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

Microstrip antennas on 3D-printed magneto-dielectric substrates: fabrication and efficiency study

“…The efficiency of the five MDAs was measured by means of the Wheeler cap [24][25][26], a repeatable and accurate differential method that calculates efficiency indirectly by comparing two different states of the antenna under test (AUT): (a) radiating in free space, and (b) placed inside a Faraday shield to choke radiation. More information on this method can be found in the Appendix.…”

“…The raw S-parameter measurements of the free-space and capped states of the antenna tell very little about η rad ; a postprocessing of raw data is in order [24][25][26]. Two fundamentally different post-processors were applied in this study.…”

“…The reason is that antennas tend to store more energy in their near fields when mounted inside the cap. The second postprocessor is a modified version of the frequency-swept variant of the improved Wheeler cap (IWC) [25,26]. After years of experience with the IWC, we have empirically concluded that it is a slightly pessimistic post-processor, i.e.…”

“…it tends to underestimate efficiency. Further details on the two post-processors can be found in [24][25][26] and the pioneering references therein. A common property of modern Wheeler cap post-processors is that the AUT is required to be neither matched, nor tuned in the band of interest.…”

“…This is the very foundation of broadband η rad measurements. Not only is the modern Wheeler cap indifferent toward AUT impedance, it can also detect very small η rad levels [26].…”