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A seamless integration of 3-D vertical cavity filters with highly efficient slot antennas is presented herein. This integration technique enables low-loss filtering and reduces co-site interference within phased arrays. A vertical two-pole cavity filter integrated with a slot antenna is demonstrated at X band. The center frequency and fractional bandwidth of the filter/antenna system are 10.16 GHz and 3.0%, respectively. Due to the near-zero transition loss achieved by this seamless integration, the efficiency of the integrated slot antenna is shown to be as high as 97%. Equivalent circuit models are developed to identify the losses in the filter/antenna, which are verified by full-wave simulations. The measured impedance matching and radiation patterns closely agree with simulation results. This technique can be extended for higher-order filters seamlessly integrated with slot antennas. In addition, this technique can be applied in all microwave, millimeter-wave, and submillimeter-wave frequency regions.