Editing Extremely high frequency (section)

== Propagation ==
[[File:Micrwavattrp.png|thumb|upright=1.7|Atmospheric attenuation in dB/km as a function of frequency over the extremely high frequency band. Peaks in absorption at specific frequencies are a problem, due to atmosphere constituents such as [[water]] vapour ({{chem2|H2O}}) and molecular [[oxygen]] ({{chem2|O2}}). The vertical scale is double logarithmic, as dB are themselves logarithmic.]] 
Millimeter waves propagate solely by [[line-of-sight propagation|line-of-sight]] paths. They are not refracted by the [[ionosphere]] nor do they travel along the Earth as [[ground wave]]s as lower frequency radio waves do.<ref name="Huang">{{cite book
 | last = Huang
 | first = Kao-Cheng
 | author2 = Zhaocheng Wang
 | title = Millimeter Wave Communication Systems
 | publisher = John Wiley & Sons
 | year = 2011
 | pages = Sections 1.1.1–1.2
 | url = https://books.google.com/books?id=kCmyPdxme1AC&pg=PP17
 | isbn = 978-1-118-10275-6}}</ref> At typical power densities they are blocked by building walls and suffer significant attenuation passing through foliage.<ref name="Huang" /><ref name="FCCbulletin70" /><ref name="Preez">{{cite book
 | last1  = du Preez
 | first1 = Jaco
 | last2  = Sinha
 | first2 = Saurabh
 | title  = Millimeter-Wave Antennas: Configurations and Applications
 | publisher = Springer
 | date   = 2016
 | pages  = 13–14
 | url    = https://books.google.com/books?id=8_B6DAAAQBAJ&pg=PA13
 | isbn   = 978-3-319-35068-4
 }}</ref> Absorption by atmospheric gases is a significant factor throughout the band and increases with frequency. However, this absorption is maximum at a few specific [[spectral line|absorption lines]], mainly those of [[oxygen]] at 60&nbsp;GHz and [[water vapor]] at 24&nbsp;GHz and 184&nbsp;GHz.<ref name="FCCbulletin70">{{cite web
  | title = Millimeter Wave Propagation: Spectrum Management Implications
  | version = Office of Engineering and Technology, Bulletin No. 70
  | publisher = Federal Communications Commission (FCC), US Dept. of Commerce
  | date = July 1997
  | url = https://transition.fcc.gov/Bureaus/Engineering_Technology/Documents/bulletins/oet70/oet70a.pdf
  | access-date = May 20, 2017}}</ref> At frequencies in the "windows" between these absorption peaks, millimeter waves have much less atmospheric attenuation and greater range, so many applications use these frequencies. Millimeter wavelengths are the same order of size as [[raindrop]]s, so precipitation causes additional attenuation due to [[scattering (optics)|scattering]] ([[rain fade]]) as well as absorption.<ref name="FCCbulletin70" /><ref name="Preez" /> The high [[free space loss]] and atmospheric absorption limit useful propagation to a few kilometers.<ref name="Huang" /> Thus, they are useful for densely packed communications networks such as [[personal area network]]s that improve spectrum utilization through [[frequency reuse]].<ref name="Huang" />

Millimeter waves show "optical" propagation characteristics and can be reflected and focused by small metal surfaces and [[dielectric lens]]es around 5 to 30&nbsp;cm (2 inches to 1 foot) diameter. Because their wavelengths are often much smaller than the equipment that manipulates them, the techniques of [[geometric optics]] can be used. [[Diffraction]] is less than at lower frequencies, although millimeter waves can be diffracted by building edges. At millimeter wavelengths, surfaces appear rougher so [[diffuse reflection]] increases.<ref name="Huang" /> [[Multipath propagation]], particularly reflection from indoor walls and surfaces, causes serious fading.<ref name="Preez" /><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   = 0-471-74368-2
 }}</ref> [[Doppler shift]] of frequency can be significant even at pedestrian speeds.<ref name="Huang" /> In portable devices, [[Fading|shadowing]] due to the human body is a problem. Since the waves penetrate clothing and their small wavelength allows them to reflect from small metal objects they are used in [[millimeter wave scanner]]s for airport security scanning.