A skip zone, also called a silent zone or zone of silence, is a region where a radio transmission can not be received located between regions both nearer and further from the transmitter where reception is possible.

When using medium to high frequency radio telecommunication

Telecommunication

Telecommunication is the transmission of information over significant distances to communicate. In earlier times, telecommunications involved the use of visual signals, such as beacons, smoke signals, semaphore telegraphs, signal flags, and optical heliographs, or audio messages via coded...

, there are radio waves which travel both parallel to the ground, and towards the ionosphere

Ionosphere

The ionosphere is a part of the upper atmosphere, comprising portions of the mesosphere, thermosphere and exosphere, distinguished because it is ionized by solar radiation. It plays an important part in atmospheric electricity and forms the inner edge of the magnetosphere...

, referred to as a ground wave and sky wave, respectively. A skip zone is an annular region between the furthest 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 actual 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 foF2. It varies mainly with this parameter, being larger for low foF2. 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 an example.

Another method of decreasing the skip zone is by decreasing the frequency of the radio waves. Decreasing the frequency is akin to increasing the ionospheric width. A point is eventually reached when decreasing the frequency results in a zero distance skip zone. In other words, a frequency exists for which vertically incident radio waves will always be refracted back to the Earth. This frequency is equivalent to the ionospheric plasma frequency and is also known as the ionospheric critical frequency, or f_oF₂.

There is a place in Mexico called the "Zone of Silence" also known as the Vertice de Trino: the place where the borders of the Mexican states of Durango, Chihuahua and Coahuila join between the twenty-sixth and the twenty-eighth parallel.

Skip zone is the subject of a film 'SKIPZONE' made in 1992 by UK artist, Peter Lee-Jones. It refers to areas in Scottish Highlands where it is difficult to obtain radio and TV reception.