High-frequency characterization of open microstrip discontinuities.

Abstract

Currently, microwave circuit applications are extending upward in frequency, and approaching the point where it will become difficult or impossible to integrate microstrip lines and distributed elements into (M)MIC design without reliable circuit models. The Space Domain Integral Equation (SDIE) technique will be presented and applied to an extensive class of microstrip discontinuity and distributed elements. This technique has demonstrated the ability to model microstrip elements at frequencies where other techniques fail. In particular, the method accounts for all radiation and electromagnetic coupling effects which have significant impact at microwave and millimeter-wave frequencies. The radiation losses encountered occur from two mechanisms, space wave and surface wave excitation. The SDIE approach involves an application of the method of moments to the electric field integral equation. Transmission line theory is then employed to arrive at a general procedure for extracting scattering parameters from multiport networks. The radiation loss is separated into space and surface wave contributions by the application of a saddle point analysis. Network parameters are presented for a microstrip step, stub, corner, T-, and cross-junction discontinuities, and results are validated by experiments performed with the Thru-Reflect-Line (TRL) de-embedding procedure. Techniques for improved microstrip design are investigated including balanced and radial stubs, and mitered bends. It is shown that in addition to exhibiting improvements in bandwidth and return loss, these structures show improved radiation performance. Finally, a novel experimental approach is used to verify the direction of surface wave propagation in the dielectric substrate. These patterns are critical for understanding electromagnetic coupling between microstrip elements in close proximity.