A defected-ground coupled line section with two shorts for wideband balun application

This paper presents a defected-ground coupled line section with two shorts (DGCL section) which consists of an offset dumb-bell-shaped defected ground structure (DGS) at the back of edge-coupled lines. An equivalent circuit model is given based on even-odd mode analysis, which shows that the characteristic impedance of coupled line section can be lowered and bandwidth can be improved by introducing a DGS at the back. A wideband balun, which consists of a quarter-wavelength microstrip line and a DGCL section, is then designed, fabricated and measured. Measurements show that the return loss at the unbalance port is better than -10 dB from 3 GHz to 9 GHz. The amplitude imbalance and phase difference between the two balance ports are within 1 dB and 180° ± 9° over the operation bandwidth, respectively.