A Novel Quad-Band Bandpass Filter Using Short Stub Loaded E-Shaped Resonators

In this letter, a novel quad-band bandpass filter (BPF) using short stub loaded E-Type resonators is proposed. Two sets of resonators are utilized and respectively designed to realize Filter 1, operating at 2.4 and 3.5 GHz, and Filter 2 with two very closely spaced passbands, operating at 5.2 and 5.8 GHz. Then the two dual-band BPFs are combined by common feed lines with interdigital capacitance structure for quad-band responses, and two more transmission zeros (TZs) are achieved by strengthening the source-load coupling. The measured result of the fabricated filter agrees well with the simulation, which shows that the proposed structure is a good candidate for quad-band BPF designs and validates the proposed design flow well.