Ionospheric absorption
Ionospheric absorption is the primary limiting factor in radio propagation.
Ionospheric absorption (ISAB) describes the loss of signal strength that happens when electromagnetic waves interact with free electrons in the ionosphere. This interaction can disrupt radio transmissions.
When planning radio networks, telecommunication systems, or linked radio systems, understanding ionospheric absorption is essential—especially for predicting how signals will travel. The ionosphere is a layer of the atmosphere that reflects or refracts shortwave radio waves back to Earth, which is vital for long-distance communication. However, this reflection also reduces some of the signal's strength, making ISAB the main factor that limits radio propagation.
The absorption only occurs during the day, when radio signals pass through the sunlit part of the ionosphere. Solar wind and radiation charge the ionosphere with electrons. At night, the atmosphere loses this charge, allowing radio signals to travel much farther with less loss. Low-frequency signals, which are heavily attenuated during the day, can be received over great distances at night.
The amount of attenuation follows the inverse-square law: the lower the frequency, the greater the absorption. A riometer is used to measure relative ionospheric absorption.
- Primary limiting factor
- ISAB is the primary limiting factor in radio propagation
- Frequency dependence
- The lower the frequency, the greater the attenuation
- Measurement instrument
- Relative ionospheric absorption can be measured using a riometer
- Attenuation law
- The specific amount of attenuation can be derived as a function of the Inverse-square law
Lore & Background
The ionosphere can be described as an area of the atmosphere in which radio waves on shortwave bands are refracted or reflected back to Earth. As a result of this reflection, which is often very crucial in the long-distance propagation of radio waves, some of the shortwave signal strength is decreased. ISAB is only a factor in the period of the day where radio signals travel through the portion of the ionosphere facing the Sun. The solar wind and radiation cause the ionosphere to become charged with electrons. At night, the atmosphere becomes drained of its charge, and radio signals can go much further with less loss of signal. In particular, low frequency signals that would be attenuated to nothing during the day will be received much further away at night.
Reader's Guide
Ionospheric absorption is of critical importance when radio networks, telecommunication systems or interlinked radio systems are being planned, particularly when trying to determine propagation conditions. The specific amount of attenuation can be derived as a function of the Inverse-square law, with lower frequencies experiencing greater attenuation. Relative ionospheric absorption can be measured using a riometer. The phenomenon is only a factor during the daytime when radio signals travel through the portion of the ionosphere facing the Sun, as solar wind and radiation charge the ionosphere with electrons. At night, the atmosphere becomes drained of its charge, allowing radio signals to travel much further with less loss of signal. Low frequency signals that would be attenuated to nothing during the day will be received much further away at night. This diurnal variation makes ISAB a primary limiting factor in radio propagation, especially for long-distance shortwave communications.
Did You Know?
- Ionospheric absorption is only a factor during the daytime when signals travel through the sunlit portion of the ionosphere.
- Low frequency signals that are attenuated to nothing during the day can be received much further away at night.
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