Fast Broadband Antenna Efficiency Measurements: The Complementary Q-Factor Method Revisited

Abstract:
A crucial factor in obtaining accurate efficiency characterization by way of the Wheeler cap is the post-processing method (PPM) that is employed to extract efficiency values from raw measured data. The complementary Q-factor method (CQFM) is the fastest amongst the six PPMs that are currently available. CQFM is an end-to-end methodology for broadband efficiency measurements, which exploits inherently wideband Q-calculation formulas that are based on frequency derivatives of antenna input impedance and on the concept of matched VSWR bandwidth. CQFM is useful both for narrow- and wide-band antennas, regardless of whether the latter exhibit closely or widely spaced multiple (anti-)resonances. Experimental data indicated that the CQFM systematically over-estimates the radiation efficiency of the antenna-under-test (AUT). Two possible reasons theoretically exist: high losses in the materials, and inaccurate calculation of the AUT Q-factor inside the Wheeler cavity. This paper proves that even a mild distortion of the near field of the AUT by the cavity produces an artificially larger capped Q-factor.