Compact step-impedance coaxial bandpass filters by using stacked dielectric blocks

Abstract

This article proposes a novel coaxial step-impedance resonator (SIR) for the design of compact bandpass filter (BPF) with wide stopband. The proposed coaxial SIR is divided into two dielectric blocks for processing. At the bottom of the upper dielectric block, there is an ungrounded metallized blind hole, while at the center of the lower dielectric block, there is a metallized through-hole. The stack of the two dielectric blocks creates the proposed quarter-wavelength coaxial SIR, which not only features a compact size and light weight, but also exhibits a very high frequency ratio between the first spurious mode and the fundamental mode. Based on the coaxial SIR, a fourth-order BPF with a passband range from 2.515 GHz to 2.675 GHz was designed and measured. The dimensions of the BPF are 31 × 10 × 4.75 mm³, which is about 1.2λ_g × 0.4λ_g × 0.18λ_g. The measured return loss is better than 17 dB and the minimum in-band insertion loss is approximately 0.7 dB. The first harmonic appears at 6.03 GHz, which is approximately 2.33f₀. The proposed BPF features a small size and a wide stopband, making it a promising option for future communication base station applications.