Distance Extrapolation Calculator
Convert radiated emission limits and measurements between different test distances (e.g., 3m, 10m, 30m) using standard inverse distance laws.
1. Test Distances
2. Field Strength (dBµV/m) Factor: 10.46 dB
Extrapolation Formula
- N = 20: Standard free-space propagation (Inverse distance law). Used by CISPR and FCC for frequencies above 30 MHz.
- N = 40: Near-field magnetic propagation. Commonly mandated by FCC Part 15 for frequencies below 30 MHz.
Common Extrapolation Factors
| Conversion | Factor (20 dB/dec) | Factor (40 dB/dec) |
|---|---|---|
| 10m to 3m (Limits) | + 10.46 dB | + 20.92 dB |
| 3m to 10m (Limits) | - 10.46 dB | - 20.92 dB |
| 30m to 3m (Limits) | + 20.00 dB | + 40.00 dB |
*Note: When converting a LIMIT from a farther distance to a closer distance, the limit value increases (you add the factor).
*Engineering Approximation vs. Physical Reality: This tool follows the engineering approximation rules mandated by standards like FCC/CISPR (e.g., 20 dB/dec for >30MHz, 40 dB/dec for <30MHz). In real-world physical environments, especially in the near-field or transition regions (where low-frequency magnetic fields may actually decay at 60 dB/dec), the actual field strength decay rate may deviate from these theoretical extrapolations.
Calculator quality evidence
Purpose of this calculator
Use this calculator for a transparent first-order translation of a field-strength reading between two distances, especially the common 3 m and 10 m EMC test geometries.
Formula and variables
For a selected slope N in dB/decade: L2 = L1 + N·log10(d1/d2). The tool exposes N = 20 or 40 dB/decade and applies the reverse relation when the second level is edited.
Physical assumptions and scope
The 20 dB/decade option represents an ideal 1/r far-field trend; 40 dB/decade can represent a different specified approximation. Test-site geometry, antenna height, ground plane, loading, and regulatory method can override this simple slope.
Valid input scope
Both distances must be finite and greater than zero; levels may be any finite dBµV/m value. Use the result as an extrapolation aid only, never as a substitute for the specified compliance distance and test method.
Independently checked test vectors
- 40 dBµV/m @ 10 m, N=20 → 50.46 dBµV/m @ 3 m(forward)
- 50.46 dBµV/m @ 3 m, N=20 → 40 dBµV/m @ 10 m(reverse)
Conversion-specific misuse to avoid
Do not use an extrapolated 3 m or 10 m value as proof of compliance when the standard requires a different site, frequency range, detector, or measurement setup. Select the slope from the applicable method, not from the desired margin.
Source and content review
- 47 CFR Part 15: radiated emission distance extrapolationSupports the regulated context in which 20 dB/decade and 40 dB/decade distance extrapolations appear, while the applicable test method remains controlling.
Content review date：