Accurate design of compact forward‐coupled microstrip filters using an integral‐equation field solver

In this article, we present forward‐coupled microstrip filters designed using an integral‐equation‐based field solver, and fabricated in high‐temperature superconducting (HTS) thin films. Two designs were studied, one a nine‐pole bandpass filter at 900 MHz with 25‐MHz bandwidth, and the other a five‐pole bandpass filter at 2 GHz with 24‐MHz bandwidth and 10‐Ω resonator characteristic impedance. Both designs were fabricated using YBa2Cu3O7(YBCO) HTS thin films. Measurements showed excellent agreements with the simulation predictions. For the nine‐pole filter design, a measured insertion loss of 0.3 dB, and a measured return loss of 16 dB were achieved in the YBCO filter at 77 K.