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An ultra-wideband (UWB) antenna is presented whose frequency characteristics can be reconfigured electronically to have either a single- or dual-band notch function to block interfering signals from C-band satellite communications systems, IEEE802.11a and HIPERLAN/2 WLAN systems for example. The proposed antenna is excited using a circular ring resonator fabricated on one side of the dielectric substrate with a circular slot etched on the ground plane. To realise a single- or dual-band notch characteristic the antenna includes a semi-circular parasitic strip located above the circular ring, and a T-shaped stub is located near the inner edge of circular ring. The parasitic strip and T-shaped stub are connected to the circular ring via PIN diodes to give the antenna reconfigurable capability by configuring the PIN diodes in a prescribed combination of ON and OFF states. It is shown that there is virtually no change in the antenna's E-plane and H-plane radiation patterns for the various PIN diode switching conditions. The proposed antenna's performance has been verified by measurements.