A dual-band dual-mode filter constructed by using floating-plate overlay structures

This paper presents the realization of dual-band dual-mode filter with adopting a floating-plate overlay feeding structures. The structure of floating-plate overlay can not only achieve a pass band bandwidth wider than the conventional features, but can realize the filter to have dual pass bands also. However, the structure of the floating-plate overlay limits the input positions of two loop resonators must be collinear, thus, the conventional dual-mode filter design method cannot be applied to implement both two pass-bands. A new set of design equations that can determine the dual mode filter with input position of feeding capacitor other than the conventional structure is proposed. For verifying the proposed design concept, an experimental filter having two designed pass-bands located at 2.4GHz and 3.6GHz, respectively, was designed and fabricated by using single layer PCB technology and two floating-plate overlays. The size of the filter is about 22×41 mm² and the measured insertion losses in the pass bands are less than 1.8 and 2.0dB, respectively.