Editing Low frequency (section)

==Propagation==
[[Image:Atmosphericnoise.PNG|thumb|260px|Atmospheric [[radio noise]] increases with decreasing frequency. At the LF band and below, it is far above the thermal noise floor injected by amplifier circuits in the receiver, so weak signals can be amplified in the receiver to compensate with no perceivable increase in the noise (''see'' [[signal to noise ratio|SNR]]). Consequently, ''for reception'', even inefficient antennas much smaller than the wavelength are adequate.]]

Because of their long [[wavelength]], low frequency [[radio wave]]s can [[Diffraction|diffract]] over obstacles like mountain ranges and travel beyond the horizon, following the contour of the Earth. This mode of propagation, called ''[[ground wave]]'', with the radio waves traveling horizontally through the atmosphere just above the surface of the Earth,  is the main mode in the LF band.<ref name=Seybold>
{{cite book
 |last  = Seybold   |first = John S.   |author-link=John S. Seybold
 |year  = 2005
 |title = Introduction to RF Propagation
 |publisher = John Wiley and Sons
 |isbn  = 0471743682
 |pages = 55–58
 |url   = https://books.google.com/books?id=4LtmjGNwOPIC&q=cross+polarization+discrimination&pg=PA57
 |access-date  = 2020-11-30  |via = Google books   |url-status   = live
 |archive-url  = https://web.archive.org/web/20210416234908/https://books.google.com/books?id=4LtmjGNwOPIC&q=cross+polarization+discrimination&pg=PA57
 |archive-date = 2021-04-16
}}
</ref> Ground waves are absorbed by the Earth as they travel, so the signal strength (power density) decreases exponentially with distance from the transmitting antenna, limiting transmission distance.  The attenuation of signal strength with distance is lower than at higher frequencies. Low frequency ground waves can be received up to {{convert|2000|km|mi}} from the transmitting antenna.  Ground waves must be [[vertical polarization|vertically polarized]] (the [[electric field]] is vertical while the [[magnetic field]] is horizontal), so vertical [[monopole antenna]]s are used for transmitting.

Low frequency waves can also occasionally travel long distances by reflecting from the [[ionosphere]] (the actual mechanism is one of [[refraction]]), although this method, called ''[[skywave]]'' or "skip" propagation, is not as common as at higher frequencies. Reflection occurs at the ionospheric [[Kennelly–Heaviside layer|E layer]] or [[F Region|F layers]]. Skywave signals can be detected at distances exceeding {{convert|300|km|mi}} from the transmitting antenna.<ref>{{cite magazine |title=Understanding LF propagation |first=Alan (G3NYK) |last=Melia |magazine=[[Radcom]] |publisher=[[Radio Society of Great Britain]] |location=Bedford, UK |issue=9 |volume=85 |pages=32}}</ref>