Q.Name the two radio waves that propagate near to the surface of the Earth.

Propagation is the forward travel of radio waves from the transmitter to the receiver. Once a signal leaves the transmitting antenna, it does not have just one way to reach the far end — it can follow one of three basic paths through the atmosphere, and which path dominates depends mainly on the signal's frequency band.

The three modes are:

1. Ground wave — the wave is guided along the earth's surface, using the ground itself as part of the path. This works best at low and medium frequencies (LF and MF).
2. Space wave — the wave travels in a straight, line-of-sight path directly between the transmitting and receiving antennas, staying in the lower atmosphere. This is the main mode at VHF and higher frequencies.
3. Sky wave — the wave is beamed upward and bent back to earth by the ionosphere, allowing very long-distance communication. This is the HF mode.

A useful way to picture it: the ground wave and the space wave both stay close to the earth in the lower atmosphere, while only the sky wave leaves that region to reflect off the ionised layers high above. For very long links beyond even sky-wave reach, satellite (space) communication relays signals via a spacecraft.

Recognising these modes is the foundation for the rest of wireless communication, because each later topic — ground-wave range, radio horizon, skip distance — is simply a detailed study of one of these paths.