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A quad-band microstrip bandpass filter (BPF) based on stub-loaded resonators (SLRs) with improved skirt selectivity is presented. A single metallic via hole in microstrip is utilised to introduce a compact inductive source-load coupling, with which two pairs of SLRs generate eight controllable transmission zeros at each side of all the quadruple passbands. As an example, a quad-band BPF with measured centre frequencies of 1.88, 2.55, 3.55 and 5.15 GHz as well as insertion losses of 1.53, 1.64, 1.79 and 2.55 dB, and respective fractional bandwidths of 4.38, 3.84, 3.44 and 2.9% is designed and fabricated. The measured response shows a good concordance with the simulated one.