Editing Skywave (section)

==Local and distant skywave propagation==

Skywave transmissions can be used for long-distance communications (DX) by waves directed at a low angle as well as relatively local communications via nearly vertically directed waves ([[Near vertical incidence skywave|near vertical incidence skywaves – NVIS]]).

===Low-angle skywaves===
[[File:PSKReporter Skip Example.jpg|thumb|Example of Skywave Propagation taken from [[PSK Reporter]].{{clarify|reason=What are "mins"?|date=February 2023}}|300x300px]]

The ionosphere is a region of the upper [[Earth's atmosphere|atmosphere]], from about 80&nbsp;km (50 miles) to 1000&nbsp;km (600 miles) in altitude, where neutral air is [[ion]]ized by solar [[photon]]s, [[solar particle event|solar particles]], and [[cosmic ray]]s. When [[high-frequency]] signals enter the ionosphere at a low angle they are bent back towards the Earth by the ionized layer.<ref>{{cite book |publisher=Sony Corporation |year=1998 |title=Wave Handbook |page=14 |oclc=734041509}}</ref> If the peak [[ionization]] is strong enough for the chosen frequency, a wave will exit the bottom of the layer earthwards – as if obliquely [[Reflection (physics)|reflected]] from a mirror. Earth's surface (ground or water) then [[Diffuse reflection|reflects]] the descending wave back up again towards the ionosphere.

When operating at frequencies just below the [[maximum usable frequency]], losses can be quite small, so the radio signal may effectively "bounce" or "skip" between the Earth and ionosphere two or more times (multi-hop propagation), even following the curvature of the Earth.  Consequently, even signals of only a few Watts can sometimes be received many thousands of miles away. This is what enables [[shortwave]] broadcasts to travel all over the world. If the ionization is not great enough, the wave only curves slightly downwards, and subsequently upwards as the ionization peak is passed so that it exits the top of the layer only slightly displaced. The wave is then lost in space. To prevent this, a lower frequency must be chosen. With a single "hop", path distances up to 3500&nbsp;km (2200 miles) may be reached. Longer transmissions can occur with two or more hops.<ref>{{cite book |author=Rawer, K. |title=Wave Propagation in the Ionosphere |publisher=Kluwer Academic Publications |location=Dordrecht |year=1993 |isbn=0-7923-0775-5}}</ref>

===Near-vertical skywaves===

Skywaves directed almost vertically are referred to as [[Near vertical incidence skywave|''near-vertical-incidence skywaves'' (''NVIS'')]]. At some frequencies, generally in the lower [[shortwave]] region, the high angle skywaves will be reflected directly back towards the ground. When the wave returns to ground it is spread out over a wide area, allowing communications within several hundred miles of the transmitting antenna. NVIS enables local plus regional communications, even from low-lying valleys, to a large area, for example, an entire state or small country. Coverage of a similar area via a line-of-sight VHF transmitter would require a very high mountaintop location. NVIS is thus useful for statewide networks, such as those needed for emergency communications.<ref>{{cite book |editor=Silver, H.L. |year=2011 |title=The ARRL Handbook for Radio Communications |edition=88th |publisher=American Radio Relay League |location=Newington, CT}}</ref> In short wave broadcasting, NVIS is very useful for regional broadcasts that are targeted to an area that extends out from the transmitter location to a few hundred miles, such as would be the case in a country or language group to be reached from within the borders of that country. This will be much more economical than using multiple FM (VHF) or AM broadcast transmitters. Suitable antennas are designed to produce a strong lobe at high angles. When short range skywave is undesirable, as when an AM broadcaster wishes to avoid interference between the ground wave and sky wave, [[anti-fading antennas]] are used to suppress the waves being propagated at the higher angles.

===Intermediate distance coverage===

[[Image:Antenna Vertical Angle vs 1 Hop Distance.png|right|thumb|350px|Antenna vertical angle required vs distance for skywave propagation]] For every distance, from local to maximum distance transmission, (DX), there is an optimum "take off" angle for the antenna, as shown here. For example, using the F layer during the night, to best reach a receiver 500 miles away, an antenna should be chosen that has a strong lobe at 40 degrees elevation. One can also see that for the longest distances, a lobe at low angles (below 10 degrees) is best. For NVIS, angles above 45 degrees are optimum.  Suitable antennas for long distance would be a high Yagi or a rhombic; for NVIS, a dipole or array of dipoles about .2 wavelengths above ground; and for intermediate distances, a dipole or Yagi at about .5 wavelengths above ground. Vertical patterns for each type of antenna are used to select the proper antenna.

===Fading===

At any distance sky waves will fade. The layer of ionospheric [[Plasma (physics)|plasma]] with sufficient ionization (the reflective surface) is not fixed, but undulates like the surface of the ocean.  Varying reflection efficiency from this changing surface can cause the reflected signal strength to change, causing "''[[fading]]''" in shortwave broadcasts.  Even more serious [[selective fading|fading]] can occur when signals arrive via two or more paths, for example when both single-hop and double-hop waves interfere with other, or when a skywave signal and a ground-wave signal arrive at about the same strength. This is the most common source of fading with nighttime AM broadcast signals.
Fading is always present with sky wave signals, and except for digital signals such as [[Digital Radio Mondiale]] seriously limit the fidelity of shortwave broadcasts.