Realisation of Four Transmission Zeroes in a Four-Pole Superconducting Microstrip Using Cross-Shaped Spiral Resonators

The realization of four finite transmission zeros with high-temperature superconducting microstrip narrow-band bandpass filter using four cross-shaped spiral resonators is described for the fully canonical case. The approach is based on the "N + 2" transversal coupling matrix, which is composed of main direct coupling as well as source/load coupling. The filter, with 0.75% bandwidth, 3.25 MHz, at 408-MHz band, is to be used for a radio astronomy application at Jodrell Bank Observatory, U.K. The measured results match well with both theoretical results and simulated results, except for a small center frequency offset.