Super high frequency

{{Short description|Range 3-30 GHz of the electromagnetic spectrum}} {{MWband | freq = 3 to 30 [[Hertz|GHz]] | wave = 10 to 1 [[Centimetre|cm]] | bands = {{startplainlist}} * {{hlist|class=nowrap |[[F band (NATO)|F]] |[[G band (NATO)|G]] |[[H band (NATO)|H]] |[[I band (NATO)|I]] |[[J band (NATO)|J]] |[[K band (NATO)|K]] (NATO)}} * {{hlist|class=nowrap |[[S band|S]] |[[C band (IEEE)|C]] |[[X band|X]] |[[Ku band|Ku]] |[[K band (IEEE)|K]] |[[Ka band|Ka]] (IEEE)}} {{endplainlist}} }}

'''Super high frequency''' ('''SHF''') is the [[International Telecommunication Union|ITU]] designation<ref name="1037B">{{cite book | title = US Federal Standard 1037B: Telecommunications, Glossary of Telecommunications Terms | publisher = Office of Technology Standards, General Services Administration | date = 3 June 1991 | pages = S-18 | url = https://books.google.com/books?id=zYLXU4fkD34C&pg=RA15-PA18}}</ref><ref name="itu-2015-acts">{{cite conference |date=2015 |title=Final Acts WRC-15 |url=https://www.itu.int/dms_pub/itu-r/opb/act/R-ACT-WRC.12-2015-PDF-E.pdf |conference=World Radiocommunication Conference |location=Geneva, Switzerland |publisher=International Telecommunications Union |pages = 4|access-date=2025-01-12}}</ref> for [[Radio frequency|radio frequencies]] (RF) in the range between 3 and 30 [[gigahertz]] (GHz). This band of frequencies is also known as the '''centimetre band''' or '''centimetre wave''' as the [[wavelength]]s range from one to ten centimetres. These frequencies fall within the [[microwave]] band, so radio waves with these frequencies are called microwaves. The small wavelength of microwaves allows them to be directed in narrow beams by [[antenna aperture|aperture antennas]] such as [[Parabolic antenna|parabolic dish]]es and [[horn antenna]]s, so they are used for [[Point-to-point (telecommunications)|point-to-point]] communication and data links<ref name="Freedman">{{cite journal |last=Freedman |first=S. |title=Two-way radio for everyone |journal=Radio News |volume=36 |issue=3 |pages=25–27 |publisher=Ziff-Davis Publications |location=New York |date=September 1946 |url=https://www.worldradiohistory.com/Archive-Radio-News/40s/Radio-News-1946-09.pdf |access-date=March 24, 2014}} This article from the beginning of the microwave era predicted the future value of microwaves for point-to-point communication.</ref> and for [[radar]]. This frequency range is used for most [[radar]] transmitters, [[wireless LAN]]s, [[satellite communication]], [[microwave radio relay]] links, [[satellite phone]]s ([[S band]]), and numerous short range terrestrial data links. They are also used for heating in industrial [[microwave heating]], medical [[diathermy]], [[hyperthermy|microwave hyperthermy]] to treat cancer, and to cook food in [[microwave oven]]s.

==Propagation== [[Image:Parabolic antennas.JPG|thumb|upright=1.6|A variety of [[parabolic antenna]]s on a [[Radio masts and towers|communications tower]] in [[Australia]] for [[point-to-point (telecommunications)|point-to-point]] [[Microwave transmission|microwave communication]] links. Some have white plastic [[radome]]s over their apertures to protect against [[rain]]. ]] [[Image:Radar antennas on USS Theodore Roosevelt SPS-64.jpg|thumb|upright=1.3|[[X-band]] (8 - 12 GHz) [[marine radar]] antenna on a [[ship]]. The rotating bar sweeps a vertical fan-shaped beam of microwaves around the water surface to the horizon, detecting nearby ships and other obstructions]]

[[Microwave]]s propagate solely by [[Line-of-sight propagation|line of sight]]; because of the small [[refraction]] due to their short wavelength, the [[groundwave]] and ionospheric reflection ([[skywave]] or "skip" propagation) seen with lower frequency radio waves do not occur.<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&dq=cross+polarization+discrimination&pg=PA57 | isbn = 0471743682 }}</ref> Although in some cases they can penetrate building walls enough for useful reception, unobstructed rights of way cleared to the first [[Fresnel zone]] are usually required. Wavelengths are small enough at microwave frequencies that the antenna can be much larger than a wavelength, allowing highly [[directional antenna|directional]] (high [[antenna gain|gain]]) [[antenna (radio)|antenna]]s to be built which can produce narrow beams. Therefore, they are used in [[point-to-point (telecommunications)|point-to-point]] terrestrial communications links, limited by the visual horizon to 30–40 miles (48–64 km). Such [[high gain antenna]]s allow [[frequency reuse]] by nearby transmitters. They are also used for communication with [[spacecraft]] since the waves are not [[refraction|refracted]] (bent) when passing through the [[ionosphere]] like lower frequencies.

The [[wavelength]] of SHF waves creates strong reflections from metal objects the size of automobiles, aircraft, and ships, and other vehicles. This and the narrow [[beamwidth]]s possible with high gain antennas and the low atmospheric attenuation as compared with higher frequencies make SHF the main frequencies used in [[radar]]. Attenuation and scattering by moisture in the atmosphere increase with frequency, limiting the use of high SHF frequencies for long range applications.

Small amounts of microwave energy are randomly scattered by water vapor molecules in the [[troposphere]]. This is used in [[troposcatter]] communications systems, operating at a few GHz, to communicate beyond the horizon. A powerful microwave beam is aimed just above the horizon; as it passes through the [[tropopause]] some of the microwaves are scattered back to Earth to a receiver beyond the horizon. Distances of 300 km can be achieved. These are mainly used for military communication.

==Antennas== [[Image:Diplexer1.jpg|thumb|Microwaves are often carried by [[waveguide]], such as this example from an [[air traffic control]] [[radar]], since other types of cable have large power losses at SHF frequencies.]]

The [[wavelength]] of SHF waves is short enough that efficient transmitting antennas are small enough to be conveniently mounted on handheld devices, so these frequencies are widely used for wireless applications. For example a [[whip antenna|quarter wave whip]] antenna for the SHF band is between 2.5 and 0.25 centimeters long. [[Omnidirectional antenna]]s have been developed for applications like wireless devices and [[cellphone]]s that are small enough to be enclosed inside the device's case. The main antenna used for these devices is the printed [[inverted F antenna]] (PIFA) consisting of a [[monopole antenna]] bent in an L shape, fabricated of copper foil on the [[printed circuit board]] inside the device. Small [[dipole antenna|sleeve dipoles]] or [[monopole antenna|quarter-wave monopoles]] are also used. The [[patch antenna]] is another common type, often integrated into the skin of aircraft.

The wavelengths are also small enough that SHF waves can be focused into narrow beams by [[high gain antenna|high gain]] [[directional antenna]]s from a half meter to five meters in diameter. Directive antennas at SHF frequencies are mostly [[aperture (antenna)|aperture antennas]], such as [[parabolic antenna]]s (the most common type), [[lens antenna|lens]], [[slot antenna|slot]] and [[horn antenna]]s. Large parabolic antennas can produce very narrow beams of a few degrees or less, and often must be aimed with the aid of a [[Antenna boresight|boresight]]. Another type of antenna practical at microwave frequencies is the [[phased array]], consisting of many dipoles or [[patch antenna]]s on a flat surface, each fed through a [[phase shifter]], which allows the array's beam to be steered electronically. The short wavelength requires great mechanical rigidity in large antennas, to ensure that the radio waves arrive at the feed point in phase.

===Waveguide=== At microwave frequencies, the types of cable ([[transmission line]]) used to conduct lower frequency radio waves, such as [[coaxial cable]], have high power losses. Therefore, to transport microwaves between the transmitter or receiver and the antenna with low losses, a special type of metal pipe called [[waveguide (electromagnetism)|waveguide]] must be used. Because of the high cost and maintenance requirements of long waveguide runs, in many microwave antennas the output stage of the transmitter or the [[RF front end]] of the receiver is located at the antenna.

==Advantages== SHF frequencies occupy a "sweet spot" in the [[radio spectrum]] which is currently being exploited by many new radio services.<ref name="Lee">{{cite book | last1 = Lee | first1 = Thomas H. | title = Planar Microwave Engineering: A Practical Guide to Theory, Measurement, and Circuits | publisher = Cambridge University Press | date = 2004 | pages = 27 | url = https://books.google.com/books?id=cnhhBAAAQBAJ&pg=PA27 | isbn = 1316175774 }}</ref> They are the lowest frequency band where radio waves can be directed in narrow beams by conveniently sized antennas so they do not interfere with nearby transmitters on the same frequency, allowing frequency reuse. On the other hand, they are the highest frequencies which can be used for long distance terrestrial communication; higher frequencies in the [[Extremely high frequency|EHF]] (millimeter wave) band are highly absorbed by the atmosphere, limiting practical propagation distances to one kilometer or less. The high frequency gives microwave communication links a very large information-carrying capacity ([[bandwidth (signal processing)|bandwidth]]). In recent decades many new solid state sources of microwave energy have been developed, and microwave [[integrated circuit]]s for the first time allow significant [[signal processing]] to be done at these frequencies. Sources of EHF energy are much more limited and in an earlier state of development.

==See also== *[[Knife-edge effect]] *[[Microwave burn]]

== References == {{Reflist}}

==External links== * Tomislav Stimac, "''[http://www.vlf.it/frequency/bands.html Definition of frequency bands (VLF, ELF... etc.)]''". IK1QFK Home Page (vlf.it). * Inés Vidal Castiñeira, "''[http://www.broadbandhomecentral.com/report/backissues/Report0308_3.html Celeria: Wireless Access To Cable Networks]''"

[[Category:Radio spectrum]] [[Category:Wireless]]