Compact ultra-wideband microstrip-to-coplanar stripline transitions

A compact planar configuration is presented for an ultra-wideband (UWB) transition between a microstrip line and a coplanar strip line (CPS). A single metallic via and a slot in the ground plane of a microstrip line are used to transform the microstrip mode into the CPS mode. Three prototype back-to-back transitions are fabricated on a 0.813 mm thick RO4003 substrate. The measured insertion losses of the back-to-back transitions are lower than 2.5 dB from 1.7 GHz to at least up to 15.3 GHz. Over this frequency range, the return loss is less than 10 dB and the group delay variation is less than 0.12 nS. The transition lengths are about 0.1 λ_g in the microstrip line direction, where λ_g is the microstrip line guided wavelength at the lowest frequency having an insertion loss of 2.5 dB. Design guidelines are also presented for other substrate parameters. The transition is most suitable for UWB communication systems.