Free RF calculator
Antenna near-field and far-field calculator
Where an antenna's reactive near field ends and its far field begins, from the frequency and the antenna's largest dimension, and which region a point at a given distance is in.
At 40.0 ft (34.61 wavelengths)
Radiating near field (Fresnel)
Past the reactive region but short of the far field. Power density here can be above or below the far-field formula, so an exposure evaluation at this point needs a near-field method or a measurement.
403.0 ft more to reach the far field.
Reactive near field: 0.0 ft to 36.9 ft
Stored energy dominates: E and H are not in phase and power density is not a meaningful quantity. Objects here couple to the antenna and detune it.
Radiating near field (Fresnel): 36.9 ft to 443.0 ft
Radiation dominates but the pattern still changes shape with distance. Gain, pattern and far-field exposure formulas are not yet valid.
Far field (Fraunhofer): 443.0 ft and beyond
The pattern is independent of distance and power density falls as 1/r^2. Where published gain, pattern plots and the OET-65 exposure formulas apply.
Runs in your browser; nothing you enter is sent anywhere. The same math the Sentinel RF product uses, tested against it. A planning figure, not an engineering sign-off.
The math
reactive near field: r < 0.62 x sqrt(D^3 / lambda)
far field: r >= 2 x D^2 / lambda
D is the antenna's largest physical dimension. Both formulas assume an antenna large against the wavelength, so for a small one the same texts set floors: the reactive region never ends closer than a wavelength over 2 pi, and the far field never begins closer than one wavelength (Balanis, Antenna Theory, chapter 2; IEEE Std 145).
Reading the result
2 D^2 / lambda is where the phase error across the aperture falls to 22.5 degrees and the pattern stops changing with distance. Published gain, pattern plots and the far-field exposure formulas in OET Bulletin 65 assume you are past it.
A pattern or gain measurement taken closer than that measures the range, not the antenna, and an exposure estimate closer than that needs a near-field method or a meter.
In Sentinel RF
In the product, RF exposure is summed across every emitter on a shared tower, each against its own limit, because sources that pass alone can be over the line together.
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