The mode of propagation used by short wave broadcast services having a frequency range from a few $\mathrm{MHz}$ up to $30 \mathrm{MHz}$ is

Sky wave propagation is used by short wave broadcast services having a frequency range from few $\mathrm{MHz}$ up to $30$ to $40 \mathrm{MHz}$

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 skywave propagation.