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A technique for enhancement of bandwidth and efficiency of a cavity-backed slot antenna is proposed. The bandwidth of the cavity-backed slot antenna depends on the Q of the slot and the cavity. The proposed technique removes the substrate under the slot to decrease the capacitance of the slot. Because a half-wavelength slot is considered a parallel resonant circuit at resonant frequency, lowered capacitance increases the bandwidth of the antenna. Antenna efficiency also can be enhanced by the proposed technique. The dielectric loss, which is produced by the E-field across the slot, is effectively decreased by removing the substrate under the slot. Various simulation results of demonstration of the proposed technique are given. The proposed antenna, which was fabricated on a 2-mm-height FR-4 substrate, shows 6.2% higher antenna efficiency and 24% wider bandwidth compared to the conventional cavity-backed slot antenna, which has a whole substrate. The proposed technique is effective in enhancing the efficiency and bandwidth of a cavity-backed slot antenna.