This is raw RSSI value report by LoRa modem, using lowest possible TX power less then 23dBm (less then 250mW), and non directional antennas.
The following formula can be used to calculate strength of received signal
RSSI (dBm) = raw RSSI – 164.
It means that lowest value (except four zeros, set by firmware in case of bad frame’s CRC) is 60 – 164 = -104dBm
We can predict RSSI for longer distances (for 23dBm RF power)
- 5 km => -104 dBm
- 10 km => -110 dBm ( 6dB lost)
- 20 km => -116 dBm ( 6dB lost)
- 40 km => -122 dBm ( 6dB lost)
The LoRa modem has sensitivity like -121 dBm, it means that signal should be received at distance 35-40 km (using lowest RF power).
Using full 30dBm RF power (7 dB more) we should reach 80km distance without problems 😉