GROUND-WAVE PROPAGATION

In radio communication, the ground wave consists of three distinct components: the direct wave (also called the line-of-sight wave), the reflected wave, and the surface wave. The direct wave travels in a straight line. It plays a significant role only when the transmitting and receiving antennas are connected by a straight geometric line entirely above the Earth’s surface. At most radiofrequencies, EM fields pass through objects such as trees and frame houses with little attenuation. Concrete-and-steel structures cause some loss in the direct wave at higher frequencies. Earth barriers such as hills and mountains block the direct wave.
A radio signal can be reflected from the Earth or from certain structures such as concrete-and-steel buildings. The reflected wave combines with the direct wave (if any) at the receiving antenna. Sometimes the two are exactly out of phase, in which case the received signal is weak even if the transmitter and receiver lie along a direct line of sight. This effect occurs mostly at frequencies above 30 MHz (wavelengths less than 10 m).
The surface wave travels in contact with the Earth, and the Earth forms part of the circuit. This happens only with vertically polarized EM fields (those in which the electrical flux lines are vertical) at frequencies below about 15 MHz. Above 15 MHz, there is essentially no surface wave. At frequencies from about 9 kHz up to 300 kHz, the surface wave propagates for hundreds or even thousands of kilometers. Sometimes the surface wave is called the ground wave, but technically this is a misnomer.