Free space is just that, free space, so there is nothing to affect the signal except attenuation of the air.
With an actual link, the ground has to be taken in to account, for two reasons:
1) The ground produces reflections of the transmitted signal. These reflections reach the receiver at different times to the direct signal and therefore cause phase cancellation which reduces or could completely cancel out the received signal. To avoid this, we have to consider the so called Fresnel clearance, which is an ellipsoid area of space between the aerials, the dimensions of which are related to path length and frequency. If there is an obstacle such as a hill which does not obstruct the line of sight between the aerials, but does infringe the Fresnel zone, then attenuation will occur. Thus, increasing the height of one or both aerials will reduce these losses.
2) The curvature of the earth effectively reduces the overall height of the path, so the longer the link, the higher the aerials need to be. (NB, talking about VHF here, not HF).
For both reasons, the higher above the earth one or both stations are, the less attenuation and interference will be experienced. There is a limit though, because at some point the free path attenuation will be too much, and this will limit the maximum range.
A formulae for working out an appproximate max range for VHF is:
1.25*sq. route (2*(transmitter height+receiver height))
Heights in feet, range in nm.
For a jet at FL350 this gives a practical max range of ~331nm, assuming the transmit/receive mast is 200' agl. - and I find I can often receive London Main Volmet at about 300-325nm out.
Last edited by Uplinker; 12th Oct 2015 at 14:35.