Ionospheric absorption
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Ionospheric absorption (or ISAB) is the scientific name for absorption occurring as a result of the interaction between various types of electromagnetic waves and the free electrons in the ionosphere, which can interfere with radio transmissions.
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[edit] Description
Ionosphere absorption is of critical importance when radio networks, telecommunication systems or interlinked radio systems are being planned, particularly when trying to determine propagation conditions. [1]
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 key in the long-distance propagation of radio waves,some of the shortwave signal strength is decreased. In this regard, ISAB is the primary limiting factor in radio propagation. [2]
[edit] Causes
(See ionosphere for more information on the D Layer)
Roughly 70% of any possible signal attenuation associated with ISAB is believed to happen in the D layer of the ionosphere. The remaining 30% attenuation that is seen transpires in the E and F layers, but this varies in terms of signal loss and is often insignificant next to the amount of attenuation in the D layer. Signal attenuation is measured in potential loss in dB.[2]
A signal enters the D layer, which is suffused with free electrons. As the signals propagate through the layer, they begin to transfer energy to these electrons, setting them in motion. This results in the electrons vibrating , which can cause them to collide with other molecules, ions, or electrons. [3]
Collisions can happen many times per second, and each one dissipates a small amount of signal strength as energy is transferred from wave to particle. The specific amount of energy, and thus signal strength, that is lost depends on a number of factors:
- The number of collisions. The more collisions, the more signal attenuation.[1]
- The frequency of the signal. As this decreases, the wavelength becomes longer, and more collisions take place. A lower wavelength transmits less energy, but there is less energy to retain signal cohesion with as well.
- The number of particles. This is mostly a factor in the lower layers of the D layer, where density and pressure of the atmosphere is higher.[3]
[edit] Attenuation mechanics
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 in the first place. At night, the atmosphere becomes drained of its charge, and radio signals can go much further with less loss of signal. In particular, low wavelength signals that would be attenuated to nothing during the day will be received much further away at night.[4][5]
The specific amount of attenuation can be derived as a function of the inverse square-law. The lower the frequency, the greater the attenuation. [2]
[edit] See also
[edit] Resources
- ^ a b Walker, J.K. ; Bhatnagar, V.P. Ionospheric absorption, typical ionization, conductivity, and possible synoptic heating parameters in the upper atmosphere. Geological Survey of Canada.http://www.osti.gov/energycitations/product.biblio.jsp?osti_id=6068491
- ^ a b c Davies, Kenneth. Ionospheric Radio. ISBN 086341186X
- ^ a b R. D. Hunsucker, J.K. Hargreaves. The high-latitude ionosphere and its effects on radio propagation.2003 Cambridge University Press. ISBN 0521330831
- ^ John Keith Hargreaves. The Solar-Terrestrial Environment: An Introduction to Geospace. 1992 Cambridge University Press. ISBN 0521427371
- ^ Leo F. McNamara. (1994) ISBN 0-89464-804-7 Radio Amateurs Guide to the Ionosphere.
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