Propagation Near Cutoff Frequency in a Lossy Rectangular Waveguide

We present a simple and fundamental technique to compute the attenuation of electromagnetic waves propagating in lossy rectangular waveguides with imperfectly conducting wall. A set of transecendental equations is derived by matching the tangential electric and magnetic fields at the surface of the wall with the constitutive properties of the wall material, expressed as surface impedance. The propagation constant of the waves can be obtained by substituting the dispersion relation into the equations and numerically solving for the roots. We have compared our results with that using the approximate perturbation method. Althoug good agreement was found at a reasonable range of frequencies above the cutoff frequency, the attenuation curves for the two methods, however, deviate below cutoff. We attribute the discrepancies to the dispersive effect in a lossy waveguide. Since the perturbation method assumes lossless fields’ expressions, such effect failed to be accounted for in the method.