Spectral-Domain Modeling of Radiation and Guided Wave Leakage in a Printed Transmission Line

Abstract

The dominant mode of an infinite transmission line printed on a layered substrate and/or ground plane configuration can be lossy due to coupling of power to radiation or to source free guided modes of the layered structure [1–3]. Such ‘non-conventional’ lossy modes of a printed transmission line are undesirable for an integrated circuit application. However, these modes can sometimes be inevitably excited for certain specific layered configurations at all frequencies, or only at higher frequencies for certain other geometries. Similar lossy modes can also occur for higher order modes of printed transmission lines which are otherwise dominantly non-leaky (bound) [4]. Unlike the dominant mode of a conventional transmission line (a regular microstrip line, for example) where the field is bound to the guiding structure, for these leaky modes the field profile of the infinite transmission line has been found to behave in a non-standard way by growing exponentially in transverse directions [3]. For such growing behavior, the Fourier transforms for various field components in the transverse dimension do not exist for real values of the spectral argument. Hence, the standard spectral-domain formulations [5–7], where existence of Fourier transform on the real axis is assumed, can not be directly used to characterize this class of leaky modes of printed transmission lines.