Space Wave Propagation

Mechanism. At frequencies too high for the ionosphere to reflect (very-high frequency, VHF, and above, roughly above 30 MHz) and too high for the ground wave to carry any meaningful distance, an electromagnetic wave can still reach a receiver directly, travelling in essentially a straight line from the transmitting antenna to the receiving antenna -- this is space-wave propagation, and a link of this kind, where the receiver lies within a direct, unobstructed straight-line path of the transmitter, is called line-of-sight (LOS) communication.

Why range is limited, and the range formula. Because a space wave travels in a straight line while the Earth's surface curves away beneath it, the maximum range of a space-wave link is set purely by the geometry of the curved Earth and the heights of the two antennas above it -- exactly like the distance to the visible horizon from the top of a tower. For a transmitting antenna of height $h_T$ alone (receiver effectively at ground level), simple geometry of the right triangle formed by the antenna height, the line-of-sight distance, and the Earth's radius $R$ (using $h_T \ll R$) gives the maximum range as $d = \sqrt{2Rh_T}$. When the receiving antenna is also raised, to a height $h_R$, the total maximum range becomes the sum of each antenna's own horizon distance, $d = \sqrt{2Rh_T} + \sqrt{2Rh_R}$ -- which is why both broadcast transmitting towers and receiving aerials are built as tall as is practical, since raising either antenna pushes its own horizon, and so the total range, further out. …