What factors influence HF Radio Propagation?

Several factors can influence the propagation of high frequency (HF) radio waves, including:

1. Solar activity: The sun's activity can affect the ionosphere and the amount of ionization, which can influence the reflection of HF radio waves.
    
2. Time of day: The ionosphere is more ionized during the day due to solar radiation, which means that HF radio waves are more likely to be reflected back to Earth at certain frequencies. At night, the ionosphere becomes less ionized, and the frequencies that are reflected back to Earth shift to lower frequencies.
    
3. Season: The ionosphere's ionization levels can vary depending on the season, which can affect the frequencies that are reflected back to Earth.
    
4. Location: The angle at which HF radio waves approach the ionosphere can affect how they are reflected back to Earth, and the distance they travel.
    
5. Weather conditions: Weather can affect HF radio propagation by causing the ionosphere to change, or by affecting the signal as it passes through the atmosphere.
    
6. Geomagnetic storms: These storms, which are caused by disturbances in the Earth's magnetic field, can cause disruptions in HF radio communication by changing the ionosphere's properties.
    
7. Signal frequency: The frequency of the HF radio signal itself can also affect its propagation, as different frequencies may be reflected or absorbed by the ionosphere in different ways.
    

You may find more detailed information in the main webpage, here. See also an index for HF Radio Propagarion.