Ionosphere

The ionosphere is the part of the atmosphere that is ionized by solar radiation. It forms the inner edge of the magnetosphere and has practical importance because it influences high-frequency (HF) (3–30 MHz) radio propagation to distant places on the Earth.

Radio Application

DX communication, popular among amateur radio enthusiasts, is a term given to communication over great distances. When using High-Frequency bands, the ionosphere is utilized to reflect the transmitted radio beam. The beam returns to the Earth's surface, and may then be reflected back into the ionosphere for a second bounce.

Related Topics:
DX communication - Amateur radio

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Radio waves "hop" from the Earth to the ionosphere and back to the Earth. When a radio wave reaches the ionosphere, the electric field in the wave forces the electrons in the ionosphere into oscillation at the same frequency as the radio wave. Some of the radio wave energy is given up to this mechanical oscillation. The oscillating electron will then either be lost to recombination or will re-radiate the original wave energy back downward again. Total reflection can occur when the collision frequency of the ionosphere is less than the radio frequency, and if the electron density in the ionosphere is great enough.

Related Topics:
Radio - Hop - Electric field - Oscillation

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The critical frequency is the limiting frequency at or below which a radio wave is reflected by an ionospheric layer at vertical incidence. If the transmitted frequency is higher than the plasma frequency of the ionosphere, then the electrons cannot respond fast enough, and they are not able to re-radiate the signal. It is calculated as shown below:

Related Topics:
Critical frequency - Incidence

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: f{critical} = 9 imes 10^{-3} sqrt{N}

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where N = electron density per cm3 and fcritical is in MHz.

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The Maximum Usable Frequency (MUF) is defined as the upper frequency limit that can be used for transmission between two points at a specified time.

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: f{muf} = rac{f{critical}}{ sin{(I)}}

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where I = angle of attack, the angle of the wave relative to the horizon, and sin is the sine function.

Related Topics:
Angle of attack - Horizon - Sine

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The cutoff frequency is the frequency below which a radio wave fails to penetrate a layer of the ionosphere at the incidence angle required for transmission between two specified points by reflection from the layer.

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