Compact ultra-wide stopband low pass filter using multimode resonators

A compact microstrip ultra-wide stopband low pass filter using multimode resonators is presented. It consists of a narrow width microstrip line with high impedance and three multimode resonators. The characteristics and equivalent circuits of the multimode resonator are investigated. Owing to the strong slow-wave and bandstop effects, the multimode resonator can be implemented to achieve an ultra-wide stopband and size reduction. At the 3 dB cut off frequency of 1.8 GHz, the measured stopband of the low pass filter with 15 dB suppression is from 2.83 to 24 GHz. The physical size of the proposed low pass filter is 0.09λ_g × 0.11λ_g. The presented simulation and measurement results are in good agreement.