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
meters
meters
2. Field Strength (dBµV/m) Factor: 10.46 dB
Extrapolation Formula
Level (D₂) = Level (D₁) + N * log₁₀(D₁ / D₂)
- 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.