HF Shortwave Radio Propagation - Basics
Shortwave (HF) propagation is possible due to the ionosphere, a region of the upper atmosphere
ranging from approximately 60 to 450 km in height.
The ionosphere is divided up into three regions, the D, E, and F layers, with the F layer
subdivided into the F1 and F2 layers.
The gas molecules in the ionosphere are ionized due to radiation from the Sun. That is, they
are stripped of some of their electrons. These regions with free electrons can reflect
radio waves back to the earth. This ionization occurs during the daytime. At night, the
ions start to recombine back into neutral molecules. The D and E layers completely recombine at
night, the F layer only partially recombines. This allows HF propagation to continue at night.
Only certain frequency ranges of radio waves (often called shortwave) can be reflected. Those which are too high in
frequency pass through the ionosphere. Those which are too low in frequency are absorbed
before they can be reflected. Generally, the HF or shortwave spectrum is the range from around
2 to 30 MHz. Under times of very high solar activity, even higher frequency can sometimes be
reflected, while under times of low activity, the maximum frequency can be much lower, sometimes
only 10 or 15 MHz at best.
The D layer is the lowest layer of the ionosphere, and the least ionized. It does not reflect
radio waves, but it does absorb and attenuate them. It only exists during the daytime. The
higher the solar activity, the more attenuation caused by the D layer. Hence, the D layer can
be thought of as setting the lowest frequency that can be propagated over a certain path, based
on the transmitter power levels, and current solar activity levels.
The E layer also exists only during the daytime, except for so called "Sporadic E" which can
occur anytime. Generally, the ionization of the E layer is insufficient to reflect radio
waves, except for Sporadic E, which has a much higher ionization level. Sporadic E is generally
caused more by weather conditions than solar activity.
The F layer builds up during the daytime, under the effect of radiation from the Sun.
It starts to recombine slowly at nightime, but never completely recombines, hence it is
available 24 hours a day for reflecting radio waves. The ionization levels vary during the
day of course so that higher frequencies tend to propagate better when their path is in
The F layer is responsible for essentially all of the shortwave radio proagation we have.
Radio waves that strike it at the correct angle are reflected back to the earth. The
range that can be covered with one such reflection (or hop) varies with the frequency,
solar conditions, radiation angle, etc. but is generally a few thousand km at most.
To cover further distances, multiple hops are required, each one of course results in a lower
signal level. As a result, the strongest signal conditions generally occur when the highest
possible frequency for a given path is used, since that requires in the fewest hops.
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