Radio Waves follow the curve of the earth’s surface. You might think that such a radio signal would shoot straight up and into outer space. It would, were it not for the mirror-like effect of something called the ionosphere.
The ionosphere makes up a large portion of the earth’s atmosphere starting at from 50 to 200 miles above sea level. The ionosphere ranges in thickness from 50 to 100 miles and is filled with electrically charged atoms known as ions. These ions bounce radio signals back to earth.
A radio signal bounces between the earth and the ionosphere as it makes its
way around the world. The ionosphere is divided into three layers, and each
layer affects different types of radio signals. The D layer is roughly 60 miles
above sea level and reflects long-wave radio broadcasting back to earth.
The E layer is 80 miles above sea level and does the same for medium-wave
broadcasting. Short-wave broadcasts are reflected back to earth by the F
layer, which is found 150 miles above sea level.