Editing Radio wave (section)

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

Radio waves are more widely used for communication than other electromagnetic waves mainly because of their desirable [[radio propagation|propagation]] properties, stemming from their large [[wavelength]].<ref name="Ellingson2">{{cite book |last=Ellingson |first=Steven W. |url=https://books.google.com/books?id=QMKSDQAAQBAJ&q=%22radio+wave%22&pg=PA16 |title=Radio Systems Engineering |date=2016 |publisher=Cambridge University Press |isbn=978-1316785164 |pages=16–17 |access-date=2020-11-19 |archive-url=https://web.archive.org/web/20240922182240/https://books.google.com/books?id=QMKSDQAAQBAJ&q=%22radio+wave%22&pg=PA16#v=snippet&q=%22radio%20wave%22&f=false |archive-date=2024-09-22 |url-status=live}}</ref>  Radio waves have the ability to pass through the atmosphere in any weather, foliage, and through most building materials.  By [[diffraction]], longer wavelengths can bend around obstructions, and unlike other electromagnetic waves they tend to be scattered rather than absorbed by objects larger than their wavelength.

The study of [[radio propagation]], how radio waves move in free space and over the surface of the Earth, is vitally important in the design of practical radio systems.  Radio waves passing through different environments experience [[Reflection (physics)|reflection]], [[refraction]], [[Polarization (waves)|polarization]], [[diffraction]], and [[Absorption (electromagnetic radiation)|absorption]].  Different frequencies experience different combinations of these phenomena in the Earth's atmosphere, making certain [[radio band]]s more useful for specific purposes than others.  Practical radio systems mainly use three different techniques of radio propagation to communicate:<ref name="Seybold-2005">{{cite book |last=Seybold |first=John S. |title=Introduction to RF Propagation |date=2005 |publisher=John Wiley and Sons |isbn=0471743682 |pages=3–10 |chapter=1.2 Modes of Propagation |access-date=2017-02-03 |chapter-url=https://books.google.com/books?id=4LtmjGNwOPIC&pg=PA6 |archive-url=https://web.archive.org/web/20240922182240/https://books.google.com/books?id=4LtmjGNwOPIC&pg=PA6#v=onepage&q&f=false |archive-date=2024-09-22 |url-status=live}}</ref>
* ''[[Line-of-sight propagation|Line of sight]]'': This refers to radio waves that travel in a straight line from the transmitting antenna to the receiving antenna.  It does not necessarily require a cleared sight path; at lower frequencies radio waves can pass through buildings, foliage and other obstructions. This is the only method of propagation possible at frequencies above 30&nbsp;MHz. On the surface of the Earth, line of sight propagation is limited by the visual [[horizon]] to about 64&nbsp;km (40&nbsp;mi). This is the method used by [[cell phone]]s, [[FM broadcasting|FM]], [[television broadcasting]] and [[radar]]. By using [[dish antenna]]s to transmit beams of microwaves, point-to-point [[microwave radio relay|microwave relay]] links transmit telephone and television signals over long distances up to the visual horizon. [[Ground stations]] can communicate with [[satellites]] and spacecraft billions of miles from Earth.
** ''Indirect propagation'': Radio waves can reach points beyond the line-of-sight by ''[[diffraction]]'' and ''reflection''.<ref name="Seybold-2005" /> Diffraction causes radio waves to bend around obstructions such as a building edge, a vehicle, or a turn in a hall. Radio waves also partially reflect from surfaces such as walls, floors, ceilings, vehicles and the ground. These propagation methods occur in short range radio communication systems such as [[cell phone]]s, [[cordless phone]]s, [[walkie-talkie]]s, and [[wireless network]]s. A drawback of this mode is ''[[multipath propagation]]'', in which radio waves travel from the transmitting to the receiving antenna via multiple paths. The waves [[interference (wave propagation)|interfere]], often causing [[fading]] and other reception problems.
* ''[[Ground wave]]s'': At lower frequencies below 2&nbsp;MHz, in the [[medium wave]] and [[longwave]] bands, due to diffraction [[vertical polarization|vertically polarized]] radio waves can bend over hills and mountains, and propagate beyond the horizon, traveling as [[surface wave]]s which follow the contour of the Earth.   This makes it possible for mediumwave and longwave broadcasting stations to have coverage areas beyond the horizon, out to hundreds of miles.  Ground waves are gradually absorbed by the Earth, so the power density of the waves decreases exponentially with distance from the transmitting antenna, limiting the range of reception.  As the frequency drops, the losses decrease and the achievable range increases. Military [[very low frequency]] (VLF) and [[extremely low frequency]] (ELF) communication systems can communicate over most of the Earth.  VLF and ELF radio waves can also penetrate water to hundreds of meters deep, so they are used to [[Communication with submarines|communicate with submerged submarines]].
* ''[[Skywave]]s'': At [[medium wave]] and [[shortwave]] wavelengths, radio waves reflect off conductive layers of charged particles ([[ions]]) in a part of the atmosphere called the [[ionosphere]]. So radio waves directed at an angle into the sky can return to Earth beyond the horizon; this is called "skip" or "skywave" propagation. By using multiple skips communication at intercontinental distances can be achieved. Skywave propagation is variable and dependent on atmospheric conditions; it is most reliable at night and in the winter. Widely used during the first half of the 20th century, due to its unreliability skywave communication has mostly been abandoned. Remaining uses are by military [[over-the-horizon radar|over-the-horizon (OTH) radar]] systems, by some automated systems, by [[radio amateur]]s, and by shortwave broadcasting stations to broadcast to other countries.
At [[microwave]] frequencies, atmospheric gases begin absorbing radio waves, so the range of practical radio communication systems decreases with increasing frequency. Below about 20&nbsp;GHz atmospheric attenuation is mainly due to water vapor. Above 20&nbsp;GHz, in the [[millimeter wave]] band, other atmospheric gases begin to absorb the waves, limiting practical transmission distances to a kilometer or less. Above 300&nbsp;GHz, in the [[terahertz band]], virtually all the power is absorbed within a few meters, so the atmosphere is effectively opaque.<ref name="Coutaz">{{cite book |last1=Coutaz |first1=Jean-Louis |url=https://books.google.com/books?id=zah8DwAAQBAJ&pg=PA18 |title=Principles of Terahertz Time-Domain Spectroscopy: An Introductory Textbook |last2=Garet |first2=Frederic |last3=Wallace |first3=Vincent P. |date=2018 |publisher=CRC Press |isbn=9781351356367 |pages=18 |access-date=2021-05-20 |archive-url=https://web.archive.org/web/20230221211545/https://books.google.com/books?id=zah8DwAAQBAJ&pg=PA18 |archive-date=2023-02-21 |url-status=live}}</ref><ref name="Siegel">{{cite web |last=Siegel |first=Peter |date=2002 |title=Studying the Energy of the Universe |url=https://www.nasa.gov/audience/foreducators/k-4/features/Peter_Siegel.html |url-status=dead |archive-url=https://web.archive.org/web/20210620092047/https://www.nasa.gov/audience/foreducators/k-4/features/Peter_Siegel.html |archive-date=20 June 2021 |access-date=19 May 2021 |website=Education materials |publisher=NASA website}}</ref>