Understanding Free Space Path Loss
Free Space Path Loss (FSPL) is the attenuation of radio energy between two antennas in an unobstructed line-of-sight environment. It follows the inverse-square law.
FSPL (dB) = 20*log10(d) + 20*log10(f) + 32.44
Where d is the distance in kilometers and f is the frequency in MHz. Every doubling of distance adds approximately 6 dB of path loss, and every doubling of frequency also adds 6 dB.