A Wideband Balanced-to-Unbalanced HTS Filtering Balun for Suppressing Radio Frequency Interference in Radio Astronomy

This article presents a wideband balanced-to-unbalanced high-temperature superconducting (HTS) filtering balun for suppressing radio frequency interference in radio astronomy. To address the challenge of the complexity in designing a high-order uniplanar wideband filtering baluns using the conventional design methods, a simplified and flexible balanced-to-unbalanced design process is utilized to design the proposed high-order multisection filtering balun with the ability of impedance transformation. Moreover, by virtue of the loaded open-ended stub and parasitic microstrip line in the circuit, several parasitic harmonic frequencies caused by this cascaded structure around 6 GHz are suppressed below −30 dB, which demonstrates a good filtering performance. For verification, a fifth-order S-band wideband HTS filtering balun with good suppression of out-of-band harmonics and common-mode noise was theoretically designed and fabricated on a 0.5-mm-thick MgO substrate with double-sided YBa₂Cu₃O₇ films. The measured return loss is better than 20 dB. The in-band maximum insertion loss is 0.17 dB with a low-amplitude imbalance of less than 0.2 dB over 1.7–4.1 GHz at a temperature of 77 K, whose fractional bandwidth is 80%. The phase imbalance between the balanced ports is measured to be within 5°. Good agreement is observed between the measured and simulated results.