What is the near field to far field transition distance and why does it matter for antenna testing?
Antenna Field Regions
The reactive near field is dominated by non-radiating energy storage. The electric and magnetic fields are not in-phase, and the Poynting vector oscillates rather than pointing radially outward. No useful radiation pattern information can be extracted from this region. It extends to approximately λ/(2π) = 0.16λ from the antenna.
The Fresnel (radiating near field) region extends from the reactive boundary to the far-field boundary at 2D²/λ. In this region, the antenna does radiate, but the wavefront curvature across the observation plane is significant. The radiation pattern measured here is a distorted version of the far-field pattern: the main beam is wider, sidelobes are merged, and nulls are not fully formed.
Near-field measurement techniques intentionally measure in the Fresnel region and use mathematical processing (near-field to far-field transformation) to compute the far-field pattern. This is more practical than far-field measurement for large antennas because the measurement distance is only a few wavelengths instead of hundreds of meters.
Frequently Asked Questions
Why can't I measure in the near field directly?
The near-field pattern does not represent the antenna's actual performance. A near-field measurement would show incorrect gain, incorrect beamwidth, and incorrect sidelobe levels. The mathematical transformation (NF-to-FF) corrects for the wavefront curvature to produce accurate far-field data.
What are the three near-field scan types?
Planar scan: probe moves on a flat plane. Best for high-gain antennas (beam fills the scan plane). Cylindrical scan: probe moves on a cylinder. Good for fan-beam antennas. Spherical scan: probe moves on a sphere. Most complete (captures full 3D pattern) but most complex mechanically.
How does CATR work?
A compact antenna test range uses a large shaped reflector (or lens) to convert the spherical wave from a feed into a plane wave, creating a far-field-like test zone in a much shorter distance (typically 10-30m). The test zone must be large enough to encompass the antenna under test with uniform amplitude and phase.