A novel compact microstrip lowpass filter with ultra-wide stopband characteristic using square ring loaded resonators is proposed. A microstrip high impedance main transmission line loaded with five square ring loaded resonators is adopted in the design of the filter. Owing to the adoption of the square ring structure, the filter achieves compact size and ultra-wide stopband. A demonstration filter with 3 dB cutoff frequency at 0.72 GHz has been designed, fabricated and measured. Results indicate that the proposed filter is able to suppress the 19th harmonic response referred to a suppression degree of 15 dB, together with a small size of 0.054λg×0.070λg, where λg is the guided wavelength at 0.72 GHz.
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