In this paper, three cavity-backed slot antennas utilizing the substrate-integrated waveguide (SIW) technology are proposed. They are single-slot, two-slot, and 2×2-slot antenna structures. Their performance is numerically evaluated by means of the method of finite elements. Using two slots backed by a circular SIW cavity, one can improve antenna gain by 2.3 dB over the gain of the single-slot antenna at the cost of a slight increase of the antenna dimensions. The maximum gain of the circular cavity backed two-slot antenna is 7.7 dB at 9.59 GHz with a broadside radiation pattern and with the desired input matching. For a 2×2-slot antenna, a gain of 10 dB at 9.58 GHz is achieved. This is 4.6 dB and 2.3 dB higher than the gain of the single and two-slot antennas, respectively. We show that its radiation pattern has low side lobe and back radiation. All the proposed antenna structures are made of a single-layer substrate and are fed by a 50Ω microstrip line. These antenna structures are compact and low-cost and have high radiation efficiency.

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