Understanding Near-Field and Far-Field for Antenna Performance
Many hams walk around an antenna with a NanoVNA or handheld probe thinking they’re comparing efficiency. In reality, they’re measuring mostly reactive fields, not radiated power. Here’s the concise, engineer-friendly explanation you can share.
TL;DR
- Efficiency is a radiated-power quantity — the fraction of accepted input power that escapes as radiation, not heat.
- Near field ≠ radiated power. It’s mostly stored energy that sloshes back and forth around the antenna.
- Only the far field is plane-wave. There, E and H are in phase and E/H ≈ 377 Ω. That’s where gain and efficiency make sense.
- Walking around with a VNA perturbs the antenna — your body and probe detune and couple to the reactive field, corrupting the measurement.
What “Near Field” and “Far Field” Actually Mean
The space around any antenna (largest dimension D, wavelength λ) divides into zones:
- Reactive near field — dominated by stored energy ∝ 1/r³. E and H are not in phase; no net radiation occurs.
- Radiating near field (Fresnel) — fields decay ∝ 1/r but the pattern still changes with distance.
- Far field (Fraunhofer) — true radiation zone; pattern constant and E/H ≈ 377 Ω.
Region boundaries (engineering approximations):
Reactive near-field end: r ≥ max( λ⁄2π, 0.62 √(D³⁄λ) ) Far-field start: r ≥ 2D²⁄λ
Rule of thumb: measure at r ≥ max(3 λ, 2D²/λ) unless you have a calibrated near-to-far-field setup.
Why “Walking Around with a NanoVNA” Fails as an Efficiency Test
- Wrong region. At arm’s length you’re still in the reactive or Fresnel zone; the field there isn’t radiated energy.
- Probe detuning. Your body and the VNA load the antenna, altering its impedance and near-field distribution.
- E- vs H-field imbalance. A whip senses E; a loop senses H. Different designs favour one — readings aren’t comparable.
- Friis doesn’t apply. That formula assumes plane waves; invalid in the near field.
- Match ≠ efficiency. A perfect SWR can hide a dummy-load radiator; a mismatch can still radiate efficiently. ηtot = ηrad(1 – |Γ|²)
- Environment dominates. Ground, walls, and people cause coupling and multipath that swamp small differences.
Typical Region Sizes (Normalized)
| D/λ | Reactive NF end (λ) | Far-field start (λ) | Good practice (λ) |
|---|---|---|---|
| 0.05 | 0.159 | 0.005 | 3.000 |
| 0.10 | 0.159 | 0.020 | 3.000 |
| 0.50 | 0.219 | 0.500 | 3.000 |
| 1.00 | 0.620 | 2.000 | 3.000 |
| 2.00 | 1.754 | 8.000 | 8.000 |
| 5.00 | 6.932 | 50.000 | 50.000 |
Even a half-wave dipole needs ≈ 3 λ spacing for valid far-field data.
Real-World Distances for a Half-Wave Dipole
| Freq (MHz) | λ (m) | Reactive NF end (m) | Far-field start (m) | Good practice (m) |
|---|---|---|---|---|
| 3.5 | 85.7 | 18.8 | 42.8 | 257 |
| 7 | 42.8 | 9.4 | 21.4 | 128 |
| 14 | 21.4 | 4.7 | 10.7 | 64 |
| 28 | 10.7 | 2.3 | 5.4 | 32 |
| 50 | 6.0 | 1.3 | 3.0 | 18 |
| 144 | 2.08 | 0.46 | 1.04 | 6.25 |
| 433 | 0.69 | 0.15 | 0.35 | 2.08 |
| 868 | 0.35 | 0.08 | 0.17 | 1.04 |
| 2400 | 0.13 | 0.03 | 0.06 | 0.37 |
Even at 2.4 GHz, a few cm away ≠ far field — keep several wavelengths of distance.
Proper Ways to Compare Antenna Efficiency
- Far-field gain range: Two antennas ≥ 3 λ apart; measure S₂₁ and correct for losses.
- Total Radiated Power (TRP): Integrate over all angles in a chamber — includes mismatch + pattern.
- Wheeler cap: Compare feed resistance with/without conductive cap to derive ηrad.
- Reverberation chamber: Fast TRP measurement for small devices.
- Calibrated NF scanner: Only valid with dense sampling and NF→FF transform.
Quick Explanation
Close to an antenna you’re mostly measuring stored energy, not radiation. Your body and probe alter the field, and E/H isn’t 377 Ω there. Efficiency and gain only make sense in the far field — a few wavelengths away — where the wave is plane and Friis applies.
Mini-FAQ
- Can I still use a VNA to compare antennas? — Yes, for S₁₁ and match, not efficiency.
- What’s the simplest field-test method? — Two antennas several λ apart, same polarization, measure S₂₁.
- Does ground proximity affect boundaries? — Slightly; it shortens distances but doesn’t make near fields radiative.
Questions or experiences to share? Feel free to contact RF.Guru.