The distance attenuation comprises
20 log10(r) + 10log10(4π)
where
r is the actual distance from source to receiver
This equation is the standard equation for spherical propagation and assumes equal energy in all directions
PEN Implementation
The distance attenuation is the normal 6 dB per doubling of distance. The distance is the calculated distance from the source to the receptor. It ignores any intervening terrain (or barriers) but places the source and receptor at the appropriate location in three dimensional space.
Note since this is an attenuation, the negative sign in the original equation 1 becomes positive.