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Explain following terms - Critical frequency, Virtual height, Maximum usable frequency.

Mumbai University > Electronics Engineering > Sem 5 > Electromagnetic Engineering

Marks: 7 & 10 Marks

Year: May 2015, May 2016

1 Answer
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i) Critical frequency:

It is that highest frequency radio wave, which sent normally towards layer of ionosphere gets reflected back & returns to the earth.

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$f α \sqrt{N} \\ f= g \sqrt{N} \\ f= g(N_{\max} )^{\frac{1}{2}}$

where, N → free electron density

ii) Virtual height:

The apparent height of a layer in the ionosphere, determined from the time required for a radio pulse to travel to the layer and return, assuming that the pulse propogates at speed of light. It is also known as equivalent height.

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Where,

H→ virtual height

h → Actual height of Reflected wave.

iii) Maximum usable frequency (MUF):

It is that highest frequency of radio waves which when sent at some angle towards ionosphere, gets reflected and returns to the surface of the earth.

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$MUF=\dfrac{f_c}{\cos ⁡i} \\ MUF=f_c.\sec⁡(i) \\ Also, MUF=f_c \sqrt{1-\dfrac{D^2}{4h^2}}$

Where , $\text{D → skip distance} \\ \text{i → Angle between normal and direction of incident wave.}$

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