Microwave dual-band bandpass planar filter using double-coupled resonating feeding sections

A new concept of microwave coupled-resonator dual-passband planar filter is described. It is based on using novel input/output double-coupled resonating feeding stages. These feeding sections, leading to a multi-coupled resonator filter topology, enables dual-band bandpass filtering profiles with close passbands and inter-band transmission zeros to be derived. Furthermore, as an added value, they are useful to increase the filter order through their synthesis as in-band resonating nodes. Some guidelines for the design of this dual-band bandpass filter are provided. Also, to experimentally demonstrate the principle, a microstrip dual-passband filter circuit is constructed and tested.