Theory of imperfect waveguides: the effect of wall impedance

Wave propagation in waveguides whose walls exhibit an arbitrary but small surface impedance is studied. It is shown that, although in the case of circular waveguides all E-and H-modes are stable, in the case of rectangular waveguides all E-modes and higher-order H-modes are unstable, leaving only H modes that can propagate without change of form. General formulae are derived from which the propagation coefficients of imperfect waveguides can be calculated, and for imperfect cavities an expression for the bandwidth and resonant frequency is obtained.