Mode Conversion and Leaky Wave Excitation at Microstrip Discontinuities.

The method of moments is used in the special domain to study mode conversion and leaky wave excitation at an open-end coupled-microstrip discontinuity. A covered structure is investigated which supports fundamental leaky modes. After solving the two-dimensional problem to obtain the complex wavenumbers of the modes, a symmetric and an asymmetric discontinuity problem is analyzed. The total current on the strips is modelled by complete-domain basis functions, piecewise sinusoidal functions, and an additional adjustable size, finite element current required to represent the asymmetric discontinuity. A non-leaky mode is incident upon the discontinuity, and the scattered currents are investigated. Results of the analysis show that leaky waves can be excited significantly at this type of discontinuity. It is also shown that to adequately model the reflected current, complete-domain basis functions are required for the leaky modes. Attempts to model the reflected current only by piecewise sinusoidal basis functions led to instabilities in the method of moments algorithm. The excited modes have relatively weak damping, and the energy is leaked over extended portions of the transmission line, rather than just from near the discontinuity. Thus, the work presented here represents a new and potentially important distributed form of surface-wave and parallel -plate mode excitation, which may lead to undesirable cross talk and may also be utilized in the design of new devices.