User:Langlock/sandbox

In high frequency (HF) or skywave radio communications, the skip distance is the minimum possible ground range that can be achieved by a radio wave of a given frequency following specular reflection by the ionosphere. The region between the radio transmitter and the skip distance is known as the skip zone; it is not possible to detect the transmitted signal within this region. The skip distance depends on both the frequency of the transmitted radio wave and the local electron density profile of the ionosphere at the time of communication. For this reason, skip distances are generally larger at night-time due to the lower ionospheric electron density, and also vary with the seasons and with the solar cycle.

Reflection of Radio Waves
It can be shown from geometrical optics that a radio wave will be reflected by the ionosphere provided that the following condition is satisfied:
 * $$ \Delta \leq \arccos{\sqrt{1-(f_c/f)^2}} $$

where When the take-off angle $$ \Delta $$ does not satisfy the above condition, the signal penetrates the ionosphere and cannot be detected by the receiver.
 * $$ \Delta $$ is the vertical take-off angle of the signal, measured from the horizontal;
 * $$f$$ is the frequency of the transmitted wave;
 * $$f_c$$ is the vertical critical frequency of the ionosphere.

Propagation Path
Radio waves from a particular transmitting antenna do not all get refracted by a particular layer of the ionosphere; some are absorbed, some refracted while a portion escapes to the next layer. At this higher layer, there is a possibility of this radio wave being bent downwards to earth again. This bending happens because each layer of the ionosphere has a refractive index that varies from that of the others. Because of the differing heights of refraction, or apparent reflection, the radio waves hit the earth surface at different points hence generating the skip distance. Skip distance is greatest during the night when the ionosphere is the highest.