What is meant by critical frequency for sky wave propagation? Write an expression for the critical frequency interms of the maximum electorn density of the ionosphere. What is the range of variation of frequency for this critical frequency?

Explanation

In sky wave propagation, critical frequency is
the highest frequency of radio waves which
when sent straight towards the layer of
ionosphere gets reflected from ionosphere and
returns to earth. The radio waves of more
than critical frequency are not reflected by
ionosphere but penetrate ionosphere. The
expression for critical frequency `v_c` of a sky wave
for reflection from a layer of ionosphere is given
by `v_c = 9(N_(max))^(1/2)`
where `N_(max)` is the maximum number density of
electorns in the layer of ionosphere in per `m^3`.
The value of `v_c` ranges form 0.3 MHz to 8 MHz.
for the layers D,E,`F_1 and F_2` of ionosphere.