Editing Critical frequency (section)

In [[telecommunications]], the term '''critical frequency''' has the following meanings:<ref>{{Cite web|url=https://www.its.bldrdoc.gov/fs-1037/dir-009/_1324.htm|title=Definition: critical frequency|website=www.its.bldrdoc.gov|access-date=2018-09-13}}</ref>

* In [[radio propagation]] by way of the [[ionosphere]], the [[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 (electrical)|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 penetrate the ionosphere and below which the waves are reflected back from the ionosphere.
It is denoted by "''f''<sub>c</sub>".
Its value is not fixed and it depends upon the electron density of the ionosphere.