Techniques for investigating spurious propagation in enclosed microstrip

The effects of discontinuities and consequent spurious propagation in enclosed microstrip have been studied using 10:1 scaled models by probing the vertical component of the electric field in the substrate and by measuring insertion loss. A number of discontinuities were studied and the results showed the higher-order modes excited in the enclosed microstrip to be perturbed versions of the LSM and LSE modes of inhomogeneous waveguide. A coefficient of coupling from the discontinuity to each mode can be defined and estimated by measuring the external Q of the enclosure. The coupling varies with the discontinuity and its position. Since these modes all have electric fields with components tangential to the air/substrate interface, they can be suppressed by layers of resistive material. The effectiveness of this is demonstrated and also that the fundamental microstrip mode is little affected. Tests on unsealed models confirm the results obtained from scaled versions.