On Wheeler's method for efficiency measurement of small antennas

The Wheeler cap method is commonly used to measure the efficiency of small antennas. This method is based on the assumption that the spherical cap does not essentially disturb the near fields surrounding the antenna. Although this method has been proved in experiments, the validity of this assumption has not been shown rigorously via field analysis. It is demonstrated, by dimensional and scaling analysis, that the power loss of a small antenna in a Wheeler cap, as well as in a certain variety of conducting cavities, is essentially the same as that of the antenna placed in free space. Consequently, one can measure the loss resistance with the cap in place.