The presence of a new surface-wave-like mode on CPWs of infinite width produces a complex transition region at the onset of leakage, involving the unusual simultaneous combination of a coupling region and a spectral gap. An examination of this region leads to a clear physical explanation of why sharp minima occur in the leakage behavior.

The sides of strip waveguides create TE-TM coupling which has so far been neglected in published analyses. For all TM" modes and for higher TE" modes on strip waveguides under suitable conditions, this coupling produces interesting new effects, such as leakage and resonance, which have implications for device performance.

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.

We report here a new mode-coupling effect that occurs on coplanar waveguides of finite width. The coupling occurs between the standard CPW dominant mode and a new dominant mode identified by us for the first time, and called here the CPW dominant surface-wave-like mode. This coupling is different from that which is known to occur when a printed-circuit waveguide is placed in a box or package; here there is no box, and the finite width by itself is sufficient to produce the coupling effect. This effect results in a complicated dispersion behavior, and we investigate it experimentally and theoretically. In addition to this new coupling effect, a new leakage effect on this class of waveguides has been investigated by us for many variant forms. The details will be discussed elsewhere, but a brief discussion of this leakage effect is also included here in order to place the new coupling effect in proper perspective. We also investigate the sets of structural dimensions needed for the coplanar waveguide to operate without leakage. Finally, we perform a set of measurements in X-band on a standard coplanar waveguide of finite width, and it is found that the experimental results agree very well with the theoretical calculations.