The major killer for long range communications is the inverse square loss. For every kilometer you traver, every signal gets dimmer and dimmer, following the inverse square law. Usually, the distance parameter is estimated by coefficent of loss, called the "free space path loss":
$FSPL = \left( \frac{4 \pi d}{\lambda} \right)^2$
Where $d$ is the distance travelled by the signal and $\lambda$ is the wave length of the signal
