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A novel multi-mode frequency reconfigurable MIMO and an ultrawideband (UWB) MIMO antenna design for cognitive radio applications is proposed in this paper. The antenna structure contains two modified triangular-shaped printed monopole antenna elements and a combination of varactor and PIN diodes. The proposed antenna has three operating modes: UWB mode for spectrum sensing, a frequency reconfigurable MIMO mode for communication over different frequencies, and UWB antenna operating in MIMO configuration. The UWB mode and reconfigurable communication mode are obtained by utilizing different switching ON/OFF states of PIN and varactor diodes. In the UWB mode, the antenna can cover the spectrum from 1 to 4.5 GHz, while it achieves the frequency reconfigurability over a wide range from 0.9 to 2.6 GHz using varactor diode tuning in the reconfigurable communication mode. The proposed antenna has a compact and planar structure with overall dimensions of $120\times 60\times1.5$ mm3. The proposed design also exhibits good MIMO performance; the minimum isolation measured is 12.5 dB, while the envelope correlation coefficient (ECC) of less than 0.19 is achieved for the whole operating band. To validate the proposed concept, the prototype of the antenna system is fabricated and measured. The simulation results are in good agreement with the measurement results.

A Reconfigurable MIMO/UWB MIMO Antenna for Cognitive Radio Applications