A novel bandpass filter with a wide stopband is presented. The proposed filter is based on a dual-plane structure consisting of a microstrip and a defected ground structure (DGS) slot with an interdigital shape. Using this structure, the filter not only creates two transmission zeros near the passband, but also exhibits a wide upper stopband. The equivalent-circuit model is derived for the filter. A filter centred at 2.5 GHz with 14.8% fractional bandwidth is designed and implemented, which shows a minimum passband insertion loss of 1.78 dB and a wide stopband with rejection better than 22 dB up to 12f0, where f0 stands for the passband centre frequency.
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Ren et al. (2009) studied this question.