Editing Shortwave radio (section)

==Propagation characteristics==
[[Image:Skip Zone Example.gif|thumb|right|Formation of a [[skip zone]]]]
Shortwave radio frequency energy is capable of reaching any location on the Earth as it is influenced by [[ionospheric reflection]] back to Earth by the [[ionosphere]], (a phenomenon known as "[[skywave]] propagation"). A typical phenomenon of shortwave propagation is the occurrence of a [[skip zone]] where reception fails. With a fixed working frequency, large changes in ionospheric conditions may create skip zones at night.

As a result of the multi-layer structure of the [[ionosphere]], propagation often simultaneously occurs on different paths, [[Scattering|scattered]] by the [[ionosphere#layers anchor|‘E’ or ‘F’&nbsp;layer]] and with different numbers of hops, a phenomenon that may be disturbed for certain techniques. Particularly for lower frequencies of the shortwave band, absorption of radio frequency energy in the lowest ionospheric layer, the [[D layer|‘D’&nbsp;layer]], may impose a serious limit. This is due to collisions of electrons with neutral molecules, absorbing some of a [[radio frequency]]'s energy and converting it to heat.<ref name=Rawer1993/> Predictions of skywave propagation depend on:
* The distance from the transmitter to the target receiver.
* Time of day. During the day, frequencies higher than approximately 12&nbsp;MHz can travel longer distances than lower ones. At night, this property is reversed.
* With lower frequencies the dependence on the time of the day is mainly due to the lowest ionospheric layer, the [[D layer|‘D’ Layer]], forming only during the day when photons from the sun break up atoms into ions and free electrons.
* Season. During the winter months of the Northern or Southern hemispheres, the AM/MW broadcast band tends to be more favorable because of longer hours of darkness.
* [[Solar flare]]s produce a large increase in [[D layer|D region]] ionization – so great, sometimes for periods of several minutes, that skywave propagation is nonexistent.