Substrate Modes in Double-Layered Coplanar Waveguides

Abstract

Leakage of energy from Coplanar Waveguides (CPW) at high frequencies has been repeatedly observed. By using basic electromagnetic theory, mathematical models have been derived to predict the critical frequencies at which a conductor-backed double-layered CPW starts to leak. Matching with the experimental results, these models can successfully explain the leakage phenomena in CPWs, and can confirm to which substrate modes, TE or TM, a standard CPW is most likely to leak to. The results of this investigation can also be used as a quantitative guideline in the selection of CPW substrate and in the calculation of the cut-off substrate thickness approximately at millimetre wave frequencies.