In sky wave or ionospheric propagation, the electromagnetic waves of frequencies between 3 MHz – 30 MHz launched by a transmitting antenna travel upwards, get reflected by the ionosphere and return to distant locations.
In this mode, the reflecting ability of the ionosphere controls the propagation characteristics of the sky wave. The ionosphere acts as an invisible electromagnetic mirror surrounding the earth - at optical frequencies it is transparent, but at radio frequencies it reflects the electromagnetic radiation back to earth.
The maximum distance along the surface of the earth that can be reached by a single ionospheric reflection ranges between 2010 and 3000 km depending on the altitude of the reflecting layer. The communication delay encountered with a single reflection ranges between 6.8 and 10 ms, a small time interval.
This mode of propagation is used for long distance (short wave) communication in the frequency range approximately between 5 and 10 MHz. Above 10 MHz, the waves pass through the ionosphere and do not reflect back to the earth.
It is, however, subject to erratic daily and seasonal changes due to variations in the number density and height of the ionized layers in the ionosphere. The composition of the ionosphere at night is different than during the day because of the presence or absence of the sun. That is why international broadcast is done at night because the reflection characteristics of the ionosphere are better at that time.