Which mode of propagation is used by short wave broadcast services having frequency range from a few MHz upto 30 MHz ? Explain diagrammatically how long distance communication can be achieved by this mode . Why is there an upper limit to frequency of waves used in this mode ?

Explanation

Sky wave propagation is used by short wave broadcast services having frequency range from a few `MH_(z)` upto 30 to 40 MHz.
Long distance communication can be achieved by ionosphere reflection of radio waves back towards the earth as shown in Fig.
This mode of propagation is called sky wave propagation .
The ionosphere extends from a height of about 65 km to about 400 km above the earth's surface . The ionospheric layer acts as a reflector for a certain range of frequencies (3 to 30 MHz) .
Electromagnetic waves of frequencies higher than 30 MHz penetrate the ionosphere and escape.