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.

Leaky waves have been known for many years in the context of leaky-wave antennas, but it is only within the past dozen years or so that it was realized that the dominant mode on printed-circuit transmission lines used in microwave and millimeter-wave integrated circuits can also leak. Such leakage is extremely important because it may cause power loss, cross talk between neighboring parts of the circuit, and various undesired package effects. These effects can ruin the performance of the circuit, so we must know when leakage can occur and how to avoid it. In most cases, these transmission lines leak only at high frequencies, but some lines leak at all frequencies. However, those lines can be modified to avoid the leakage. This paper explains why and when leakage occurs, and shows how the dominant mode behaves on different lines. The paper also examines certain less well known but important features involving unexpected new physical effects. These include an additional dominant mode on microstrip line that is leaky at higher frequencies, and a simultaneous propagation effect, which is rather general and which occurs when the line's relative cross-sectional dimensions are changed. The final section of the paper is concerned with three important recent developments: (a) the new effects that arise when the frequency is raised still higher and leakage occurs into an additional surface wave, (b) a basic and unexpected discovery relating to improper real modes, which are nonphysical but which can strongly influence the total physical field under the right circumstances, and (c) the important practical issue of how leakage behavior is modified when the circuit is placed into a package.