Novel Multilayered Ultra-Broadband Bandpass Filters on High-Impedance Slotline Resonators

This paper presents a novel class of multilayered ultra-broadband bandpass filters based on high-impedance slotline resonators. The equivalent circuit and electrical properties of the proposed high-impedance slotline resonator realized on a low-permittivity substrate are first discussed. To largely remove the radiation losses of the devised high-impedance slotline resonator, two identical shunt-connected folded low-impedance slotline sections are utilized to replace one short-circuited high-impedance slotline section. Through the synthesis design of an ultra-broadband microstrip-to-microstrip vertical transition, the effective impedance of the conceived shunt-connected low-impedance slotline sections is confirmed and the impedance variations of this proposed high-impedance slotline resonator are quantitatively determined by the means of the extracted turns ratios of the employed transformers. To further demonstrate the effectiveness of the engineered slotline resonator in terms of radiation reduction and verify its equivalent circuit model, a new type of multilayered high-order bandpass filters with ultra-broadband Chebyshev equiripple filtering response is theoretically presented. Furthermore, for experimental validation purposes, a 2-GHz two-layered bandpass filter prototype is fabricated and measured. It exhibits a seven-pole ultra-broadband filtering response, high in-band amplitude/group-delay flatness, highly reduced radiation losses, and sharp-rejection upper-stopband characteristics.