[Andy Fletcher]

Hi Tim,

HF transmissions skip off layers in the atmosphere (D and E - if I remember correctly) and so can bounce between these layers and the earth and cover great distances. HF communication can be a bit hit and miss as the propogation properties of the layers vary depending on the time of day. Nowadays an equipment called a Chirp Sounder can ascertain the best HF frequency to use.

VLF transmissions use the "ground wave" effect and can cover even greater distances. With ground wave transmissions the signal actually hugs the ground. The Omega navigation system uses VLF to receive signals from the eight Omega "beacons" around the world (of which it needs at least three to get an accurate position).

VHF (and above) transmissions are line of sight so are affected by the curvature of the earth, range being dependant on height and power.

All RF transmissions will eventually decay.

It's been a long time since I did comms theory so I might not be 100% correct but I think the basics are more or less correct.

Best Regards

Andy Fletcher