This paper presents a study on the radiation efficiency of Ultra-Wideband (UWB) antennas. Measurement were made using a cavity-based method with source stirring. A novel post-processing filtering technique of the measured data is introduced. The approach was tested on a coplanar-line-fed UWB monopole. The efficiency of this antenna was also computed with two commercial electromagnetic packages. Simulations and measurement agree to predict a radiation efficiency of over 95% through the 3.1-10.6 GHz operating bandwidth.