Critical frequency

In telecommunication, the term critical frequency has the following meanings:

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.

It is given by:

${\displaystyle f_{c}=9{\sqrt {Nmax}}}$

where Nmax is maximum electron density.

http://www.spacew.com/www/fof2.html (refresh every five minutes)

http://www.sws.bom.gov.au/HF_Systems/6/9/1 (refresh every five minutes)

 This article incorporates public domain material from the General Services Administration document "Federal Standard 1037C" (in support of MIL-STD-188).

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