EMC Question of the Week: September 8, 2025

Two devices in an EMI test setup contribute to radiated emissions at 85 MHz. One produces a radiated field strength of 40 dB(μV/m) at the measurement location. The other produces a radiated field strength of 34 dB(μV/m) at the same location. What is the measured field strength from both sources?  

1. 41 dB(μV/m)
2. 43.5 dB(μV/m)
3. 71 dB(μV/m)
4. 74 dB(μV/m)

Answer

The best answer is “a.” The field strength from the first device is 40 dB(μV/m) = 100 μV/m. The field strength from the second device is 34 dB(μV/m) = 50 μV/m. The sources are independent (i.e., they do not have a fixed phase difference), so the test receiver measures the sum of their power densities. The measured field strength is the rms sum of the field strengths, 

$\sqrt{{\left(100\text{μV/m}\right)}^2+{\left(50\text{μV/m}\right)}^2}=112\text{μV/m}\text{or}\text{41}\text{dB(μV/m)}\text{.}$

Note that if the sources had been correlated (e.g., two signals derived from the same clock), they might have added in-phase, out-of-phase, or anything in between. In this case, the resulting emissions could have been anywhere from 0 to 150 μV/m.

If both sources were highly directional with different angles of maximum radiation (unlikely at 85 MHz), the measured field strength would be somewhere between 40 dB(μV/m) and 41 dB(μV/m).

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