Upper frequency limit of the power loss method for the estimation of ohmic losses in hollow metallic waveguides

The ohmic losses of hollow metallic waveguides are normally estimated by the power loss method. This approximate method is based on using the solution of the ideal problem (with perfect conductors) and the Leontovich or impedance boundary condition. In this paper the highest frequency where the power loss method can be used with accuracy is investigated. This frequency is determined by both, the cross-sectional dimension of the waveguide and the behaviour of the metallic conductors. Based on the analytical exact solution for the circular waveguide a general transcendental equation is proposed. The estimation of the upper frequency obtained from this formulation is presented in this paper, very useful and with practical consequences from an engineering point of view.