Optimized design of the flexible stepped impedance lowpass filter based on defected ground structure

Since the performances of the common flexible microstrip filters change in bending state, this paper applies rectangular defected ground structure (DGS) units to reduce the change. The proposed filter which is designed on a polyimide substrate consists of the stepped impedance structure and the rectangular DGS units. Simulation results demonstrate that the designed flexible lowpass filter has 30%to 83% smaller cutoff frequency offset in bending state compared with the common stepped impedance lowpass filter. At the same time, the insertion loss and the return loss in the passband maintain original level after the circuit is bent. The outband suppression changes within limited range.