Electromagnetic Field Regions
Understanding EM Field Regions
Understanding field regions is critical for antenna measurement, safety (SAR compliance), and system design. The field behavior is fundamentally different in each region.
Field Region Definitions
| Region | Boundary | Field Behavior |
|---|---|---|
| Reactive Near-Field | R < lambda/(2pi) | Fields store energy. E and H not in phase. Very complex. |
| Radiating Near-Field (Fresnel) | lambda/(2pi) < R < 2D^2/lambda | Fields radiate but pattern is range-dependent. |
| Far-Field (Fraunhofer) | R > 2D^2/lambda | Fields radiate. Pattern is independent of range. Power ~ 1/R^2. |
D = 1m dish at 10 GHz: R_ff = 2(1)^2/0.03 = 67 m
D = 0.3m at 28 GHz: R_ff = 2(0.3)^2/0.0107 = 17 m
D = 3m at 12 GHz: R_ff = 2(9)/0.025 = 720 m!
Frequently Asked Questions
What are the antenna field regions?
Reactive near-field (< lambda/2pi): stored energy, complex fields. Radiating near-field (to 2D^2/lambda): radiation but pattern changes with distance. Far-field (> 2D^2/lambda): stable pattern, 1/R^2 power decay.
Why does the far-field distance depend on antenna size?
Larger antennas have wavefronts that are curved in the near field. The far-field boundary is where the phase error across the aperture due to wavefront curvature drops below lambda/16 (22.5 degrees). Larger aperture = farther to planar wavefront.
Can I measure gain in the near field?
Not directly. Near-field patterns are range-dependent. But you can measure near-field amplitude and phase, then mathematically transform to far-field pattern. This is the basis of near-field antenna measurement ranges.