Editing Radar (section)

===Frequency bands===
{{Main|Radio spectrum#IEEE}}

Antennas generally have to be sized similar to the wavelength of the operational frequency, normally within an [[order of magnitude]]. This provides a strong incentive to use shorter wavelengths as this will result in smaller antennas. Shorter wavelengths also result in higher resolution due to diffraction, meaning the shaped reflector seen on most radars can also be made smaller for any desired beamwidth.

Opposing the move to smaller wavelengths are a number of practical issues. For one, the electronics needed to produce high power very short wavelengths were generally more complex and expensive than the electronics needed for longer wavelengths or did not exist at all. Another issue is that the [[radar equation]]'s effective aperture figure means that for any given antenna (or reflector) size will be more efficient at longer wavelengths. Additionally, shorter wavelengths may interact with molecules or raindrops in the air, scattering the signal. Very long wavelengths also have additional diffraction effects that make them suitable for [[over the horizon radar]]s. For this reason, a wide variety of wavelengths are used in different roles.

The traditional band names originated as code-names during World War II and are still in military and aviation use throughout the world. They have been adopted in the United States by the [[Institute of Electrical and Electronics Engineers]] and internationally by the [[International Telecommunication Union]]. Most countries have additional regulations to control which parts of each band are available for civilian or military use.

Other users of the radio spectrum, such as the [[broadcasting]] and [[electronic countermeasures]] industries, have replaced the traditional military designations with their own systems.

{| class="wikitable"
|+ '''Radar frequency bands'''
|- style="background:#ccc;"
!Band name!!Frequency range!!Wavelength range!!Notes
|-
|[[High frequency|HF]]||3–30 [[Hertz|MHz]]||10–100 [[metre|m]]||Coastal radar systems, [[over-the-horizon]] (OTH) radars; 'high frequency'
|-
|[[VHF]]||30–300&nbsp;MHz||1–10 m||Very long range, ground penetrating; 'very high frequency'. Early radar systems generally operated in VHF as suitable electronics had already been developed for broadcast radio. Today this band is heavily congested and no longer suitable for radar due to interference.
|-
|P||<&nbsp;300&nbsp;MHz||>&nbsp;1&nbsp;m||'P' for 'previous', applied retrospectively to early radar systems; essentially HF + VHF. Often used for remote sensing because of good vegetation penetration.
|-
|[[UHF]]||300–1000&nbsp;MHz||0.3–1 m||Very long range (e.g. [[Ballistic Missile Early Warning System|ballistic missile early warning]]), ground penetrating, foliage penetrating; 'ultra high frequency'. Efficiently produced and received at very high energy levels, and also reduces the effects of [[nuclear blackout]], making them useful in the missile detection role.
|-
|[[L band|L]]||1–2 [[Hertz|GHz]]||15–30 [[centimetre|cm]]||Long range air traffic control and [[surveillance]]; 'L' for 'long'. Widely used for long range [[early warning radar]]s as they combine good reception qualities with reasonable resolution.
|-
|[[S band|S]]||2–4&nbsp;GHz||7.5–15&nbsp;cm||Moderate range surveillance, Terminal air traffic control, long-range weather, marine radar; 'S' for 'sentimetric', its code-name during WWII. Less efficient than L, but offering higher resolution, making them especially suitable for long-range [[ground controlled interception]] tasks.
|-
|[[C band (IEEE)|C]]||4–8&nbsp;GHz||3.75–7.5&nbsp;cm||Satellite transponders; a compromise (hence 'C') between X and S bands; weather; long range tracking
|-
|[[X band|X]]||8–12&nbsp;GHz||2.5–3.75&nbsp;cm||[[Missile]] guidance, [[marine radar]], weather, medium-resolution mapping and ground surveillance; in the United States the narrow range 10.525&nbsp;GHz ±25&nbsp;MHz is used for [[airport]] radar; short-range tracking. Named X band because the frequency was a secret during WW2. Diffraction off raindrops during heavy rain limits the range in the detection role and makes this suitable only for short-range roles or those that deliberately detect rain.
|-
||[[Ku band|K<sub>u</sub>]]||12–18&nbsp;GHz||1.67–2.5&nbsp;cm||High-resolution, also used for satellite transponders, frequency under K band (hence 'u')
|-
|[[K band (IEEE)|K]]||18–24&nbsp;GHz||1.11–1.67&nbsp;cm||From [[German language|German]] ''kurz'', meaning 'short'. Limited use due to absorption by [[water vapor]] at 22&nbsp;GHz, so K<sub>u</sub> and K<sub>a</sub> on either side used instead for surveillance. K-band is used for detecting clouds by meteorologists, and by police for detecting speeding motorists. K-band operates at 24.150 ± 0.100&nbsp;GHz.
|-
|[[Ka band|K<sub>a</sub>]]||24–40&nbsp;GHz||0.75–1.11&nbsp;cm||Mapping, short range, airport surveillance; frequency just above K band (hence 'a') Photo radar, used to trigger cameras which take pictures of license plates of cars running red lights, and by police for detecting speeding motorists. Operates at 34.300 ± 0.100&nbsp;GHz.
|-
|mm||40–300&nbsp;GHz||1.0–7.5&nbsp;[[millimetre|mm]] ||[[Millimetre band]], subdivided as below. Oxygen in the air is an extremely effective attenuator around 60&nbsp;GHz, as are other molecules at other frequencies, leading to the so-called propagation window at 94&nbsp;GHz. Even in this window the attenuation is higher than that due to water at 22.2&nbsp;GHz. This makes these frequencies generally useful only for short-range highly specific radars, like [[power line]] avoidance systems for [[helicopter]]s or use in space where attenuation is not a problem. Multiple letters are assigned to these bands by different groups. These are from Baytron, a now defunct company that made test equipment.
|-
|[[V band|V]]||40–75&nbsp;GHz||4.0–7.5&nbsp;mm || Very strongly absorbed by atmospheric oxygen, which resonates at 60&nbsp;GHz.
|-
|[[W band|W]]||75–110&nbsp;GHz||2.7–4.0&nbsp;mm||Used as a visual sensor for experimental autonomous vehicles, high-resolution meteorological observation, and imaging.
|}