A miniaturized ultra-wideband microstrip filter Using interdigital capacitive loading and interresonator tapped-in coupling

Design of miniaturized wideband filter in combline configuration using interresonator taps and interdigitally-coupled capacitive loading is described. The design is based on increasing the effective capacitance of the original quarter-wavelength combline resonator by engineering the loading interdigital capacitance at the open end of resonator, thus achieving the same resonant frequency by using a shorter physical length. The resonator structure is designed to confine the electric field in a smaller real estate to decrease losses and coupling interference with adjacent components. The filter is physically compact and has wide passband. An ultra-wideband UHF filter having bandwidth more than 20% is designed using full-wave solving software and fabricated in microstrip technology. Experimental results are in good agreement with the theoretical and simulated results.