↑ How does the ionosphere affect HF Radio Propagation?

By Doron Tal, 4X4XM

The Earth's atmosphere plays an important role in high-frequency (HF) radio propagation. In particular, the ionosphere, the upper atmosphere from about 60 km to about 700 km above the Earth's surface, is the key factor in the behavior of HF radio waves.

Ionosphere is part Earth's Atmosphere

The ionosphere is ionized by solar radiation. Because of this ionization, radio waves can be refracted and reflected back to Earth, which enables long-range communication.

Refractions and reflections by the Ionosphere

The density of the atmosphere decreases with height. The intensity of EUV solar radiation is very high above the F layer, but there are few molecules to be ionised. Because of EUV absorption at the F and E layers, the intensity of EUV radiation is very low below the E layer. As a result, there is an ionisation density peak at intermediate heights, as shown below.

The graph below is based on research from U.C.Berkeley Bob Brown, NM7M, Ph.D.
Plasma Density and height

The density and height of the ionosphere layers vary with the time of day, season, and location, which impacts the behavior of HF radio waves. For example, during the day, the ionosphere is more ionized and denser, which makes it more reflective to higher frequency radio waves. At night, the ionosphere becomes less dense and is more reflective to lower frequency radio waves. Similarly, the location of the transmitting and receiving stations can affect the angle at which the HF radio waves approach the ionosphere, which affects their reflection and propagation.

The Earth's atmosphere can also affect HF radio propagation in other ways. For example, atmospheric phenomena such as thunderstorms and precipitation can cause attenuation or scattering of HF radio waves. Similarly, the Earth Magnetic field can interact with the ionosphere and affect HF radio wave propagation during geomagnetic storms.

Illustration of Geonagnetic storms as seen from earth close to polar regions (public domain images):

Overall, understanding the behavior of the Earth's atmosphere and its effect on the ionosphere is critical for optimizing HF radio communication and ensuring reliable transmission over long distances.

Read more about The role of the ionosphere. See also an index for HF Radio Propagarion.

The Understanding HF Propagation Project provides radio amateurs with a detailed overview and tutorials on several aspects of HF propagation.

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