PCB Loop (Differential-Mode) Radiated Field Estimate
Estimate the maximum radiated field from a PCB differential-mode loop from frequency, loop area, loop current and distance.
Worked example
Input: Frequency 100 MHz, Loop area A 10 cm², Loop current I 10 mA, Measurement distance r 3 m
Result: Maximum radiated field E 438.953 µV/m, Maximum radiated field E 52.8484 dBµV/m, Wavelength λ 2.99792 m, Far-field boundary λ/(2π) 0.477135 m, Loop perimeter (square assumed) / λ 0.0421929
Result: Maximum radiated field E 438.953 µV/m, Maximum radiated field E 52.8484 dBµV/m, Wavelength λ 2.99792 m, Far-field boundary λ/(2π) 0.477135 m, Loop perimeter (square assumed) / λ 0.0421929
Formula
E = 1.316×10⁻¹⁴ · f² · A · I / r (f[Hz], A[m²], I[A], r[m])
Conditions: perimeter < λ/10, r > λ/(2π), free-space maximum direction
Over a ground plane, reflection can raise it by up to 2× (+6 dB)
How it works
For an electrically small loop (perimeter below λ/10), the free-space maximum field is E = 1.316×10⁻¹⁴·f²·A·I/r (f in Hz, A in m², I in A, r in m). The constant comes from η₀·π/c². Because it scales with f², doubling the frequency adds 12 dB. With the example values (100 MHz, 10 cm², 10 mA, 3 m) the result is about 439 µV/m, or 52.8 dBµV/m.
Practical tipThis is an idealized estimate for the strongest direction in the far field (r > λ/2π). Over a ground plane reflection can add up to +6 dB, and the loop current and area are usually estimates themselves. Use it for relative comparisons such as "half the area is −6 dB", not for compliance decisions; confirm with measurement. The most reliable fix is to shrink the loop by routing the signal and its return close together.
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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