Thin Film Ferroelectric Compact Branch-Line Coupler Based on D-CRLH Unit Cell and YBCO HTS Microstrip

In this paper, two new compact branch-line couplers (BLCs) are proposed. The first is designed and implemented using D-CRLH unit cells made of copper microstrip line. The new design occupies only 52 % of the area of the conventional BLC that operates at 1.8 GHz. The structure has been fabricated and the experimental results have been compared to the simulated ones and very good agreement has been obtained. Advanced design system circuit simulator and the CST full wave EM simulator have been used to simulate the proposed BLCs. The second BLC is designed using YBCO high-temperature superconducting (HTS) microstrip rather than copper to achieve low insertion loss, and more compact size. Three cases of this coupler with different substrate materials are studied. The design of YBCO on lanthanum aluminate (LAO) substrate attains an optimum BLC performance compared to the one using D-CRLH. All parameters of the second design are enhanced compared to the first one, the insertion loss is enhanced from −0.1268 to −0.007 dB, return loss is enhanced from −35 to −41 dB, isolation is enhanced from −25 to −36 dB, and the area is reduced from 246 to 53.55 mm². Moreover, the tunable frequency BLC is designed using thin film layer of BST ferroelectric material, the layer is sandwiched between the LAO substrate and HTS microstrip line, the response shows a variation of the center frequency work with the applied electric field, and the ferroelectric thickness.