In this paper, the modal method is applied to analyze coated circular waveguide terminated by a perfect electric conductor (PEC) plate. The key to this method is the accurate calculation of the propagation constants of modes in coated circular waveguide. To overcome numerical difficulties, such as overflow, encountered in solving characteristic equation, the characteristic equation is modified using Hankel function of the second kind instead of Bessel function of the first kind in the coated layers. The modified characteristic equation can be accurately solved to obtain the propagation constants even for very large circular waveguide with highly lossy coatings. To verify the modified characteristic equation, the attenuation and scattering property of circular waveguide structure have been simulated. Simulation results agree well with the reference results.
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