In telecommunication, the term critical frequency has the following meanings:
- In radio propagation by way of the ionosphere, the limiting frequency at or below which a wave component is reflected by, and above which it penetrates through, an ionospheric layer.
- At near vertical incidence, the limiting frequency at or below which incidence, the wave component is reflected by, and above which it penetrates through, an ionospheric layer.
Critical Frequency changes with time of day, atmospheric conditions and angle of fire of the radio waves by antenna.
The existence of the critical frequency is the result of electron limitation, i.e., the inadequacy of the existing number of free electrons to support reflection at higher frequencies.
Critical frequency is the highest magnitude of frequency above which the waves penetrates the ionosphere and below which the waves are reflected back from the ionosphere. It is denoted by "fc". Its value is not fixed and it depends upon electron density of ionosphere.
It is given by: fc=9√Nmax Nmax is maximum electron density.
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