Q.Explain the concept of space wave propagation.

Space-wave propagation, also called line-of-sight propagation, is the main way that VHF and higher-frequency signals travel. The space wave is made up of two parts that arrive together at the receiver: the direct wave, which travels straight from the transmitting antenna to the receiving antenna, and the ground-reflected wave, which bounces off the earth on the way. Together these form the space wave.

Because the wave travels in a straight line and does not bend around the earth or reflect off the ionosphere, its reach is limited by the earth's curvature — the idea of the horizon. The farthest point visible to the naked eye is the optical horizon (OH). Radio waves bend very slightly and so travel a little farther, out to the radio horizon (RH), which is about four-thirds of the optical horizon:

$RH \approx \frac{4}{3}\,OH$ …