Supercompact Balanced Wideband Bandpass Filter Using Capacitor-Loaded Three-Line Coupled Structure

A supercompact balanced wideband bandpass filter (BPF) based on capacitor-loaded one-eighth wavelength three-line coupled structure is proposed. Multiple differential-mode (DM) transmission poles, DM transmission zeros, and common-mode (CM) transmission zeros can be obtained and controlled by loading multiple capacitors on the three-line coupled structure, which makes the filter achieve wideband DM filtering response and CM suppression. Compared with the state-of-the-art balanced wideband BPFs, the proposed design has the advantages of compact size and simple structure. A prototype centered at 0.66 GHz with the 3-dB fractional bandwidth of 43.8% is demonstrated, which exhibits the supercompact size of0.003~λ ²g(λ gis the guided wavelength at the center frequency) and the minimum insertion loss of 0.43 dB.