At very high frequencies this means that in normal circumstances once a receiver moves over the horizon from a transmitter that’s it, you’re out of range and there can be no communication. But at lower frequencies this is not the case. As you move through the lower end of the VHF into the HF (Short Wave) portion of the spectrum and below, the radio signal routinely travels far further than the horizon, and at the lower HF frequencies it starts to reach other continents, even as far as the other side of the world.
If you are familiar with radio propagation you’ll know that radio waves do not naturally bend around the earth. Like light and indeed all electromagnetic radiation if they are given a free space they will travel in a straight line.
via Blog – Hackaday http://ift.tt/2but1Oh