Test distance field strength extrapolation
Extrapolate radiated emission field strength between 3m, 10m and other test distances using the far-field 1/r law.
Formulas
E₂ = E₁ · (d₁ / d₂)
E₂(dBµV/m) = E₁(dBµV/m) + 20·log₁₀(d₁/d₂)
Engineering background
In the far field, radiated field strength decays as 1/r, allowing extrapolation between 3m, 10m and other distances. The rule does not apply in the near field; standards may specify their own distance rules.
Key benefits
Enter a known field strength and two distances to instantly get the field at the target distance.
Based on the far-field 1/r decay law, suitable for 3m/10m limit comparison.
Avoids manual 20·log₁₀(d₁/d₂) logarithm errors.
How to use
- 1 Enter the field strength E₁ at the source distance d₁ (dBµV/m).
- 2 Enter the source distance d₁ and target distance d₂ (in meters).
- 3 Read the extrapolated field strength E₂ at the target distance.
Use cases
- › Extrapolate a 3m semi-anechoic-chamber measurement to the 10m limit for comparison.
- › Convert between limits specified at different test distances.
- › Radiated-emission pre-assessment.
FAQ
Can this formula be used in the near field?
No. Near-field strength does not follow 1/r decay; a more detailed electromagnetic model is required.
How much attenuation from 3m to 10m?
20·log₁₀(3/10) ≈ −10.46 dB, so the field at 10m is about 10.46 dB lower than at 3m.