A higher mode tri-plate strip transmission line, in which the first higher mode propagates, was developed to realize mass production of millimeter-wave integrated circuits for application in intelligent transport systems, and its transmission characteristics were investigated. The design diagram of this guided mode was determined and a higher mode tri-plate strip transmission line was fabricated at 30 GHz. The dispersion curve was found to be similar to that of a rectangular waveguide and a low transmission loss of less than 10 dB/m was obtained. For construction of some functional devices, two types of basic reactance components, such as a gap and a slot, were expressed by equivalent circuits. The former was expressed by capacitive parameters, and the latter was expressed by an ideal transformer with inductive parameters. The gap-coupled circuit was successfully employed for a 3-pole 0.1 dB Chebyshev ripple band-pass filter with a small excess insertion loss of less than 1 dB at a center frequency of 32 GHz, as well as no spurious response in a bandwidth from 26.5 GHz to 40 GHz. The slot element acted as a matching circuit and a suppressor of the lowest mode, which is the TEM mode in the tri-plate strip transmission line. Moreover, this element was applied to a mode transformer between the lowest mode and the first higher mode.

A complex coefficient filter obtained by directly exchanging several reactance elements included in a real coefficient BPF for imaginary valued resistors is described. By using the proposed method, we obtain four varieties of complex coefficient filters. The stability problem is examined.