Quasi-Absorptive Substrate-Integrated Bandpass Filters Using Capacitively-Loaded Coaxial Resonators

This paper reports on highly-selective substrate-integrated (SI) bandpass filters (BPFs) with two-port symmetric quasi-absorptive characteristics. The proposed concept is based on a two-pole/one-transmission-zero (TZ) bandpass (BP) filtering section that is loaded at its input/output ports with resistively-terminated second-order bandstop (BS) sections. It features size compactness and low insertion loss (IL) that are obtained through: i) high-quality-factor SI capacitively-loaded coaxial resonators and ii) a BP section that is shaped by two resonators and a frequency-dependent coupling. Various synthesized examples and practical implementation aspects are discussed through synthesis and linear simulations. For proof-of-concept demonstration, a prototype shaped by six resonators was manufactured and measured. It exhibited a highly-selective symmetric quasi-absorptive transfer function with one TZ and the following characteristics: center frequency: 3.68 GHz, 3 dB-bandwidth: 590 MHz (16%), minimum IL: 1 dB, in-band group delay: 1.79±0.5 ns, and return loss<10 dB for frequencies between 2.9 GHz and 4.9 GHz.