Compact wideband balanced filter for eliminating radio-frequency interference on differentially-fed antennas

For compact wireless devices, radio-frequency interference (RFI) usually occurs when the common-mode (CM) noises from digital cirucits couple to differentially-fed antennas (DFAs). To tackle this problem, a wideband balanced filter with high-level CM suppression is presented. By combining with the pseudo left-handed transmission line (LHTL) and defected ground structure (DGS), a wideband differential-mode (DM) bandpass response can be achieved; meanwhile, the DGS also improves the CM suppression up to 10 GHz. For verification, this balanced filter is implemented on two-layered PCB and the circuit size is 0.42 × 0.24 square guided-wavelengths. Simulated and measured results are in good agreement with each other. The measured result shows that the -3 dB DM transmission coefficient is up to 103 %, from 1 to 3.17 GHz, and CM transmission coefficient is less than -30 dB within the passband. The proposed balanced filter is finally integrated with a wideband bowtie-shaped DFA. The operating frequency of the filter-integrated DFA is from 1.77 to 3.44 GHz, where the DM reflection coefficient is lower than -10 dB. Meanwhile, within the operating band, the CM reflection coefficient is higher than -3 dB. Thanks to high-level CM suppressions provided by the balanced filter, undesired CM radiation patterns can be reduced more than 20 dB.