Abstract-The frequency response characteristics of a basic microstrip lowpass filter improved using H-shaped defected ground structures are presented. The proposed defected ground structures behave as a resonant element at high frequency and thus eliminate the stopband frequencies to achieve wide stopband rejection. The 3 dB cutoff frequency of the filter is 1.935 GHz. Due to the defects etched in the ground plane of the basic structure, the harmonic rejection is improved from 5th to 10th order along with low insertion loss and voltage standing wave ratio together with good selectivity. The compact filter has a size of 0.0338λ 2 g , with λ g = 85.18 mm being the guided wavelength at the cutoff frequency. The characteristics of the lowpass filter are verified through simulation and measurement. Consistent and stable results are obtained.

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A compact microstrip lowpass filter with ultra-wide stopband characteristics and high suppression level is presented. To achieve compact size and wide stopband suppression along with improved impedance matching, symmetrically loaded resonant patches, open stubs and stair shaped high impedance stub are introduced in the filter. The measurement results show good agreement with the simulations. The 3 dB cutoff frequency of the filter is 2.44 GHz. The stopband with attenuation level better than 22 dB is extended from 2.84 GHz to 16 GHz, hence an ultra-wide stopband with 6th harmonic suppression is achieved. The proposed filter has low insertion loss and high return loss in the passband, together with compact size of 0.257λ g × 0.148λ g , where λ g is the guided wavelength at cutoff frequency. The relative stop bandwidth of the proposed design is identified to be 139.5%.

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