An Improved Stopband Dual-Band Filter Using Quad-Mode Stub-Loaded Resonators

In this paper, we present a dual-band microstrip bandpass filter (BPF) with an improved stopband, which was constructed by a quad-mode stub-loaded resonator (SLR). Since the SLR is able to produce multiresonance within a single unit, the area saving is significant. The proposed quad-mode SLR was implemented by two stubs allocated at symmetry places, thus the even-/odd-mode can be applied to analysis the resonance. Moreover, to shift the spurious passband, the step impedance structure was applied to shift the harmonic resonance to the higher frequency. Design procedure for high performance dual-band BPF is proposed, and filter examples were designed for wireless local area network (WLAN) of 2.4/5.2 GHz. The measured insertion losses, return losses and fractional bandwidths (FBW) are 1.43 dB, 10 dB, and 14.8% at 2.4 GHz and 1.34 dB, 10 dB, and 12.9% at 5.2 GHz. Moreover, by applying two quarter-wavelength stubs on the input/output ports, the passband selectivity with an isolation of 40 dB can be achieved. The simulation and measurement have a close match, verifying the design concept.