Vialess vertical microstrip-to-microstrip transition using open slot-line

A new wide-band, low-loss and compact vertical microstrip-to-microstrip transition is presented. The proposed structure consists of two quarter-wavelength open-circuited microstrip lines and a quarter-wavelength open slot-line etched on the common ground plane. The theoretical analysis based on equivalent-circuit model reveals that the proposed transition exhibits ultra-wide passband with three transmission poles. To effectively enhance the bandwidth, the optimised parameters are obtained by the transmission poles separations. Finally, a transition having a wide fractional bandwidth of 114% is achieved with the compact size of 8.75 mm × 4.2 mm. Predictions are well verified by measurements over a wide frequency range.