Leakage of the stripline dominant mode produced by a small air gap

It is demonstrated that a dominant complex improper (leaky) mode exists in general on a conventional stripline with a small air gap above the strip. This improper mode has a zero cutoff frequency, as does the proper mode. The improper (leaky) mode, rather than the proper mode, is the one that has fields resembling those of the conventional TEM (transverse electromagnetic) homogeneous stripline mode, and is therefore the mode that is expected to be more strongly excited by a common stripline feed. Its presence may cause spurious transmission-line effects along such stripline, because it leaks power and may therefore interact with other circuit components, as well as interfere directly with the fields of the proper mode. This spurious behavior has been demonstrated experimentally.<>