Quad-band filter with high skirt selectivity using stub-loaded nested dual-open loop resonators

A quad-band filter with compact size and high skirt selectivity utilising stub-loaded nested dual-open loop resonators is presented. By loading open-ended stubs at the centre of the two nested resonators, two odd modes as well as two even modes can be independently adjusted to realise a flexible quad-band filtering response. Moreover, both a pair of zero-degree feed lines and the frequency transformation method are adopted to introduce multiple transmission zeroes, which rightly enhance the out-of-band selectivity. According to the topology circuit design and coupling matrix synthesis, a quad-band filter operating at 1.58/1.81/2.50/2.66 GHz is finally designed and fabricated.