Near Field
Understanding the Near Field
The near field is fundamentally different from the far field. In the near field, the electric and magnetic fields are not in phase, the wave impedance is not 377 ohms, and the radiation pattern changes with distance. Measurements must account for these effects or be performed in the far field.
Near-Field Regions
- Reactive near field: Extends to ~lambda/(2pi) from antenna. Dominated by reactive (stored) energy. Electric and magnetic fields out of phase.
- Radiating near field (Fresnel): From ~lambda/(2pi) to 2D^2/lambda. Pattern shape changes with distance. Far-field equations not valid.
- Far field (Fraunhofer): Beyond 2D^2/lambda. Pattern shape is independent of distance. Standard antenna specifications apply.
Near-Field Distance Examples
- 30 cm dish at 10 GHz: 2(0.3)^2/0.03 = 6 m to far field.
- 1 m dish at 10 GHz: 2(1)^2/0.03 = 67 m to far field.
- 3 m dish at 10 GHz: 2(9)/0.03 = 600 m to far field!
Frequently Asked Questions
What is the near field?
The near field is the region close to an antenna where the radiation pattern changes with distance and contains reactive energy. The boundary to far field is 2D^2/lambda. In the near field, standard antenna gain and pattern do not apply.
Why does near field matter?
Antenna patterns are defined in the far field, but for large antennas or high frequencies, the far field may be very distant. Near-field measurement techniques (planar, cylindrical, or spherical scanning) measure the near field and mathematically transform it to the far field.
What is near-field measurement?
Near-field measurement scans the field amplitude and phase over a surface close to the antenna (typically 3-10 wavelengths away). A mathematical transform (near-field to far-field transformation) then computes the far-field pattern. This enables accurate pattern measurement indoors in a small chamber.