Editing Skip zone (section)

==Cause==
When using medium to high-frequency radio [[telecommunication]], there are [[radio wave]]s which travel both parallel to the ground, and towards the [[ionosphere]], referred to as a [[ground wave]] and [[sky wave]], respectively. A skip zone is an annular region between the farthest points at which the ground wave can be received and the nearest point at which the refracted sky waves can be received. Within this region, no signal can be received because, due to the conditions of the local ionosphere, the relevant sky waves are not reflected but penetrate the ionosphere.

The skip zone is a [[natural phenomenon]] that cannot be influenced by technical means. Its width depends on the height and shape of the ionosphere and, particularly, on the local ionospheric maximum electron density characterized by critical frequency f<sub>o</sub>F<sub>2</sub>. It varies mainly with this parameter, being larger for low f<sub>o</sub>F<sub>2</sub>. With a fixed working frequency it is large by night and may even disappear by day.  Transmitting at night is most effective for long-distance communication but the skip zone becomes significantly larger. 
[[Very high frequency]] waves and higher normally travel through the ionosphere wherefore communication via skywave is exceptional. A highly ionized Es-Layer that occasionally may appear in summer may produce such [[Sporadic E propagation]].