Compact Split-Type Dual-Band Bandpass Filter Based on $\lambda /4$ Resonators

This letter presents a compact split-type dual-band bandpass filter based on the λ/4 resonators, whereas the filter´s dual-band frequency response is achieved by creating a stopband inside a virtual wide passband. Besides the uniform microstrip λ/4 resonators, dimensions of both required J and K inter-resonator inverters for the proposed coupling topology are analytically derived based on the synthesized coupling matrix. In parallel, the hook-type coupled line is designed not only to provide the desired external and cross I/O ports´ couplings, but also to manipulate the associated finite transmission zero as demanded in getting an improved upper stopband. To justify our proposed idea, a split-type dual-band BPF with central frequencies at 1.8 and 2.45 GHz, and fractional bandwidths of both 5.0% are designed. These closely proximate dual passbands are primarily realized by properly allocating two finite transmission zeros between them at 2.13 and 2.29 GHz. Moreover, an extra pair of transmission zeros at 1.53 and 3.04 GHz, contributed by cross I/O ports´ coupling, is produced at lower and higher sides of virtual wide passband, thus enhancing the out-of-band rejection ratios. The final measured and simulated results show good consistence with the theoretical counterparts.