Ground-Wave Propagation

Ground wave works only with vertical polarization. One side of the antenna is the metal vertical

radiator and the other side of the antenna is the earth ground. The surface wave in the air travels faster

than the part of the wave flowing through the ground. The surface of the earth is curved like the curved

part of a racetrack. On the curved track, a car on the outside of the track has to travel faster than the

car on the inside lane to stay even, and the two cars travel in a curved path. Although the wave in the air

travels faster than the wave on the ground, the two parts of the wave cannot be separated. Because of

this, the radio wave also travels in a curved path that follows the curvature of the earth.

The AM broadcast stations use ground wave propagation during the day and skywave propagation at

night. Since radio waves at lower frequencies conduct better through the ground, an AM broadcast

station on 540 kHz will be many dB stronger than a station on 1600 kHz, if both run the same power.

This fact is important in understanding why ground