A balanced bandpass filter with two transmission zeros based on square patch resonators

Inspired by the excellent properties of a patch element, the square patch resonator is considered for the implementation of a balanced bandpass filter with two transmission zeros. Two square patch resonators with proper port location are cascaded for the realization of dual mode differential bandpass filtering characteristics. By adding open stubs and spur-lines to the basic configuration, a balanced bandpass filter with good differential-mode selectivity and good common-mode rejection can be implemented. For demonstration, a balanced bandpass filter operating at 2.4 GHz is designed, fabricated and measured. A reasonable agreement between simulation and measurement can be found.