Editing High frequency (section)

==Propagation characteristics==
[[File:ICOM IC-M700PRO.jpg|thumb|A modern ''Icom M700Pro'' two-way radio for marine HF radio communications.]]

The dominant means of long-distance communication in this band is [[skywave]] ("skip") propagation, in which radio waves directed at an angle into the sky [[Refraction|refract]] back to Earth from layers of [[ionization|ionized]] atoms in the [[ionosphere]].<ref name="Seybold">{{cite book
 | last1  = Seybold
 | first1 = John S. 
 | title  = Introduction to RF Propagation
 | publisher = John Wiley and Sons
 | date   = 2005
 | pages  = 55–58
 | url    = https://books.google.com/books?id=4LtmjGNwOPIC&q=cross+polarization+discrimination&pg=PA57
 | isbn   = 0471743682
 }}</ref>  By this method HF radio waves can travel beyond the horizon, around the curve of the Earth, and can be received at intercontinental distances.  However, suitability of this portion of the spectrum for such communication varies greatly with a complex combination of factors:
*Sunlight/darkness at site of transmission and reception
*Transmitter/receiver proximity to solar [[Terminator (solar)|terminator]]
*[[Season]]
*[[Solar cycle|Sunspot cycle]]
*[[Sunspot|Solar activity]]
*[[Aurora|Polar aurora]]
At any point in time, for a given "skip" communication path between two points, the frequencies at which communication is possible are specified by these parameters:
*[[Maximum usable frequency]] (MUF)
*[[Lowest usable high frequency]] (LUF) and a
*[[Frequency of optimum transmission]] (FOT)
The maximum usable frequency regularly drops below 10&nbsp;MHz in darkness during the winter months, while in summer during daylight it can easily surpass 30&nbsp;MHz. It depends on the angle of incidence of the waves; it is lowest when the waves are directed straight upwards, and is higher with less acute angles. This means that at longer distances, where the waves graze the ionosphere at a very blunt angle, the MUF may be much higher. The lowest usable frequency depends on the absorption in the lower layer of the ionosphere (the D-layer). This absorption is stronger at low frequencies and is also stronger with increased solar activity (for example in daylight); total absorption often occurs at frequencies below 5&nbsp;MHz during the daytime. The result of these two factors is that the usable spectrum shifts towards the lower frequencies and into the [[Medium frequency|Medium Frequency]] (MF) range during winter nights, while on a day in full summer the higher frequencies tend to be more usable, often into the lower [[Very high frequency|VHF]] range.{{Citation needed|date=January 2012}}

When all factors are at their optimum, worldwide communication is possible on HF. At many other times it is possible to make contact across and between continents or oceans. At worst, when a band is "dead", no communication beyond the limited [[Surface wave#Electromagnetic waves|groundwave]] paths is possible no matter what powers, [[Antenna (radio)|antennas]] or other technologies are brought to bear. When a transcontinental or worldwide path is open on a particular frequency, [[Template:Digital modes|digital]], [[Single-sideband modulation|SSB]] and [[Morse code]] communication is possible using surprisingly low transmission powers, often of the order of milliwatts, provided suitable antennas are in use at both ends and that there is little or no [[Q code#Amateur radio|artificial]] or [[Q code#All Services (QRA–QUZ)|natural interference]].<ref>{{cite web 
|url=http://www.qrparci.org/content/view/58/118/
|title=Solar Activity & HF Propagation 
|author=Paul Harden
|publisher=QRP Amateur Radio Club International
|date=2005
|access-date=2009-02-22 }}</ref>  On such an open band, interference originating over a wide area affects many potential users.  These issues are significant to military, safety<ref>{{cite web|url=http://www.arrl.org/pio/emergen1.html |title=Amateur Radio Emergency Communication |publisher=American Radio Relay League, Inc. |date=2008 |access-date=2009-02-22 |url-status=dead |archive-url=https://web.archive.org/web/20090129111343/http://www.arrl.org/pio/emergen1.html |archive-date=January 29, 2009 }}</ref> and [[amateur radio]] users of the HF bands.

There is some propagation by [[ground wave]]s, the main propagation mode in the lower bands, but transmission distance decreases with frequency due to greater absorption in the earth.  At the top end of the band ground wave transmission distances are limited to 10-20 miles.<ref name="Carr">{{cite book
 | last1  = Carr
 | first1 = Joseph
 | last2  = Hippisley
 | first2 = George
 | title  = Practical Antenna Handbook, 5th Ed.
 | publisher = McGraw-Hill
 | date   = 2012
 | url    = https://wiki.la-com.org/lib/exe/fetch.php?media=technical:carr_j_j_hippisley_g_w_-_practical_antenna_handbook_5th_edition_-_2012.pdf
 | id     = 
 | isbn   = 9780071639590
 }}</ref>{{rp|p.38}}  Short range communication can occur by a combination of [[line-of-sight]] (LOC), ground bounce, and ground wave paths, but [[multipath propagation|multipath interference]] can cause [[fading]].