A modified Wheeler cap method for radiation efficiency measurement of balanced electrically small antennas

Measurement of radiation efficiency for ultra small antennas represents a great challenge due to influence of the feeding cable. The Wheeler cap method is often used to measure the radiation efficiency of small antennas. However, it is well applicable for antennas on a ground plane, but not for balanced antennas like loops or dipoles. In this paper, a modified Wheeler cap method is proposed for the radiation efficiency measurement of balanced electrically small antennas and a three-port network model of the Wheeler cap measurement is introduced. The advantage of the modified method is that it is wideband, thus does not require any balun, and both the antenna input impedance and radiation efficiency can be obtained. An electrically small loop antenna and a wideband dipole were simulated and measured according to the proposed method and the results of measurements and simulations are presented. The obtained results are also compared with the results from a spherical near-field measurements in an anechoic chamber and a good agreement is observed.