A Modified Interdigital Coupling Structure for Narrow Bandpass Filters Design

This article presents a modified interdigital coupling structure by using quarter- and half-wave resonators alternately, and a narrowband bandpass filter operating at X-band has been designed proving the feasibility of the proposed structure. The weak coupling required for narrowband filter is realized by arranging five quarter- and half-wave resonators alternately. In this article, simulated and measured result for a narrowband filter with fifth-order Chebyshev response is proposed. The filter is fabricated on 0.508-mm-thick Rogers RO4350B (Chandler, AZ, USA) with dielectric constant 3.66 using standard photolithography technique. Configurations produce very small and compact filter size, at 28.30 x 10.63 mm(2). The simulated frequency responses have demonstrated a good narrowband property from 9.84 to 10.25 GHz (3-dB bandwidth), which is a fractional bandwidth of 4.1% at the center frequency, 10 GHz. The measured passband insertion loss is less than 4.56 dB and the passband return loss is better than -14.68 dB. Very small and compact filter is achieved where measured results show good agreement with the simulated responses.