Survey of the Fixed-Geometry Wheeler Cap Method: The Myth of a Simple Antenna Efficiency Measurement Technique

Rigorous experimental estimation of antenna efficiency is challenging and so the simplicity of the fixed-geometry Wheeler cap method has made it an attractive low-cost option. Although experimental results have revealed multiple issues, the method continues to be applied and attracts much research attention well over a half century since it was published. Wheeler’s original proposal was for electrically small antennas, but the method has been expanded and applied to wideband, multiport, and millimeter wave antennas. Ongoing publications have pointed out successes and shortfalls but have seldom been conclusive as to a range of validity for the method. Modern simulation offers insight that cannot be easily found through physical experimentation and is used here to investigate the artifacts of experimental results. This paper offers a survey of the extraordinary research efforts on the Wheeler method and presents new results showing its applicable range to be extremely limited. The antenna must be low-loss and electrically small enough to avoid antenna characteristic modes – in addition to the Wheeler cap being small enough to exclude cavity modes. However, the antenna must also be electrically large enough so that its radiation resistance is sufficient to avert instrumentation measurement errors. Given these constraints and the many potential sources of errors, the method should be considered impracticable for general antennas.