# Critical frequency

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.

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.

## Critical Frequency and F layer of the Ionosphere

• All long-distance HF Radio Communications use HF Radio signals that are obliquely incident on the ionosphere, If the HF frequency is above Critical Frequency, the radio signals are passing through the ionosphere at an angle instead of head-on.
• The Critical Frequency is changing continuously and the F layer of the Ionosphere is the mostly responsible for the refraction of radio waves back to Earth,
• The other layers(D) interact in other ways - absorption of frequency and During the day, the D Layers forms, and the F layer splits into F1 and F2 layers.
• Because of changing the Ionosphere during day and night, during daytime higher frequency bands under critical Frequency work best, but during nighttime the lower frequency bands work best.
• The D layer is present during the day and it is a good absorber of radio waves, increasing losses, Higher frequencies are absorbed less, so higher frequencies tends to perform better during daytime.

• The actual F2-Layer Critical Frequency Map link below

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

• The Ionosphere and the Practical Maximum Usable Frequencies (MUFs) Map link below

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