Antenna Gain Calculator (dBi)
Calculate antenna gain in dBi and dBd, effective radiated power, and compare antenna types for ham radio operations.
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.