Question

In sky-wave propagation, skip distance depends on
(A) Frequency of EM wave transmitted
(B) Critical frequency of layer
(C) Height of layer above the earth’s surface
(D) All of the above

Answer 1

Hint We will see that skip-distance is the distance between the ionosphere and the layer above earth’s surface. It depends on various factors such as frequency, critical frequency and height.

Complete step by step answer
Sky wave: It refers to the signals that travel from earth’s surface towards the ionosphere.
Skip-distance: Minimum distance on the earth’s surface where an EM wave of definite frequency can reach the ionosphere via reflection from the transmitter. It is also known as the dead zone or silent zone. We don’t receive radio transmission.
Formula
${D_{skip}} = 2h\sqrt {\dfrac{{{f^2}}}{{f_c^2}} - 1} $
It depends on various factors -
 Frequency (f): As the wave travels from the lower portion, it starts forming an angle. Lower angle starts increasing as frequency starts to increase and returns back from the ionosphere in shorter duration in the beginning. As frequency starts increasing and reaches maximum, it starts showing reflections and rare-fraction by the region of ionosphere.
More the frequency more will skip distance and vice versa.
Critical frequency $\left( {{f_c}} \right)$:
This wave is reflected back and receiver receives on the same site of the transmitter.
Height (h): Distance between maximum point and minimum point.

Since it satisfies all 3 options above, so option D is correct.

Note
We need to keep in mind that there is a critical difference in frequency and frequency. More the critical frequency more is the skip distance whereas more the critical frequency less will be the skip-distance.