Radio link path loss
A radio wave spreads like light from a bare bulb: double the distance and the same energy covers four times the area, so each square gets a quarter, which is 6 dB less.
FSPL = 20 log10(d) + 20 log10(f) − 147.55 dB (d in m, f in Hz)
Free space only, line of sight, no antenna gain. Compare the result with transmit power plus antenna gains minus receiver sensitivity to see if a link closes; add 10–30 dB of margin for walls, foliage, rain and fading. Check local regulations before transmitting.