Ground-Wave Propagation:

In the radio communication, ground wave contains three distinct components: the direct wave (also termed as the line-of-sight wave), the surface wave, and the reflected wave. The direct wave moves in a straight line. It plays an important role only if the transmitting and receiving antennas are joined by a straight geometric line wholly above the Earth's surface. At most radiofrequencies, the EM fields pass via objects like trees and frame houses with small attenuation. The concrete-and-steel structures cause a few losses in the direct wave at higher frequencies. The earth barriers like hills and mountains obstruct the direct wave.

The radio signal can be reflected from Earth or from some structures like concrete-and-steel buildings. The reflected wave joins with the direct wave (if any) at the receiving antenna. At times the two are precisely out of phase, in which situation the received signal is weak even when the transmitter and receiver lie all along a direct line of sight. This effect takes place mostly at frequencies above 30 MHz (i.e., wavelengths less than 10 m).

The surface wave travels in contact with the Earth, and the Earth forms portion of the circuit. This occurs only with vertically polarized EM fields (those in which the electrical flux lines are vertical) at frequencies below around15 MHz. Above 15 MHz, there is necessarily no surface wave. At frequencies from around 9 kHz to 300 kHz, the surface wave propagates for hundreds or even thousands of kilometers. At times the surface wave is termed as the ground wave, though technically this is a misnomer.