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This paper describes a modified planar monopole antenna able to cover a broad spectrum, well beyond the requirements of IEEE's 802.15.4 standard or FCC's UWB regulation. Our design in based on the well-known circular planar monopole and in order to minimize the return loss, an impedance adapter is used in the microstrip feed line and slits are added to the monopole antenna. Simulation results show that our modified circular monopole improves the available bandwidth as well as the return loss of the antenna. Given its extremely large bandwidth, the antenna is suitable for spectrum sensing applications for future cognitive radio systems.