Editing Ultra high frequency (section)

== Propagation characteristics ==
{{Main article|Radio propagation}}

Radio waves in the UHF band travel almost entirely by [[line-of-sight propagation]] (LOS) and ground reflection; unlike in the [[High frequency|HF]] band there is little to no reflection from the [[ionosphere]] ([[skywave]] propagation), or [[ground wave]].<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> UHF radio waves are blocked by hills and cannot travel beyond the horizon, but can penetrate foliage and buildings for indoor reception. Since the [[wavelength]]s of UHF waves are comparable to the size of buildings, trees, vehicles and other common objects, reflection and [[Knife-edge effect|diffraction]] from these objects can cause [[fading]] due to [[multipath propagation]], especially in built-up urban areas. Atmospheric moisture reduces, or [[Attenuation|attenuates]], the strength of UHF signals over long distances, and the attenuation increases with frequency. UHF TV signals are generally more degraded by moisture than lower bands, such as [[Very high frequency|VHF]] TV signals.

As the visual horizon sets the maximum range of UHF transmission to between 30 and 40&nbsp;miles (48 to 64&nbsp;km) or less, depending on local terrain, the same frequency channels can be reused by other users in neighboring geographic areas ([[frequency reuse]]). Radio [[repeater]]s are used to retransmit UHF signals when a distance greater than the line of sight is required.

Occasionally when conditions are right, UHF radio waves can travel long distances by [[tropospheric ducting]] as the atmosphere warms and cools throughout the day.