dBi stands for deciBel Isotropic. It is the gain of an antenna relative to that of an ideal isotropic antenna that radiates energy equally in all directions.
dBm is the power relative to one milliwatt. dBm can be directly converted to Watt and vice versa.
This tool uses the
- Power at the output of a transmitter (dBm)
- Cable loss (dB) and
- EIRP (dBm)
to find the required antenna gain in dBi.
dBi to dBm Formula
GAnt = EIRP – PTx + LC
Example Application
The maximum EIRP for a LoRa device operating at 922 MHz in Japan is +27 dBm.
Assuming a cable loss of 1 dB and a maximum output power from the transmitter of +15 dBm, the maximum allowable antenna gain is 13 dBi.
dBm to dBi Table
The FCC specifies a max EIRP of +36 dBm for Wi-Fi systems. The table below shows the max antenna gain (dBi) for a given transmit power (dBm) with no cable loss.
| Antenna Gain (dBi) | Transmit Power (dBm) |
| 56 | -20 |
| 55 | -19 |
| 54 | -18 |
| 53 | -17 |
| 52 | -16 |
| 51 | -15 |
| 50 | -14 |
| 49 | -13 |
| 48 | -12 |
| 47 | -11 |
| 46 | -10 |
| 45 | -9 |
| 44 | -8 |
| 43 | -7 |
| 42 | -6 |
| 41 | -5 |
| 40 | -4 |
| 39 | -3 |
| 38 | -2 |
| 37 | -1 |
| 36 | 0 |
| 35 | 1 |
| 34 | 2 |
| 33 | 3 |
| 32 | 4 |
| 31 | 5 |
| 30 | 6 |
| 29 | 7 |
| 28 | 8 |
| 27 | 9 |
| 26 | 10 |
| 25 | 11 |
| 24 | 12 |
| 23 | 13 |
| 22 | 14 |
| 21 | 15 |
| 20 | 16 |
| 19 | 17 |
| 18 | 18 |
| 17 | 19 |
| 16 | 20 |
| 15 | 21 |
| 14 | 22 |
| 13 | 23 |
| 12 | 24 |
| 11 | 25 |
| 10 | 26 |
| 9 | 27 |
| 8 | 28 |
| 7 | 29 |
| 6 | 30 |
| 5 | 31 |
| 4 | 32 |
| 3 | 33 |
| 2 | 34 |
| 1 | 35 |
| 0 | 36 |
| -1 | 37 |
| -2 | 38 |
| -3 | 39 |
| -4 | 40 |
| -5 | 41 |
| -6 | 42 |
| -7 | 43 |
| -8 | 44 |
| -9 | 45 |
| -10 | 46 |
| -11 | 47 |
| -12 | 48 |
| -13 | 49 |
| -14 | 50 |