A narrowband absorptive band-stop filter based on a resistor-loaded compact resonator

Abstract

This paper presents a narrowband absorptive band-stop filter (ABSF) based on L-shaped microstrip (MS) resonators loosely coupled with main transmission line. Three loosely-coupled MS resonators are connected in a cascaded manner to form a reflective band-stop filter circuitry. In order to improve the return loss of the filter, the first stage resonator is loaded with a lumped resistor at a proper position to form a lossy resonator. Accordingly, the 3-staged reflective band-stop filter is converted into an absorptive band-stop filter. Furthermore, an open radial patch is adopted to effectively reduce the size of the circuit. An equivalent circuit model has been established for quickly determine the resonant frequency of the resistively loaded resonators. After that, full-wave simulation with Ansys HFSS is performed for optimizing the coupling coefficients as well as the resistance of the lumped resistor. A prototype of narrowband absorptive band-stop filter with three-staged resonators has been designed for a centre frequency of 2.23 GHz. The MS circuit is fabricated on Rogers RO4350B substrate with a thickness of 20 mil. Measured suppression to the transmitted signal near the center frequency is more than 20dB over 15 MHz operation band. Measured input return loss is better than 30 dB in the specified band. This absorptive band-stop filter also features a compact size of 0.5A S χ 0.18A S by the adoption of a radial patch instead of straight MS stubs.