Editing Microwave (section)

== Propagation ==
{{main|Radio propagation}}
[[File:Atmospheric Microwave Transmittance at Mauna Kea (simulated).svg|right|thumb|upright=1.3|The atmospheric [[attenuation]] of microwaves and far infrared radiation in dry air with a precipitable water vapor level of 0.001&nbsp;mm. The downward spikes in the graph correspond to frequencies at which microwaves are absorbed more strongly. This graph includes a range of frequencies from 0 to 1 THz; the microwaves are the subset in the range between 0.3 and 300 gigahertz.]]Microwaves travel solely by [[line-of-sight propagation|line-of-sight]] paths; unlike lower frequency radio waves, they do not travel as [[ground wave]]s which follow the contour of the Earth, or reflect off the [[ionosphere]] ([[skywave]]s).<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 = 978-0471743682 }}</ref>  Although at the low end of the band, they can pass through building walls enough for useful reception, usually rights of way cleared to the first [[Fresnel zone]] are required.  Therefore, on the surface of the Earth, microwave communication links are limited by the visual horizon to about {{convert|30|-|40|miles}}. Microwaves are absorbed by moisture in the atmosphere, and the attenuation increases with frequency, becoming a significant factor ([[rain fade]]) at the high end of the band.   Beginning at about 40&nbsp;GHz, atmospheric gases also begin to absorb microwaves, so above this frequency microwave transmission is limited to a few kilometers.   A spectral band structure causes absorption peaks at specific frequencies (see graph at right). Above 100&nbsp;GHz, the absorption of electromagnetic radiation by Earth's atmosphere is so effective that it is in effect [[Opacity (optics)|opaque]], until the atmosphere becomes transparent again in the so-called [[infrared]] and [[optical window]] frequency ranges.

=== Troposcatter ===
{{main|Tropospheric scatter}}
In a microwave beam directed at an angle into the sky, a small amount of the power will be randomly scattered as the beam passes through the [[troposphere]].<ref name="Seybold" />  A sensitive receiver beyond the horizon with a high gain antenna focused on that area of the troposphere can pick up the signal.  This technique has been used at frequencies between 0.45 and 5&nbsp;GHz in [[tropospheric scatter]] (troposcatter) communication systems to communicate beyond the horizon, at distances up to 300&nbsp;km.