Antenna Gain Calculator (dBi)
Calculate antenna gain in dBi and dBd, effective radiated power, and compare dipole, Yagi, and directional antenna types for ham radio and RF link budgets.
Understanding antenna gain:
Antenna gain measures how effectively an antenna focuses radiated power in a particular direction compared to a reference antenna. It does not create power — it redirects power from unwanted directions into the desired direction, like a flashlight focuses a light bulb’s output into a beam.
Gain units explained:
| Unit | Reference | Conversion |
|---|---|---|
| dBi | Isotropic radiator (theoretical point source) | — |
| dBd | Half-wave dipole | dBi = dBd + 2.15 |
A half-wave dipole has a gain of 2.15 dBi (0 dBd). Every 3 dB of gain doubles the effective power in the favored direction.
Common antenna gains:
| Antenna Type | Gain (dBi) | Gain (dBd) | Pattern |
|---|---|---|---|
| Isotropic | 0 | -2.15 | Perfect sphere |
| Quarter-wave vertical | 2.15 | 0 | Omnidirectional |
| Half-wave dipole | 2.15 | 0 | Figure-8 |
| Ground plane (elevated) | 3–5 | 0.85–2.85 | Omnidirectional |
| 3-element Yagi | 7–8 | 4.85–5.85 | Directional |
| 5-element Yagi | 10–11 | 7.85–8.85 | Directional |
| 10-element Yagi | 13–14 | 10.85–11.85 | Highly directional |
| Quad loop (2 el) | 7–8 | 4.85–5.85 | Directional |
| Log-periodic | 6–8 | 3.85–5.85 | Broadband directional |
| Dish (parabolic) | 20–40+ | 17.85–37.85+ | Pencil beam |
Effective Radiated Power (ERP):
ERP (watts) = Transmitter power × Feedline efficiency × Antenna gain (linear)
Antenna gain (linear) = 10^(Gain_dBi / 10)
Feedline efficiency = 10^(-Loss_dB / 10)
Example calculation:
A 100-watt transmitter with a 5-element Yagi (10.5 dBi) and 1.5 dB feedline loss:
- Gain linear: 10^(10.5/10) = 11.22
- Feedline efficiency: 10^(-1.5/10) = 0.708
- ERP: 100 × 0.708 × 11.22 = 794 watts ERP
This means a 100W station with a good Yagi has the same signal strength as a 794W station with an isotropic antenna.
The 3 dB rule:
Every 3 dB of gain doubles the effective power:
- +3 dBi = 2× power
- +6 dBi = 4× power
- +10 dBi = 10× power
- +20 dBi = 100× power
Gain vs. beamwidth tradeoff:
Higher gain means narrower beamwidth. A 3-element Yagi has ~60° beamwidth. A 10-element Yagi narrows to ~30°. Dish antennas can have beamwidths under 5°. Narrow beamwidth requires accurate aiming but provides better signal and noise rejection.
Practical considerations:
Adding 3 dB of antenna gain is equivalent to doubling transmitter power — and far cheaper. A $50 antenna upgrade often outperforms a $500 amplifier. Antenna height and clear line-of-sight matter as much as gain for real-world performance.