Near field versus far field is not a formality in a textbook. Close to a source, the electric and magnetic fields are largely independent, store energy rather than carry it away, and fall off far faster than distance alone would suggest. A probe reading taken there cannot be scaled outward to predict what a receiver will hear across the street.
Two regions with different rules
Very near a radiating structure, the field is dominated by reactive components tied to the charge and current distribution on the conductor. Electric and magnetic components are not locked in a fixed ratio, the wave impedance depends on whether the source is a voltage-driven or current-driven structure, and the amplitudes fall with the square or even the cube of distance. Farther out, those reactive terms have decayed and what remains is a propagating wave in which electric and magnetic fields are perpendicular, in phase, and in a fixed ratio set by the impedance of free space. There the amplitude falls inversely with distance and the pattern shape stops changing. Almost everything intuitive about radio applies only to that second region.
Where the boundary sits depends on the antenna
For a small source, the transition is commonly placed around a sixth of a wavelength, so at a hundred megahertz it is roughly half a meter and at two and a half gigahertz it is a couple of centimeters. For an electrically large antenna such as a dish or a long array, the relevant criterion instead depends on the aperture size squared divided by the wavelength, which can push the far field hundreds of meters away. This is why testing a large antenna properly requires a long range, a compact range with a shaped reflector, or a near-field scanner with mathematical transformation. It also explains a common confusion: a dish can be in its own near field at a distance where a small whip at the same frequency would be comfortably in the far field.
Near-field probes are comparative tools
A small loop or stub probe held over a circuit board is invaluable for troubleshooting because it is local: it tells you which trace, which component, which seam owns a particular harmonic. It is useless for prediction because its coupling depends on orientation, height and geometry in ways that no calibration factor captures. Used correctly, the workflow is relative. Find the peak in a chamber or on a spectrum analyzer, then hunt with the probe for the spot on the board where that frequency is loudest, make a change, and see whether the probe reading at that spot drops. Then re-measure at distance to confirm the change actually helped. Treating a probe number as an emissions level is the standard beginner error.
Exposure assessment uses different quantities near and far
Radio frequency exposure limits are expressed in terms that suit the region involved. In the far field, where the fields are locked together, a single power density figure describes the situation adequately. Close to a transmitting antenna, the electric and magnetic fields must be evaluated separately because their ratio is not fixed, and for devices operated against the body the relevant quantity is the rate at which energy is absorbed in tissue rather than a field strength in air. This is why handset compliance is assessed with a body-worn absorption measurement while a broadcast tower site is assessed with field surveys at distance. The FCC publishes the applicable limits and explains how they are derived and applied to different categories of equipment.
Practical consequences for installations
Several everyday observations follow directly. A receiver antenna placed very close to a transmitting one couples through reactive fields that no free-space path calculation predicts, which is why isolation between co-located antennas is measured rather than computed from separation. Handheld devices detune noticeably when gripped, because a hand sits squarely in the reactive region and loads it. Antenna measurements made in a small room are contaminated by reflections and, if the room is short relative to the far-field distance, by near-field effects as well. When a result has to be trusted, either provide the distance the antenna requires or use a facility designed to compensate for not having it.
What to check before you act
- Work out the far-field distance for your antenna size and frequency.
- Use near-field probes for locating sources, never for predicting levels.
- Measure isolation between co-located antennas rather than calculating it.
- Follow the applicable exposure assessment method for the region involved.
Common questions
Does the near field extend further at lower frequencies?
For small sources, yes, because the boundary scales with wavelength. For large apertures the aperture dimension dominates instead.
Is near-field energy wasted?
Not exactly. Reactive fields store energy that returns to the source each cycle. They contribute to coupling and detuning rather than to radiation.
The practical takeaway
Know which region you are measuring in before you interpret a number. Use close-in probing to find sources, use proper distance or a designed facility to quantify radiation, and follow the published assessment method when the question is about exposure rather than performance.
References and further reading
- FCC: radio frequency safety questions including field regions and measurement
- FCC: radio frequency energy and exposure evaluation overview
- FCC: wireless devices, exposure limits and consumer questions
Numerical scenarios are illustrative unless identified otherwise. Follow the exact product instructions; component ratings and local installation requirements can differ.
