Skywave

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Illustration of reflection of radio waves by the Earth's upper atmosphere. Note that the outgoing (in red) signal from point A is partly reflected by the F1-layer, then refracted by the E and D layers to reach point B.

Skywave is the propagation of radio waves bent (refracted) back to the Earth's surface by the ionosphere. As a result of skywave propagation, a night-time broadcast signal from a distant AM broadcasting or shortwave radio station (or rarely, a TV station) can sometimes be heard as clearly as local stations. Most long-distance HF radio communication (between 3 and 30 MHz) is a result of skywave propagation. For decades Amateur radio operators, limited to lower transmit power than commercial radio, have taken advantage of skywave for distance or DX communication.

1 Role of the ionosphere
2 Other considerations
3 See also
4 External links

[edit] Role of the ionosphere

The ionosphere is a region of the upper atmosphere, where neutral air is ionized by solar photons and cosmic rays. When radio waves reach the ionosphere at a shallow angle, they are partly reflected by the surface. The ionosphere can also be similar to a prism refracting light; different frequencies are "bent" by different amounts.

Much as the surface of the ocean interacts with the wind, the condition of the ionosphere is constantly changing due to interaction with incoming radiation. When signals have "bounced" off this irregular surface, they may fade in and out and have the "phasing", "flanging" or "fluttery" character familiar to listeners of shortwave music broadcasts.

Depending on the transmitting antenna, the signals may reach the ionosphere at a steep angle (vertical incidence) and be reflected almost straight down. Alternately the antenna may "aim" the signal at the horizon; the signal reaches the ionosphere at a shallow angle and returns to earth at a great distance.

Under some conditions, the Earth's surface (ground or water) may reflect the incoming wave back toward the ionosphere again. As a result, like a rock "skipping" across water, the wave may actually "bounce" or "skip" between the earth and ionosphere several times. This phenomenon is known as "skip" or multihop propagation. Signals of only a few watts can sometimes be received thousands of miles away as a result.

Rough plot of Earth's atmospheric transmittance (or opacity) to various wavelengths of electromagnetic radiation, including radio waves.

[edit] Other considerations

Signals with frequencies above about 30 MHz (VHF and UHF for example) are progressively not returned to the Earth's surface, because they penetrate the ionosphere. (This includes most communications with spacecraft and satellites.) Exceptions include rare occasions of E-skip, when FM and TV signals are reflected. Skywave may be disrupted during geomagnetic storms.

Low to mid frequencies below approximately 10 MHz (longer than 30 meters), including broadcasts in the mediumwave and shortwave bands (and to some extent longwave), travel most efficiently by skywave at night. Frequencies above 10 MHz (shorter than 30 meters) travel better during the day. Frequencies lower than 3 kHz have a wavelength longer than the distance between the Earth and the ionosphere. The Maximum usable frequency for skywave propagation is strongly influenced by sunspot number.

Because the lower-altitude layers (the E-layer in particular) of the ionosphere largely disappear at night, the refractive layer of the ionosphere is much higher above the surface at night. This leads to an increase in the "skip" or "hop" distance of the skywave at night.