Study on band broadening of planar balun with the defected ground structure

The simplified equivalent circuit models are set up for coupled microstrip lines without/with the defected ground structure(DGS) in this paper. Then using the equivalent circuit models, the effect of the DGS on even-and odd-mode impedances of coupled lines is analysed and the band broadening method for balun is summarized. Based on this theory, an improved DGS is presented to broaden the bandwidth of the planar balun. Finally, a sample of planar balun with the proposed DGS is designed, fabricated and measured. The measured return loss is better than -10dB from 1.3GHz to 4.5GHz, or 145% bandwidth, and shows good agreements with the simulated result. The results show that the balun with the DGS proposed here has a wider bandwidth and is smaller in size.