dBm Explained
Edit on GitHubWhat dBm means, the powers the beacon uses, and how to think about -9 to +22 dBm.
dBm Explained
dBm is the unit of RF power used everywhere in this project. It looks intimidating and is actually simple.
The definition
dBm is power relative to 1 milliwatt, on a logarithmic scale:
- 0 dBm = 1 mW
- +10 dBm = 10 mW
- +20 dBm = 100 mW
- +30 dBm = 1 W
- -10 dBm = 0.1 mW
Every +10 dB is a 10x power increase. Every +3 dB is a 2x increase.
The beacon’s numbers
| Setting | Power | Use |
|---|---|---|
| -9 dBm | ~0.13 mW | Minimum (test, short range) |
| +17 dBm | ~50 mW | BEACON default |
| +22 dBm | ~160 mW | EMERGENCY, and RadioLib’s SX1262 max |
| +30 dBm | 1 W | E22 PA output (module capability, not used by the beacon’s settings) |
Why logarithms
Radio links involve huge dynamic ranges: a receiver might hear -50 dBm from a nearby beacon and -120 dBm from a distant one. That’s a factor of 10 billion, which is unmanageable in linear units. dBm compresses it to a 70-number scale.
RSSI vs power
RSSI (received signal strength indicator) is also in dBm but describes what the receiver sees, which depends on distance, antenna, and obstacles. A beacon transmitting at +17 dBm can produce RSSI anywhere from -40 dBm (a few meters) to -120 dBm (beyond range).
Rules of thumb
- 3 dB is barely noticeable to a listener; 10 dB is clearly louder.
- Doubling the distance costs roughly 6 dB on flat ground (inverse square for the path).
- To double range in free space, you need 4x power (+6 dB).
In the dashboard
The TX power slider ranges -9 to +22 dBm. The firmware caps it at +22 dBm regardless of what the E22’s PA could do, because RadioLib’s SX1262 driver is the limit and because legal power limits are usually far lower.