R
Member
Measuring antenna efficiency on a small lab budget is one of those tasks that looks simple until you try it. We needed efficiency numbers for a 2.4 GHz IoT antenna, no anechoic chamber in sight. I tried two approaches and here's what I learned.
Wheeler cap method: the idea is clean — measure input impedance in free space and inside a conducting cap, efficiency falls out of the resistance ratio. In practice:
- The cap has to be electrically small but not too close; I used a cylindrical can roughly λ/2π radius at the test frequency. Too tight and the cap itself detunes the antenna.
- Contact and seam losses in the cap matter. I lined the seams with copper tape and re-measured until the numbers stopped moving.
- It only gives you total efficiency (mismatch included), so pair it with a good S11 measurement and de-embed mismatch if you want radiation efficiency alone.
Reverberation chamber sanity check: a colleague had access to a small reverberation setup. Results agreed with the Wheeler cap within about 0.5 dB, which gave me confidence — but the chamber needed careful stirring and averaging, and below its lowest usable frequency the numbers were garbage.
Takeaway: for sub-6 GHz small antennas, a well-built Wheeler cap gets you within half a dB of chamber results at nearly zero cost. The failure mode is almost always the cap construction, not the theory.
What's your go-to method when there's no chamber available?
Wheeler cap method: the idea is clean — measure input impedance in free space and inside a conducting cap, efficiency falls out of the resistance ratio. In practice:
- The cap has to be electrically small but not too close; I used a cylindrical can roughly λ/2π radius at the test frequency. Too tight and the cap itself detunes the antenna.
- Contact and seam losses in the cap matter. I lined the seams with copper tape and re-measured until the numbers stopped moving.
- It only gives you total efficiency (mismatch included), so pair it with a good S11 measurement and de-embed mismatch if you want radiation efficiency alone.
Reverberation chamber sanity check: a colleague had access to a small reverberation setup. Results agreed with the Wheeler cap within about 0.5 dB, which gave me confidence — but the chamber needed careful stirring and averaging, and below its lowest usable frequency the numbers were garbage.
Takeaway: for sub-6 GHz small antennas, a well-built Wheeler cap gets you within half a dB of chamber results at nearly zero cost. The failure mode is almost always the cap construction, not the theory.
What's your go-to method when there's no chamber available?
