Design and analysis of an HTS coplanar waveguide branch-line coupler

This paper presents the design and performance of a high temperature superconductor (HTS) coplanar waveguide (CPW) 3 dB, 90° branch-line hybrid coupler. The coupler was designed to operate over a 500 MHz bandwidth centered at 11.95 GHz. The design was performed using in-house software based on a mixed potential integral equation formulation combined with the method of complex images for CPW with air-bridges. Experimental results show the transmission and coupling responses are ¿3.5dB and ¿3.1dB, respectively, at the centre frequency. The return loss and isolation are both below ¿15dB over the bandwidth, and both reach a minimum value of less than ¿40dB.