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.

Reference only. Verify against your calibrated setup, antenna calibration data, and the applicable standard before making compliance decisions.

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.

Distance d₁

Distance d₂

Field at d₁

dBµV/m

Field at d₂

dBµV/m

Field at d₁

V/m

Field at d₂

V/m

Why EMC tests change with distance

CISPR Class B radiated limits are typically specified at 3 m; Class A often at 10 m. Chambers and OATS do not always match the limit distance. The usual free-space engineering estimate is 20 dB per decade of distance: a 3 m reading is 10.5 dB higher than the same source at 10 m if the 1/r assumption holds.

Use this converter to normalize a plot before you panic about a Class A/B mismatch. Then remember the caveats: near-field coupling, ground-plane reflection, and chamber validation can move the real delta away from 20 dB/decade.

Frequently asked questions

Quick answers about using this calculator

What is the 20 dB/decade rule?

In the far-field 1/r region, field strength falls 20 dB when distance increases by 10×. From 3 m to 10 m the factor is 20 × log₁₀(10/3) ≈ 10.5 dB.

Can I use this to pass a failed 3 m test?

No. It is an estimate for planning and comparison. The accredited method names the distance. If you measured at 3 m, report 3 m unless the standard allows extrapolation.

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