GROUND WAVE PROPAGATION

Ground wave (or surface wave) propagation is the mode in which electromagnetic waves radiated by the transmitter glide directly along the surface of the Earth to reach the receiver, rather than travelling upward into the sky or in a straight line through open space. Because the wave must stay close to the ground for its entire journey, both the transmitting and the receiving antennas need to be close to the Earth's surface, and the size of the antenna plays a major role in how efficiently the signal is radiated and picked up in the first place. As a ground wave travels, it is progressively attenuated by three distinct effects. First, increasing distance itself weakens the signal, and how quickly this happens depends on the transmitter's power, the transmitted frequency, and the physical condition of the Earth's surface along the path. Second, the Earth absorbs energy from the wave: as the electromagnetic wave stays in contact with the ground, it induces charges in the Earth and sets up a current, so the Earth effectively behaves like a leaky capacitor draining energy out of the travelling wave. Third, as the wave progresses, its wavefront gradually tilts to follow the curvature of the Earth, and this increasing tilt steadily reduces the wave's effective electric field strength until, at some distance, the surface wave simply dies out from accumulated energy loss. Because high-frequency waves suffer noticeably more of this atmospheric absorption, ground waves are mostly restricted to frequencies below about 2 MHz -- which is also why medium-wave (AM) radio s …