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This paper presents a planar antenna for employment in overlay and underlay ultra-wideband cognitive radio (UWB-CR). The antenna can be operated as ultra-wideband, for underlay CR and for sensing in the overlay CR. When used in the overlay CR mode, the proposed antenna can selectively have one, two or three notches in the WLAN, WiMAX, and U-NII bands, to prevent interference to primary users operating in these bands. The band notches are induced using complementary split-ring resonators (CSRRs), and are controlled using three electronic switches mounted over the CSRRs.