Antenna Efficiency
Understanding Antenna Efficiency
Antenna efficiency directly determines how much of the transmitter power actually radiates, and on receive, how much of the intercepted power reaches the receiver. An antenna with 50% efficiency loses half the power as heat before it radiates.
Efficiency Components
- Radiation efficiency (e_r): Accounts for conductor and dielectric losses in the antenna structure.
- Mismatch efficiency (e_m): Accounts for power reflected due to impedance mismatch. e_m = 1 - |Gamma|^2.
- Total efficiency: e_total = e_r x e_m. This is the overall power transfer from feed to radiation.
Efficiency by Antenna Type
| Antenna | Efficiency |
|---|---|
| Wire dipole/monopole | 95-99% |
| Horn antenna | 50-70% |
| Microstrip patch | 70-95% |
| Chip antenna | 30-60% |
| Electrically small | 10-50% |
| Parabolic reflector | 55-75% (aperture eff) |
Frequently Asked Questions
What is antenna efficiency?
Antenna efficiency is the ratio of radiated power to input power. It accounts for losses in the antenna structure (conductor and dielectric). Gain = Directivity x Efficiency. Typical values range from 10% (small antennas) to 99% (wire antennas).
How is antenna efficiency measured?
Methods include Wheeler cap (place antenna inside a small metal enclosure to measure loss separately from radiation), gain comparison (compare measured gain to calculated directivity), and pattern integration (integrate the measured radiation pattern).
Why do small antennas have low efficiency?
Electrically small antennas have very high currents for a given radiated power, because their radiation resistance is very low. These high currents flowing through the finite conductor resistance dissipate significant power as heat, reducing efficiency.