Finding the Actual Horizon
When propagating in the line-of-sight (LOS), knowing the distance to the actual horizon is especially important. The equations here are as a function of antenna height, since two stations attempting to propagate paths must have a direct line to each other's antennas (in this case, we will assume the ionospheric refraction is 0).
For the above, r=Radius(Earth), h=Height above surface. The radius of the Earth is generally accepted as 3,960 miles.
Finding the Radio Horizon
For LOS calculations, we use an Earth radius that is 4/3 of the actual radius. This is because radio waves are refracted, even in LOS, by the ionosphere. To find the radio horizon, the following equation can be used, where d=miles, h=antenna height (ft.).
