Compact Filtering Balun With Wide Stopband and Low Radiation Loss Using Hybrid Microstrip and Substrate-Integrated Defected Ground Structure

In this letter, a novel hybrid microstrip and substrate-integrated defected ground structure (SIDGS) are proposed for filtering balun design. Coupled microstrip and SIDGS resonators can not only achieve 180° phase imbalance with filtering response but also realize wide stopband and wideband low radiation loss. In addition, the feature of stacked substrate packaging could reduce the size of the circuit and be flexible for integration. To verify this mechanism, a filtering balun operating at 3.08 GHz with 3-dB fractional bandwidth (FBW) of 45% is proposed based on hybrid microstrip and SIDGS, which exhibits the in-band amplitude- and phase-imbalances of ±0.5 dB and ±0.4°, respectively. The stopband extends to 18 GHz with the rejection level of 20 dB, whereas the measured total loss (i.e., including radiation, metal, and substrate loss) is less than 15% up to 15.3 GHz.