On the capacity of Wheeler Cap measurements for detecting very low antenna efficiency levels

The Wheeler Cap is the go-to solution for fast, broadband and accurate characterization of the radiation efficiency of compact antennas. Its ability to accurately measure high and medium levels of efficiency is well-documented; however, its capacity for detecting low levels of radiated power remained unknown. Starting from a highly-efficient, canonical antenna, this experimental study added increasing losses by means of precision fixed attenuators, thus creating a radiator with progressively lower, but tightly-controlled, efficiency. The recorded pairs of free-space and shielded measurements were treated by two independent post-processors. The results revealed that the Wheeler cap is capable of measuring very low efficiency levels at least down to −20 dB, or 1%; the worst case deviation from actual efficiency was 0.3 dB. Hence, the Wheeler cap becomes suitable for less common efficiency measurement scenarios, e.g., antennas immersed in lossy and dispersive materials.