Two-Pole Narrow-Band Filter Utilizing λ/2 Resonators and Source-Load Coupling

In this paper a simple and compact narrow-band filter with symmetric frequency response is proposed. A pair of transmission zeros (TZs) is obtained from a 2nd-order (N=2) filter by utilizing the source-load (S/L) coupling. The design is based on the 'N+2' transversal network coupling matrix (CM) and the number of transmission zeros obtained is 'N'. The proposed two-pole bandpass filter (BPF) employs two magnetically coupled half-wavelength (lambda/2) resonators to achieve good skirt-selectivity. A prototype is designed and fabricated for a fractional bandwidth (FBW) of 2.54% at 9.825 GHz. Two TZs are obtained, one on either side of the passband at 9.16 GHz and 10.65 GHz, respectively. Minimum insertion loss (IL) of 1.69 dB and return loss (RL) greater than 12dB is obtained in the passband. The proposed filter also possesses worthy characteristics like fairly flat response (0.7 dB ripple) and low group delay (less than 0.6 ns) in the operational frequency band. A good agreement between the measured and simulated results validates the design method.