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In this paper, we describe the operation of a low profile, 3 mm deep, circular disk antenna which requires a small ground plane, and incorporates a shorting bar which can be rotated. The position of the shorting bar determines the return loss and radiation characteristics of the antenna element. In one mode at 2.34 GHz the antenna exhibits a return loss of better than -25 dB, 5 dBi gain, and a conical/monopole beam pattern suitable for radar tracking/mobile communications applications. When the short circuit is positioned at 90 degrees the return loss at 3.52 GHz becomes -18 dB, 7 dBi gain, and the antenna exhibits normal linear polarized broadside cosine radiation characteristics. In addition when the short circuit is positioned at zero degrees at 3.52 GHz the antenna gives -1 dB return loss and by reciprocity will act as a reflector to incoming energy. Theoretical results are provided in support of the experimental results observed and are used to explain the operating mechanisms of the antenna.