Skywave 276281 215397063 2008-05-28T00:30:29Z 69.140.237.119 /* Role of the ionosphere */ '''Skywave''' is the [[Radio propagation|propagation]] of [[radio waves|electromagnetic waves]] bent (refracted) back to the Earth's surface by the [[ionosphere]]. As a result of skywave propagation, a [[Broadcasting|broadcast]] signal from a distant [[AM broadcasting]] station at night, or from a [[shortwave radio]] station (or during [[Sporadic E propagation|sporadic e season]], a low band [[TV station]]) can sometimes be heard as clearly as local stations. Most long-distance [[High frequency|HF]] radio communication (between 3 and 30 MHz) is a result of skywave propagation. Since the early 1920s [[Amateur radio]] operators, limited to lower transmitter power than commercial radio, have taken advantage of skywave for long distance or [[DX communication]]. == Role of the ionosphere == The ionosphere is a region of the upper [[Earth's atmosphere|atmosphere]], where neutral air is [[ion]]ized by solar [[photon]]s and [[cosmic ray]]s. 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 [[Refraction|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 (radio)|antenna]], signals below approximately 10 MHz during the day and 5 MHz at night may reach the ionosphere at a steep angle (vertical incidence) and be reflected almost straight back down to Earth. Alternately, if the antenna "aims" the signal close to the horizon; the signal reaches the ionosphere at a shallow angle and returns to Earth at a medium to long distance. The Earth's surface (ground or water) reflects 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 two or more times. This phenomenon is known as [[multihop propagation]]. Signals of only a few watts can sometimes be received many thousands of miles away as a result. [[Image:Atmospheric electromagnetic transmittance or opacity.jpg|thumb|300px|Rough plot of Earth's atmospheric transmittance (or opacity) to various wavelengths of electromagnetic radiation, including radio waves.]] == Other considerations == [[Very high frequency|VHF]] signals with frequencies above about 30 MHz usually penetrate the ionosphere and are not returned to the Earth's surface. [[E-skip]] is a notable exception, where VHF signals including FM broadcast and VHF TV signals are frequently reflected to the Earth during late Spring and early Summer. E-skip rarely affects [[UHF]] frequencies, except for very rare occurrences below 500 MHz. [[frequency|Frequencies]] below approximately 10 MHz (wavelengths longer than 30 meters), including broadcasts in the [[mediumwave]] and [[shortwave]] bands (and to some extent [[longwave]]), propagate most efficiently by skywave at night. Frequencies above 10 MHz (wavelengths shorter than 30 meters) typically propagate most efficiently 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. Skywave propagation is usually degraded -- sometimes seriously -- during [[geomagnetic storm]]s. Skywave propagation on the sunlit side of the Earth can be entirely disrupted during [[Sudden Ionospheric Disturbance|sudden ionospheric disturbances]]. 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 ofthe Earth at night. This leads to an increase in the "skip" or "hop" distance of the skywave at night. ==See also== {{col-begin}} {{col-2}} *[[Radio propagation]] *[[MW DX]] *[[TV-FM DX]] *[[Near Vertical Incidence Skywave]] *[[F region | F-layer]] *[[Over-the-horizon radar]] *[[Groundwave]] *[[Skip (radio)]] *[[Skip zone]] {{col-2}} *[[Radio frequency]] *[[Clear channel]] *[[Tropospheric ducting]] *[[Geomagnetic storm]] *[[History of radio]] *[[Amateur radio history]] *[[List of electronics topics]] {{col-end}} == External links == * [http://www.radionetherlands.nl/features/media/practical/mwpropagation.html Radio Netherlands: Long Distance MW Listening: Propagation] * [http://www.skywaves.info Skywaves: Long Distance (DX) radio and television reception. Also medium wave DX, propagation, aerials, etc. for the hobbyist.] * [http://www.fas.org/man/dod-101/navy/docs/es310/propagat/Propagat.htm Navy - Propagation of Waves] * [http://www.lerc.nasa.gov/WWW/MAEL/ag/agprop1.htm NASA - Types of Propagation] * [http://www.eham.net/DX/propagation?ehamsid=06a1a6696bb180ea3e70b99ababe7032 EHamNet propagation page] * [http://ecjones.org/propag.html Radio wave propagation basics] * [http://www.dxlc.com/solar Solar Terrestrial Activity Report] * [http://www.hfradio.org/forums/index.php?c=2 HFRadio Propagation forums] * [http://www.eurekalert.org/pub_releases/2006-02/su-rgf021706.php Rare gamma-ray flare disturbed ionosphere] * [http://www.df5ai.net/Material/articles.html Articles on sporadic E and 50 MHz Radio Propagation] [[Category:Radio frequency propagation]] [[Category:Broadcast engineering]] [[de:Raumwelle]]