<title>Loss and dispersion at subterahertz frequencies in coplanar waveguides with varying ground-plane widths</title>

Abstract

The characteristics of a family of coplanar transmission lines have been studied at frequencies extending to the terahertz range. Traditional wide-ground coplanar waveguides and coplanar strip lines were investigated together with a coplanar waveguide with narrow ground planes. The technique of nonuniform gap illumination was used to excite subpicosecond electrical pulses as a testing tool of transmission lines for the first time. It is shown that this method is versatile and convenient for testing ultrafast devices and circuits. The experimental results, extracted by both time- and frequency-domain analyses, indicate several interesting features. In the subterahertz frequency range, the 50-micrometers transmission lines are dominated by dispersion, while the narrower 10-micrometers lines are dominated by loss. The characteristics of traditional (wide-ground) coplanar waveguides and coplanar strips are in agreement with theory and comparable to each other up to very high frequencies. The implementation of narrow ground planes can considerably reduce attenuation and dispersion in coplanar waveguides. In some geometries, radiation loss can be eliminated completely. The reduction in radiation is attributed to the change of field patterns at the dielectric interface, which leads to reduced coupling between the coplanar waveguide mode and radiative substrate modes.