When a radio wave leaves a transmitting antenna, it travels in all directions. Part of the signal travels along the ground and is called the ground wave. Part of the signal travel through the lower atmosphere in a directing parallel to the ground. Another part travels at an angle to the ground. The part travelling through the lower atmosphere is called the tropospheric wave; the part travelling at an angle to the ground – the ionospheric wave.
In the high – frequency part of the radio spectrum (3-30mc.) the ground and tropospheric wave components travels short distances, not more than 25 or 30miles. The ionospheric wave component of the signal can travel to great distances, making long – distance short – wave communication possible.
As the ionospheric wave leaves the transmitting antenna, it starts traveling and reaches a region called the ionosphere.
This region begins at a height of about 60miles above the ground. On reaching the ionosphere, radio waves supply the free electrons of the region with electromagnetic energy. As result, the free electrons start vibrating and radiate this energy the way the electrons in a transmitting antenna radiate energy.