What is the difference between antenna gain, directivity, and efficiency?
Gain and Directivity
Directivity is a theoretical property of the antenna's radiation pattern. It describes how well the antenna concentrates energy in a particular direction regardless of how much total power is actually radiated. A very directive antenna can still have low gain if most of the input power is lost to ohmic heating in the antenna structure.
| 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
Which do I use in a link budget?
Use realized gain for the most accurate link budget because it includes all losses. If the antenna is well-matched (VSWR < 2:1): use gain (the mismatch loss is < 0.5 dB and often included as a separate line item). Never use directivity in a link budget unless you separately account for all efficiency factors.
How do I measure gain?
Compare the antenna under test to a reference antenna with known gain (gain transfer method): G_DUT = G_ref + (P_DUT - P_ref). Standard gain horn antennas with calibrated gain are commonly used as references. Alternatively, use the three-antenna method with three unknown antennas to determine all three gains.
What efficiency is typical?
Well-designed metallic antennas at microwave frequencies: η > 90% (loss < 0.5 dB). Microstrip patch antennas on lossy substrates: η = 70-90%. Electrically small antennas: η = 10-50% (dominated by radiation resistance vs loss resistance). Low-loss waveguide-fed horns: η > 95%.