Ultra low frequency 887766 189279889 2008-02-05T16:17:07Z Adamantios 650655 remove wikitable (use radio_spectrum template instead) and deprecated "----" The '''Ultra Low Frequency''' (ULF) is the [[frequency]] range between 300 [[hertz]] and 3 kilohertz. Many types of waves in the ULF frequency band can be observed in the [[magnetosphere]] and on the ground. These waves represent important physical processes in the near-Earth plasma environment. The speed of the ULF waves is often associated with the [[Alfven velocity]] that depends on the ambient magnetic field and plasma mass density. This band is used for communications in mines, as it can penetrate the earth. <ref>[http://www.weather.nps.navy.mil/~psguest/EMEO_online/module3/module_3_1.html HF and Lower Frequency Radiation - Introduction<!-- Bot generated title -->]</ref> == Earthquakes == Some monitoring stations have reported that [[earthquakes]] are sometimes preceded by a spike in '''ULF''' activity. A remarkable example of this occurred before the [[Loma Prieta Earthquake]] in [[California]] in [[1989]]. Researchers are attempting to learn more about this correlation to find out whether this method can be used as part of an early warning system for earthquakes. ==Earth Mode Communications== ULF has been used by the military for [[secure communications]] through the ground. NATO [[AGARD]] publications from the 1960s detailed many such systems, although one suspects the contents of the published papers left a lot unsaid about what actually was developed [[classified information|secretly]] for defense purposes. Communications through the ground using conduction fields is known as "Earth Mode" communications and was first used in [[World War I|WWI]]. [[Radio amateurs]] and electronics hobbyists have used this mode for limited range communications using audio power amplifiers connected to widely spaced [[electrode]] pairs hammered into the soil. At the receiving end the signal is detected as a weak [[electric current]] between two further pairs of electrodes. Using weak signal reception methods with PC based [[Digital signal processing|DSP]] filtering with extremely narrow bandwidths it is possible to receive signals at a range of a few kilometers with a transmitting power of 10-100W and electrode spacing of around 10-50m. ==See also== *[[Voice frequency]] ==References== <references/> ==External articles== * Tomislav Stimac, "''[http://www.vlf.it/frequency/bands.html Definition of frequency bands (VLF, ELF... etc.)]''". IK1QFK Home Page (vlf.it). * [http://spaceweather.com/glossary/inspire.html NASA live streaming ELF -> VLF Receiver] * Amateur Radio Below 10kHz "[http://homepage.ntlworld.com/lapthorn/earthmode.htm G3XBM's page on Earth Mode Communication]" * Review of Earth Mode Communications "[http://stinet.dtic.mil/oai/oai?&verb=getRecord&metadataPrefix=html&identifier=AD0640319 1966 abstract about Earth Mode Comms by Ames, Frazier and Orange]" * Radio communications within the earth's crust "[http://ieeexplore.ieee.org/xpl/freeabs_all.jsp?arnumber=1138037 Abstract of article by Burrows written in 1963]" {{radio_spectrum}} {{EMSpectrum}} [[Category:Radio spectrum]] {{tech-stub}} [[es:Ultra baja frecuencia]] [[fr:Ultra basse fréquence]] [[hi:अत्यन्त निम्न आवृत्ति (ULF)]] [[it:Ultra low frequency]] [[nl:ULF (radiospectrum)]] [[ja:極超長波]] [[no:Ultralav frekvens]] [[pl:ULF]] [[zh:特低頻]]