In this paper, a novel compact coplanar waveguide (CPW) ultra-wideband (UWB) antenna with an electronically reconfigurable band is developed for interweave cognitive radio (CR) system and future software defined radio (SDR). The center frequency of the tunable band can be electronically altered by adjusting the electrical length of the microstrip resonator at the bottom layer, which is accomplished by incorporating two varactor diodes at the resonator edges. Simulation results show that the proposed design is capable of tuning over a continuous wide operating band of 1.15 GHz covering a wide band for interweave CR communication from 4.54 to 5.69 GHz. Furthermore, the radiation patterns are steady over the interesting frequency bands. The proposed antenna is fabricated and measured at a fixed capacitance for verification purposes. Good agreement is found between the simulated and the measured results.