A new microstrip lowpass filter using radial stub loaded resonators is presented. A microstrip high impedance main transmission line loaded with five radial stub loaded resonators by five high impendence transmission lines is introduced in the design of the filter. Owing to the introduction of the radial stub structure, the filter achieves compact size and ultra-wide stopband. A demonstration filter with 3 dB cutoff frequency at 0.8 GHz has been designed, fabricated and measured. Results indicate that the proposed filter is able to suppress the 17th harmonic response referred to a suppression degree of 15 dB, together with a small size of 0.060λg×0.080λg, where λg is the guided wavelength at 0.8 GHz.
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