Compact microstrip lowpass filter with ultra-wide rejection-band

A lowpass filter with wideband stopband is developed using microstrip line and proposed in this paper. It consists of shunt stepped-impedance open-circuited stubs and unit elements. The electrical length of the open-circuited stubs is chosen to be twice the length of the unit elements at a specific frequency. As a result, the filter can exhibit additional transmission zeros inside the rejection band and close to the cut-off frequency. Hence, the filter can provide an equal ripple performance with very high selectivity. The proposed design is verified by EM-simulations and experiment. A good agreement is attained between the calculated, simulated and measured results. The measured filter shows excellent performance with an extended stopband.