Post-processing accuracy enhancement of the improved Wheeler cap for wideband antenna efficiency measurements

Accurate efficiency characterization by way of the Wheeler cap, especially in a wide band, relies heavily on the post-processing method (PPM) used to extract efficiency values from raw measured data. One of the best established PPMs is the Improved Wheeler Cap (IWC), which treats the antenna-under-test (AUT) as a two-port network and requires circle-fitting of measured data. Its accuracy is affected by the way the data are positioned on the Smith chart. This paper revisits the IWC and enhances its accuracy, while maintaining a wideband measurement capacity. The modified IWC is demonstrated by broadband measurements on a small antenna, and the results are compared against the current state of IWC and three other Wheeler-cap- related PPMs: McKinzie's rotation, the complementary Q-factor method and the equivalent-circuit Wheeler cap.