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A rectangular aperture of Ax×Ay, cut in the top conducting plate of a triplate transmission line and backed by a cavity, radiates a tilted beam off the direction normal to the aperture. The mechanism of the radiation is explained using the Poynting vector distribution above the aperture and the phase distribution of the electric field over the aperture. The tilt angle is calculated as a function of side length Ax for a representative value of Ay=18 mm=0.747λ12.45, where λ12.45 is the wavelength at a test frequency of 12.45 GHz. A tilted beam of approximately 27° is realized at Ax/Ay=8/9 with a gain of approximately 8 dB. Using this value of Ax/Ay, an array antenna composed of rectangular cavity-backed aperture elements is investigated. The array forms a tilted fan beam without phase shifters. The frequency responses of the gain and input impedance are discussed.