Compact Tri-band Bandpass Filter Based on Asymmetric Step Impedance Resonators for WiMAX and RFID Systems

In this article, a simple method is developed to design a highly miniaturized tri-band bandpass filter (BPF) utilizing two asymmetric coupled resonators with one step discontinuity and one uniform impedance resonator (UIR) for worldwide interoperability for microwave access (WiMAX) and radio frequency identification (RFID) applications. The first and second passbands located at 3.7 GHz and 6.6 GHz are achieved through two asymmetric coupled step impedance resonators (SIRs), while the third passband, centered at 9 GHz, is achieved using a half-wavelength UIR, respectively. The fundamental frequencies of this BPF are implemented by tuning the physical length ratio (a) and impedance ratio (R) of the asymmetric SIRs. The proposed filter is designed and fabricated with a circuit dimension of 13.69 mm X 25 mm (0.02 λ_g X 0.03 λg), where λg represents the guided wavelength at the first passband. The experimental and measured results are provided with good matching.