Maximum Usable Frequency Prediction Depending on the Solar Activity and the Ionosphere

The Sun constantly emits radiation that affects the level of ionization in the Ionosphere. Sun also emits ionized particles in the form of the solar wind and coronal mass ejections (CMEs) that can cause geomagnetic storms and affects radio communication. Solar activity follows an 11-year cycle and can be roughly predicted to minimize its negative effects. This paper analyzes the influence of the Sun on the Ionosphere and its influence on the propagation of radio waves, primarily HF waves that use diffraction from the Ionosphere to propagate over long distances. The analysis is based on the prediction of the maximum usable frequency (MUF) that can be used for communication in certain circumstances of solar activity, that is, at different times of the day and at different phases of the solar cycle. The VOACAP software (propagation prediction program) was used to predict the MUF for Zagreb as the transmitter site.