Design of HTS 3 dB Hybrid Coupler Using Lumped Elements for Radio Astronomy

In this work, we report on a highly miniaturized 3 dB coupler for radio astronomy. To do this, we made the equivalent circuits with normal branch line couplers with eight impedances. Then, we analyzed them using an ABCD matrix under even and odd mode excitation, and successfully designed miniaturized 3 dB couplers with the lumped elements of four inductors and four capacitors. To avoid the increasing of surface resistance due to the narrow line width, we used a high-Tc superconducting thin film. The designed high-Tc superconducting 3 dB couplers were fabricated using YBCO thin film on MgO substrate. Experimental results including bandwidth, center frequency, and phase difference between |S₁₂| and |S₁₃| are reported. Especially, our couplers were about five times smaller than normal branch line couplers, and had a significant broad band of 15 MHz.