Compact dual-mode dual-band balun filter using double-sided parallel-strip line

Proposed is a compact dual-band balun filter adopting a double-sided parallel-strip line (DSPSL). The middle conductor plane can be treated as a virtual ground. Then, two sets of short-stub loaded dual-mode resonators on the top and the bottom layer act as two dual-band filters. They are fed by the DSPSL at the input port, and the filters can be excited simultaneously. Meanwhile, an out-of-phase feature is obtained between the two output ports. In addition, the proposed dual-band balun filter has good frequency selectivity which is due to multiple transmission zeros. Finally, a dual-band balun filter sample was designed and fabricated with good agreement obtained between its measured and simulated S-parameters.