cutoff frequency of 50 Omega TEM cells is presented.