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.
| Parameter | Low Gain | Medium Gain | High Gain |
|---|---|---|---|
| Gain Range | 2-6 dBi | 6-15 dBi | 15-45 dBi |
| Beamwidth | 60-360° | 15-60° | 1-15° |
| Typical Types | Dipole, monopole, patch | Yagi, helical, horn | Parabolic, array, Cassegrain |
| Bandwidth | Narrow to wide | Moderate | Narrow to moderate |
| Complexity | Low | Medium | High |
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.