The structure of microstrip circuit media as a whole, most notably the arrangement of a planar dielectric substrate and ground plane, has the physical character of a grounded dielectric slab. The latter is well known to be capable of supporting a large variety of surface wave modes of electromagnetic signal propagation. Some of these modes can be suppressed in microstrips simply by choosing a substrate sufficiently thin so as to be beyond the natural cut-off wavelength of certain particular modes. Not all spurious modes, however, can be suppressed by such a technique. Thus, in spite of observing such precautions, spurious modes still tend to be excited, particularly at higher frequencies. When loosely coupled to the circuit medium, these modes tend to give rise to direct signal radiation. Radiation of this sort is highly undesirable both from a standpoint of increased insertion loss and the possibility of undesired coupling to other circuit elements.
Older types of waveguide to microstrip transitions capable of wideband operation do not contain any specific provision for suppression of spurious surface wave modes which tend to become particularly problematical at millimeter wavelengths. Certain other transition apparatus designs which are inherently less susceptible to excitation of spurious modes because of their physical configuration, e.g., a containing probe or loop connected to an intermediate structure supporting a pure TEM mode, contain frequency sensitive circuit elements which restrict their bandwidth to considerably less than that attainable with the ridgeline transformer section used in the present invention. In terms of available performance, earlier types of transition apparatus fail to combine wideband capability with low VSWR and low insertion loss throughout a broad band of frequencies as permitted by the present invention.