This paper proposes a simple technique for measuring the radiation efficiency of broadband antennas by using a tunable spherical Wheeler cap made of brass with an inner diameter of 70 mm. The proposed method utilizes a multimode tuner designed as an adjustable brass stub with a ring to compensate the undesirable reductions of the measured efficiency caused by both TM and TE mode resonances of the cap. The required tuning levels for minimizing the undesirable reduction will depend on the Q factors of the cap resonances. The experimental results of a UWB slot antenna, performed at the frequency range of 3 to 10 GHz, show that the efficiency reduction can be minimized by enveloping over all maxima of efficiency curves measured for five different tuner positions.
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