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Conventional antenna radiation efficiency measurement techniques, such as the Wheeler cap, are generally narrowband and, thus, well suited for resonant antennas. In this work, the Johnston-Geissler (J-G) method based on the Wheeler cap principle is used in a wide frequency range. First, a good agreement between measurement and simulation in an ultrawide frequency band is shown on a straight wire monopole antenna with different attenuators emulating antenna losses. Then, this technique is used to measure the radiation efficiency of several ultrawideband (UWB) antennas.