Compact reconfigurable dual-mode microstrip square ring filter

A compact dual-mode reconfigurable band pass filter is presented. A square ring microstrip structure with additional small square patches attached to the inner corners of the ring is used. The ring is loaded with four varactor diodes connected at its corners to control the center frequency of the filter. A tunable bandpass filter based on this structure is designed and implemented to tune over the range from 1.66 GHz to 1.95 GHz. The instantaneous relative bandwidth is about 6% in the entire tuning range. Measurements are in good agreement with simulations.