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Design concept of a novel combined antenna for cognitive radio applications is proposed and numerically verified. The antenna is composed of a wideband circular monopole and a narrowband dipole. The impedance bandwidth of the wideband monopole antenna is from 4.25-10GHz. The narrowband dipole antenna's central frequency is 5.8GHz. The isolation between the two elements is higher than 50dB, which indicates that the two elements independently co-exist within a small printed circuit board (PCB). The design concept proposed in this paper should be useful to the design and realization of compact, planar antennas and antenna modules having spectrum sensing function.