The theory of surface-wave and space-wave leaky-mode excitation on microstrip lines

This paper discusses the excitation and physical validity of both surface-w ave and space-wave leaky modes on microstrip lines. This is done by analyzi ng the discrete modal spectrum excited by a realistic source on or near an infinite microstrip line. A semi-analytical three-dimensional (3-D) Green's function is used for this purpose, which provides the current excited on t he conducting strip due to the source. The 3-D Green's function is in the f orm of a spectral integration (inverse Fourier transform) in the longitudin al wavenumber plane. The poles of the integrand directly determine the exci tation amplitudes of the modes on the structure that are launched by the so urce. The integrand also has different types of branch points, and the loca tion of the poles on the various Riemann sheets is used to determine the ph ysical significance of the leaky modes. Although the theory presented here is illustrated for a microstrip line, the conclusions apply in general to o pen printed-circuit structures.