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Tri-band bandpass filters based on a dual-plane structure with improved band allocation is proposed in this letter. The dual-plane structure is consisting of both a microstrip and a defeated ground structure (DGS) slot. The slot with interdigital shape in the DGS is designed to obtain dual-frequency responses and transmission zeros are introduced to split one of the single bands into two. Two design examples with different frequency responses and band allocations are presented. All results obtained from the equivalent-circuit model and full-wave simulation are conformed by measurement.