Packaged printed transmission lines: modal phenomena and relation to leakage

It is pointed out that the dispersion curves of the modes on shielded printed transmission lines often interact with the dispersion curves of box (package) guided modes in a classical coupled-mode manner. It is shown that this effect is related directly to the phenomenon of leakage. It is demonstrated that the dominant mode associated with a shielded printed transmission line can interact with box guided modes in a coupled-mode manner only if the corresponding dominant mode is leaky when the same transmission line is placed on a substrate of infinite transverse extent. It is also shown that the modes of a shielded transmission line may support fields that are leaky in the sense that they are not confined to the designed guiding region (strip or slot), even though all te modes of the structure have either purely real or imaginary propagation wavenumbers. Numerical and experimental results are presented to show the effects of these phenomena.<>