The free space path loss equation in decibel form is:
Free space loss (dB) = 32.44 + 20 log f (MHz) + 20 log d (km)
and Friis' equation is:
Set the transmitter and receiver gains, the transmitter power and the loss factor equal to 1, because they don't have any effect on the signal travelling through the air, which gives you:
PR = λ2 / 16 . π2 . d2
PR = [λ / 4 . π . d]2
substituting λ = c / f
PR = [c / 4 . π . d . f]2
substitute f in MHz, d in km and c in m/s:
PR = [3 . 108 / (4 . π . d . 103 . f . 106)]2
PR = [3 / (40 . π . d. . f)]2
converting this to logarithmic form:
PR (dB) = 20 log [3 / (40 . π . d. . f)]
but loss is negative gain, so invert the equation to give:
Path loss (dB) = 20 log [40 . π . d. . f / 3]
Path loss (dB) = 20 log (40 . π / 3) + 20 log d + 20 log f
Path loss (dB) = 32.44 + 20 log d (km) + 20 log f (MHz)