Skip distance is defined as the distance b/w transmitting antenna and a point where that wave is first received after reflection from ionosphere.

$f_{muf}= f_c \sqrt{1+\dfrac{D^2}{4h^2}} \\ \dfrac{f_{muf}}{f_c} =\sqrt{1+\dfrac{D^2}{4h^2}} \\ \bigg(\dfrac{f_{muf}}{f_c}\bigg)^2- 1=\dfrac{D^2}{4h^2} \\ D^2=4h^2 \bigg\{\bigg(\dfrac{f_{muf}}{f_c} \bigg)^2- 1\bigg\} \\ D_{Skip}=2h \sqrt{\bigg(\dfrac{f_{muf}}{f_c} \bigg)^2- 1}$