Measurement Distance Extrapolation
Extrapolate radiated field strength between measurement distances using the 20 dB/decade free-space rule.
Use this when: You are converting 3 m CISPR measurements to 10 m Class A limits, or normalizing site results.
Key formula
E₂ (dBµV/m) = E₁ (dBµV/m) + 20 × log₁₀(d₁/d₂)Example: 40 dBµV/m at 3 m → 29.54 dBµV/m at 10 m (−10.46 dB)
Assumes far field free space conditions. Results may differ in real test environments with reflections and ground planes.
Related: CISPR / FCC emission limit checker · Radiated field calculator
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