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The radiation of cavity-backed slot (CBS) antennas through a two-layer superstrate is studied in this paper. A novel transmission-line model is proposed to explain the gain-enhancement effect of the superstrate on cavity-backed slot antennas. Experimental result showed that a single cavity-backed slot antenna can achieve a high gain of 16.7 dBi through a properly designed superstrate, and this gain-enhancement property is valid for array configurations, as well. A rotationally arranged four-slot array, covered by the superstrate, is also investigated, to demonstrate that good circular polarization and high gain can be obtained from the superstrate-covered slot antennas.