Design of Compact Superconducting Diplexer with Spiral Short-Circuited Stubs

Three types of impedance-matching structures for diplexer design are analyzed and calculated in this paper. Calculations show that different matching structures should be selected for size reduction, depending on the input impedance of the channel filters in the diplexer. Two channel filters in the superconducting microstrip diplexer at the P-band are designed with spiral resonators and capacitively coupled feedlines. The diplexer is then constructed with an impedance-matching structure of spiral short-circuited stubs. The compact diplexer has a size of 22.9 mm × 21.7 mm on an MgO substrate, which amounts to only 0.101 λ_g0 × 0.096 λ_g0, where λ_g0 is the guided wavelength of the 50-Ω line on the substrate at the midband frequency. The fabricated device exhibits high performance, and its measurements match the simulations well. The experimental insertion loss of both channels is less than 0.13 dB. The common-port return loss is greater than 18.9 dB, and the isolation between the two channels is greater than 60 dB.