Equivalent circuit of an evanescent-mode inductive element

By the use of a partial-fraction expansion of the propagation constant in terms of the angular frequency, a successive series of equivalent-circuit approximations is obtained for the parallel element in the II representation of a length of evanescent-mode waveguide. The first of these approximations simply adds a parallel capacitance to the low-frequency inductance, an element which can be readily absorbed into any tuning capacitor which may be required. The third-order approximation can be put in various forms, some of which similarly exhibit a parallel capacitive element.