Wideband balanced bandpass filter based on three-line coupled structure

A compact wideband balanced filter based on a three-line coupled structure is proposed. Two finite-frequency transmission zeros close to the passband for the differential mode and wide stopband for the common mode can be realised by the three-line coupled structure. A prototype with 3 dB fractional bandwidth of 61.5% for the differential mode and insertion loss greater than 15 dB for the common mode (0-19 GHz) has been designed and fabricated.