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