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How is antenna efficiency measured?

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Antenna radiation efficiency is measured by integrating 3D gain patterns in an anechoic chamber (efficiency = total radiated power/accepted power), by the Wheeler cap method for small antennas, which compares input resistance with and without a conducting enclosure that suppresses radiation, or in a reverberation chamber by comparison with a reference antenna of known efficiency.[1][2][3]

How does the Wheeler cap method work?

A conducting cap or sphere placed around a small antenna, beyond its reactive near field (roughly the radiansphere, λ/2π) but close enough not to resonate, removes radiation without changing loss. For a series-resonant antenna, efficiency η = (Rfree − Rcap)/Rfree.[1] Cavity resonances of the cap and multimode antennas limit the method.

How does a reverberation chamber measure efficiency?

A shielded room with mechanical stirrers creates a statistically uniform field; averaged over stirrer positions, received power is proportional to total efficiency, so comparing the AUT to a reference antenna of known efficiency yields the AUT efficiency. It is fast and accounts for all directions and polarizations.[3]

Which method is most accurate?

Each has uncertainties of a few tenths of a dB to about 1 dB when carefully done. Pattern integration also gives directivity and patterns; Wheeler caps are cheap but limited to small antennas; reverberation chambers excel for small devices and MIMO metrics.[2][3]

Frequently asked questions

Can I measure efficiency with only a VNA?

With a Wheeler cap, approximately, for small antennas.

What is total efficiency?

Radiation efficiency multiplied by mismatch efficiency.

Do reverberation chambers give patterns?

No; they give total, direction-averaged quantities.

References

Online sources accessed October 21, 2026; printed works cited by edition. See all sources.

  1. “The Radiansphere Around a Small Antenna”. H. A. Wheeler, Proceedings of the IRE, vol. 47, no. 8, pp. 1325-1331. August 1959.
  2. “IEEE Recommended Practice for Antenna Measurements (IEEE Std 149-2021)”. IEEE Antennas and Propagation Society. 2022.
  3. “Foundations of Antenna Engineering: A Unified Approach for Line-of-Sight and Multipath”. P.-S. Kildal, Artech House. 2015.
Categories: Measurement