Even with a single hop path the ionosphere is not considered a stable medium that would allow the use of one frequency over the year, or even over 24 hours. The ionosphere varies with the solar cycle, the seasons and the time of day. So, a frequency which may provide successful propagation now, may not do so an hour later, as it is susceptible to what we call “Ionospheric variations”. Due to the distance covered in a multiple-hop path the following variations may well apply.
Variations due To the Solar Cycle occur when the Sun goes through a periodic rise and fall in activity which affects HF communications; solar cycles vary in length from 9 to 14 years. At solar minimum, where fewer electrons are produced, only the lower frequencies of the HF band will be supported by the ionosphere, while at solar maximum, where more electrons are being produced, the higher frequencies will successfully propagate.…show more content… Around solar maximum there is a greater likelihood of large solar flares occurring. Flares are huge explosions on the Sun which emit radiation that ionises the D region causing increased absorption of HF waves. Since the D region is present only during the day, only those communication paths which pass through daylight will be affected. The absorption of HF waves travelling via the ionosphere after a flare has occurred is called a short wave fade-out. Fade-outs occur instantaneously and affect lower frequencies the most. Lower frequencies are also the last to recover. If it is suspected or confirmed that a fade-out has occurred, it is advisable to try using a higher frequency, if possible. The duration of fade-outs can vary between about 10 minutes to over an hour depending on the duration and intensity of the
