A 7.5-GHz uniplanar 180° hybrid coupler on flexible polyimide substrate

A uniplanar hybrid rat-race coupler has been fabricated on a flexible polyimide film for wearable and flexible RF applications. To enhance the reliability and repeatability of its operation in flexible environment, the coupler is designed using an asymmetric coplanar strip (ACPS) structure. Compared to the microstrip line or coplanar waveguide (CPW) structures, the ACPS requires no via holes through the substrate and reduces the number of air bridges. The dimension of the ACPS is determined by both analytic design equations and full electromagnetic simulation. The coupler was fabricated through photolithography and e-beam evaporation processes. The measurement shows that the insertion loss and phase error are less than 6 dB and 7°, respectively, from 7.4 to 7.9 GHz. The port matching and isolation performance are better than 13.4 dB over the same frequency band.