What is the three-antenna method for absolute antenna gain measurement?
Three-Antenna Gain Method
The three-antenna method is the fundamental basis for all antenna gain calibration. Every calibrated standard gain horn traces its calibration back to a three-antenna measurement performed at a national metrology lab.
| 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 |
- Performance verification: confirm specifications against the application requirements before finalizing the design
- Environmental factors: temperature range, humidity, and vibration affect long-term reliability and parameter drift
- Cost vs. performance: evaluate whether the application demands premium components or standard commercial grades
- Interface compatibility: verify impedance, connector type, and mechanical form factor match the system architecture
- Margin allocation: include sufficient design margin to account for manufacturing tolerances and aging effects
Frequently Asked Questions
Why not just two antennas?
With two identical antennas: you can determine their gain (since G_A = G_B, the Friis equation gives: G = (S_AB/2) - FSPL/2). But: you must know that the antennas are identical (which requires a separate measurement to confirm), and any asymmetry (manufacturing variation, different cable losses) introduces an error that cannot be separated from the gain. With three different antennas: no assumption of identity is needed, and the three independent measurements provide enough information to solve for all three gains uniquely.
What is the typical accuracy?
The three-antenna method accuracy depends on: range reflection level (must be below -40 dB for 0.1 dB gain accuracy), distance measurement accuracy (±1 mm at 10 GHz contributes approximately 0.02 dB uncertainty), power ratio measurement accuracy (±0.05-0.1 dB for a high-quality VNA measurement), and antenna alignment (pointing errors contribute gain uncertainty). At national metrology labs (NIST): the three-antenna method achieves ±0.1-0.2 dB uncertainty. In well-equipped commercial labs: ±0.3-0.5 dB. This is the most accurate antenna gain measurement technique available.
Can I use this at any frequency?
Yes: the three-antenna method works at any frequency from VHF to mmW. The practical challenges change with frequency: at low frequencies (below 1 GHz): the far-field distance is very large (for a 1 m antenna at 300 MHz: far-field distance = 2(1)²/1 = 2 m, which is short). The challenge is: obtaining a reflection-free range at low frequencies (large wavelength requires a very large anechoic chamber). At high frequencies (above 40 GHz): the far-field distance is short but the alignment tolerance is very tight (beam pointing must be accurate to fractions of a degree). Atmospheric attenuation at mmW frequencies must also be accounted for.