In this paper, we present a new frequency reconfigurable antenna for cognitive radios that can operate in both Ultra Wide Band (UWB) mode and in narrowband mode where the operating frequency can be easily tuned. The antenna is composed of a disc monopole that can be excited to operate at the range of hundreds of MHz for the narrowband mode and at the range 1 to 10 GHz in the UWB mode. The total antenna dimensions are only 50 by 55 mm, which makes a good candidate for compact cognitive radios. To present the work, simulated and measured results will be presented and discussed.