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This paper describes a method to improve the cutoff capability of an ultra wideband (UWB) planar antenna at the out-of-band frequencies using a meandered slot. In the presented UWB design, aimed for operation between 3.1 and 10.6 GHz, the antenna is formed by a planar monopole and a ground plane both of half circle shape, with a meandered-shape slot made in the monopole. In order to assess the antenna's cutoff capability for the out-of-band frequency range (below 3.1 GHz and above 10.6 GHz), two antennas, without and with a slot, are designed and developed. The simulated and measured results show that the meandered slot improves the cutoff capability of the antenna by decreasing the return loss in the 23 GHz band from 610 to 1 dB, and in the 10.611 GHz from 11 to 5.5 dB. At the same time the antenna's performance in the passband is unaffected both in terms of the return loss and radiation pattern. It is also shown that the antenna is useful for the pulsed UWB applications as the measured impulse response of the antenna indicates a distortionless pulse transmission.