Miniaturized wideband bandpass filter using capacitor-loaded coupled-lines

Abstract

A compact wideband bandpass filter (BPF) based on capacitor-loaded parallel coupled-lines and open/short stubs is presented in this brief. The proposed BPF contains four lumped capacitors that can reduce the circuit size, which are connected in series or parallel with the coupled microstrip lines and open/short stubs. Owing to the symmetry of the circuit topology structure, odd- and even-mode methods are used for analysis, then the 3-dB fractional bandwidth (FBW) and return loss (RL) can be calculated by MATLAB using impedance deduction of each branch circuit. To demonstrate the feasibility of the proposed design, a wideband BPF prototype with a center frequency of 0.23 GHz and a 3-dB FBW of 117.8 % is fabricated and measured, which exhibits a miniaturized size of 0.026λg × 0.079λg (λg: guided wavelength at the center frequency). The results of experiment are very consistent with theoretical calculations, circuit and electromagnetic simulations, validating the concept of the proposed technique.