EMC antenna distance tool

Antenna near-field and far-field region plotter

Enter the known antenna aperture and frequency range. The plot shows the reactive near-field boundary, the Fraunhofer far-field boundary, and where your selected test distances sit.

Known Antenna Values

Aperture width, mm Aperture height, mm Start frequency, GHz End frequency, GHz Main test distance, m Compare distance, m Plot maximum distance, m Frequency sample points

Use largest aperture dimension Use aperture diagonal

Popular antenna presets

For DO-160-style justification, the conservative value is normally the largest transmitting aperture dimension. Presets are convenient starting points; use the actual antenna datasheet or calibration record for a formal test justification.

Formulas used
Wavelength: lambda = c / f
Conservative reactive boundary: R = max(lambda / 2pi, 0.62 sqrt(D^3 / lambda))
Conservative EMC far-field boundary: R = max(3 lambda, 2D^2 / lambda)
Maximum aperture for selected distance: Dmax = sqrt(R lambda / 2)

Region Plot

Reactive near field Radiating near field / Fresnel Fraunhofer far field Main distance Compare distance

Check	Frequency	Wavelength	Reactive boundary	Far-field boundary	At main distance

Reactive Near Field

The closest region to the antenna. Stored electric and magnetic energy is significant, the E/H relationship is not yet plane-wave-like, and small position changes can strongly affect measured level.

Fresnel / Radiating Near Field

The field is radiating, but the wavefront still has meaningful curvature and the antenna pattern is still forming. This is the awkward middle ground for EMC justification unless the standard, customer, or field-uniformity evidence supports it.

Fraunhofer Far Field

The antenna pattern is effectively formed, the field behaves more like a plane wave, and distance scaling is more predictable. This page uses the conservative envelope R = max(3 lambda, 2D^2 / lambda).

Being outside the calculated Fraunhofer distance does not automatically mean a test is impossible, but it does mean the simple far-field argument is weaker. For a compliance test, support a closer distance with the standard wording, antenna manufacturer data, measured field uniformity, or an agreed deviation.

Log-periodic note
The 2D^2 / lambda aperture expression is less tidy for log-periodics than for horns. A log-periodic has an active region that moves along the boom with frequency, so this tool also applies a conservative 3 lambda floor. For formal EMC work the better evidence remains the antenna manual/calibration data, the standard's required separation, and measured field uniformity.

References