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ultra low frequency (ULF)
Cycles per second: 300 Hz to 3000 Hz Wavelength: 1000 km to 100 km

Ultra Low Frequency (ULF) is the frequency range between 300 hertz and 3000 hertz. This band is used for communications in mines, as it can penetrate the earth. ^[1]

Contents 1 Earthquakes 2 Earth Mode Communications 3 See also 4 References 5 External articles

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.

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 secretly for defence purposes. Communications through the ground using conduction fields is known as "Earth Mode" communications and was first used in WW1. 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 current between two further pairs of electrodes. Using weak signal reception methods with PC based DSP filtering with extremely narrow bandwidths it is possible to receive signals at a range of a few kilometres with a transmitting power of 10-100W and electrode spacing of around 10-50m.

• Voice frequency

1. ^ http://www.weather.nps.navy.mil/~psguest/EMEO_online/module3/module_3_1.html

• Tomislav Stimac, "Definition of frequency bands (VLF, ELF... etc.)". IK1QFK Home Page (vlf.it).
• NASA live streaming ELF -> VLF Receiver
• Amateur Radio Below 10kHz "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]"

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Radio spectrum
ELF	SLF	ULF	VLF	LF	MF	HF	VHF	UHF	SHF	EHF
3 Hz	30 Hz	300 Hz	3 kHz	30 kHz	300 kHz	3 MHz	30 MHz	300 MHz	3 GHz	30 GHz
30 Hz	300 Hz	3 kHz	30 kHz	300 kHz	3 MHz	30 MHz	300 MHz	3 GHz	30 GHz	300 GHz

v • d • e The Electromagnetic Spectrum
(Sorted by wavelength, short to long) Gamma ray • X-ray • Ultraviolet • Visible spectrum • Infrared • Terahertz radiation • Microwave • Radio waves
Visible (optical) spectrum	Violet • Blue • Green • Yellow • Orange • Red
Microwave spectrum	W band • V band • K band: Ka band, Ku band • X band • C band • S band • L band
Radio spectrum	EHF • SHF • UHF • VHF • HF • MF • LF • VLF • ULF • SLF • ELF
Wavelength designations	Microwave • Shortwave • Mediumwave • Longwave