Cable Common-Mode Current Radiated Field Estimate
Estimate the maximum radiated field of a cable from frequency, cable length, common-mode current and distance.
Worked example
Input: Frequency 50 MHz, Cable length L 0.5 m, Common-mode current I 5 µA, Measurement distance r 10 m
Result: Maximum radiated field E 15.708 µV/m, Maximum radiated field E 23.9224 dBµV/m, Wavelength λ 5.99585 m, Cable length / λ 0.083391
Result: Maximum radiated field E 15.708 µV/m, Maximum radiated field E 23.9224 dBµV/m, Wavelength λ 5.99585 m, Cable length / λ 0.083391
Formula
E = 1.257×10⁻⁶ · f · L · I / r (f[Hz], L[m], I[A], r[m])
Conditions: L < λ/10, r > λ/(2π), includes ×2 ground-plane reflection (maximum)
In free space: 6.28×10⁻⁷ (−6 dB)
How it works
Treating the cable as a short antenna (L < λ/10), the maximum field over a ground plane is E = 1.257×10⁻⁶·f·L·I/r (f in Hz, L in m, I in A, r in m). It is the free-space value η₀/(2c) = 6.28×10⁻⁷ times 2 for ground-plane reflection. With the example values (50 MHz, 0.5 m, 5 µA, 10 m) the result is about 15.7 µV/m, or 23.9 dBµV/m. A few microamps of common-mode current can already approach the limit.
Practical tipAbove L = λ/10 this formula no longer holds, so check that the L/λ shown is 0.1 or less. The most accurate way to get the common-mode current is to measure it with a current probe. Typical fixes are ferrite cores, 360° termination of the cable shield and common-mode chokes.
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Results and summaries are for reference. For certification and test reports use the latest official standard text and calibrated instrument data. Last updated: 2026-10-10