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A novel synthesis technique to integrate high-Q 3-D filters with highly efficient slot antennas is presented in this paper. This technique allows for compact integration of 3-D filters and antennas with very high antenna efficiency and significantly reduced form factor of integrated RF front ends. Prototype four-pole Chebyshev cavity filters integrated with slot antennas are demonstrated at X-band using both coaxial and coplanar waveguide feeding. The center frequency and fractional bandwidth of the filter/antenna system with coaxial feeding are 9.96 GHz and 6.0%, respectively. Due to the high-Q factor (~850) of the cavity resonator, the efficiency of this filter/antenna system is measured to be 89%, compared with the measured S21 of -0.5 dB (89%) for an identical filter. This means a near 100% efficient slot antenna is achieved within this integrated filter/antenna system. The measured impedance matching, efficiency, gain, and radiation pattern closely agree with simulation results. Equivalent-circuit models of the integrated filter/antenna system are developed and verified with full-wave simulations. This technique can be applied for filter/antenna integration in all microwave, millimeter-wave, and submillimeter-wave frequency regions.