Compact Microstrip Quasi-Elliptic Bandpass Filter Using Open-Loop Dumbbell Shaped Defected Ground Structure

A novel square open-loop dumbbell-shaped defected ground structure (DGS) unit is proposed. This unit provides a quasi-elliptic bandpass characteristic and the two transmission zeros near the passband edges can be controlled by the dimensions of DGS. Two quasi-elliptic bandpass filters using one and two DGS units; centered at 1.5 GHz were designed and implemented. Both the simulation and experimental results show that the DGS filter response is in good accordance with ideal quasi-elliptic model. The prototype filter with two DGS units yields higher order quasi-elliptic filtering and reports the measured 0.72 dB as insertion loss, 34 dB as matching, 51.8 % fractional bandwidth and about 20 dB stopband attenuation up to 10 GHz