Compact microstrip broadband filter using multimode stub-loaded resonator

A new compact microstrip broadband filter based on the multimode stub-loaded resonator is investigated. A third-order bandpass response can be realised by properly incorporating the first three resonator modes of the proposed resonator at distant frequencies into the passband. Thus, the filter can be designed with only one resonator, resulting in an extremely small size. For demonstration, a compact broadband filter with a size of 0.5λ_g by 0.07λ_g has been designed and fabricated. Good agreement is achieved between simulation and measurement.