Dual-mode dual-band microstrip filters

A compact dual-band microstrip bandpass filter is proposed using dual-mode resonators. Two microstrip loop resonators, one within the other, are employed to obtain dual-mode dual-band filter responses. Each loop resonator uses two loading elements to excite dual modes. Distinct characteristics of these new types of dual-mode filters are investigated using full-wave electromagnetic simulations. It is also found that there is three finite-frequency transmission zeros. One of them is inherently associated with the filter configuration. Four dual-band filters using dual-mode loop resonators are demonstrated with different asymmetric responses, which result from different locations of transmission zeros. Both simulated and measured results are presented and exhibit dual-mode dual-band bandpass responses with high selectivity.