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A new method to enhance the isolation between two ports in a dual-frequency proximity coupled patch antenna is presented. The dual-polarized patch antenna has a multilayer substrate configuration to achieve a compact design. In addition, two periodic structures have been included under the two feeding lines to have stopband behavior in the other frequency port. These electromagnetic bandgap (EBG) filters have been realized by introducing mushroom type resonators underneath the antenna feeding lines. Measurements show how the isolation between the feeding ports can be improved by selecting a suitable size for the resonators. The proposed antenna design achieves an isolation higher than 55 dB in the first frequency band (2.1 GHz) and better than 40 dB in the second one (2.45 GHz). Radiation patterns are not affected by the proposed structures showing low cross-polarization levels in both planes at the two frequency bands.