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 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.
In signal processing the critical frequency it is also another name for the Nyquist frequency.
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.
Famous quotes containing the words critical and/or frequency:
“I know that I will always be expected to have extra insight into black texts—especially texts by black women. A working-class Jewish woman from Brooklyn could become an expert on Shakespeare or Baudelaire, my students seemed to believe, if she mastered the language, the texts, and the critical literature. But they would not grant that a middle-class white man could ever be a trusted authority on Toni Morrison.”
—Claire Oberon Garcia, African American scholar and educator. Chronicle of Higher Education, p. B2 (July 27, 1994)
“One is apt to be discouraged by the frequency with which Mr. Hardy has persuaded himself that a macabre subject is a poem in itself; that, if there be enough of death and the tomb in one’s theme, it needs no translation into art, the bold statement of it being sufficient.”
—Rebecca West (1892–1983)