Editing Very low frequency (section)

==Amateur use==
The frequency range below 8.3&nbsp;kHz is not allocated by the [[International Telecommunication Union]] and in some nations may be used license-free.
Radio amateurs in some countries have been granted permission (or have assumed permission) to operate at frequencies below 8.3&nbsp;kHz.<ref>{{Cite web |title=Sub 9&nbsp;kHz spectrum in the Amateur Service |url=http://www.cept.org/Documents/wg-fm/24788/fm-15-119_sub-9-khz-spectrum-in-the-amateur-service |access-date=13 May 2017}}</ref>

Operations tend to congregate around the frequencies 8.27&nbsp;kHz, 6.47&nbsp;kHz, 5.17&nbsp;kHz, and 2.97&nbsp;kHz.<ref>{{Cite web |title=Some recent milestones with amateur radio experiments at VLF |url=http://abelian.org/vlf/amateur-radio |access-date=13 May 2017}}</ref> Transmissions typically last from one hour up to several days and both receiver and transmitter must have their frequency locked to a stable reference such as a [[GPS disciplined oscillator]] or a [[rubidium standard]] in order to support such long duration coherent detection and decoding.

===Amateur equipment===
Radiated power from amateur stations is very small, ranging from 1&nbsp;μW to 100&nbsp;μW for fixed base station antennas, and up to 10&nbsp;mW from kite or balloon antennas. Despite the low power, stable propagation with low attenuation in the [[Earth-ionosphere waveguide|earth-ionosphere cavity]] enable very narrow bandwidths to be used to reach distances up to several thousand kilometers. The modes used are [[QRP operation#QRSS|QRSS]], [[Multiple frequency-shift keying|MFSK]], and coherent [[Phase-shift keying|BPSK]].

The transmitter generally consists of an audio amplifier of a few hundred watts, an impedance matching transformer, a [[loading coil]] and a large wire antenna. Receivers employ an electric field probe or magnetic loop antenna, a sensitive audio preamplifier, isolating transformers, and a PC [[sound card]] to digitise the signal. Extensive [[digital signal processing]] is required to retrieve the weak signals from beneath [[Electromagnetic interference|interference]] from [[Harmonics (electrical power)|power line harmonics]] and [[Radio atmospheric|VLF radio atmospherics]]. Useful received signal strengths are as low as {{val|3|e=-8}}&nbsp;volts/meter (electric field) and {{val|1|e=-16}}&nbsp;tesla (magnetic field), with [[signaling rate]]s typically between 1 and 100&nbsp;bits per hour.

===PC based reception===
[[File:VLF 18.1 kHz spectrogram.svg|thumb|Timing diagram of a frequency-shift keyed 18.1&nbsp;kHz VLF signal, picked up using a small [[loop antenna]] and a sound card. The Morse code says "..33376.."; the vertical stripes are distant lightning strikes.]]
VLF signals are often monitored by [[radio amateur]]s using simple homemade VLF [[radio receiver]]s based on personal computers (PCs).<ref>{{Cite book |last=Renato Romero, IK1QFK |title=Radio Nature |publisher=[[Radio Society of Great Britain]] |year=2008 |isbn=9781-9050-8637-5 |pages=77}}</ref><ref>{{Cite conference |last=Mardina Abdullah |display-authors=etal |year=2013 |title=Development of UKM-SID teaching module for space science education |conference=6th International Forum on Engineering Education (IFEE 2012) |volume=102 |pages=80–85 |doi=10.1016/j.sbspro.2013.10.716 |doi-access=free |orig-year=2012 |journal=Procedia - Social and Behavioral Sciences}}</ref> An aerial in the form of a coil of insulated wire is connected to the input of the soundcard of the PC (via a jack plug) and placed a few meters away from it. [[Fast Fourier transform]] (FFT) software in combination with a sound card allows reception of all frequencies below the [[Nyquist frequency]] simultaneously in the form of [[spectrogram]]mes.

Because CRT monitors are strong sources of noise in the VLF range, it is recommended to record the spectrograms with any PC CRT monitors turned off. These spectrograms show many signals, which may include VLF transmitters and the horizontal electron beam deflection of TV sets. The strength of the signal received can vary with a [[Sudden Ionospheric Disturbance|sudden ionospheric disturbance]]. These cause the ionization level to increase in the ionosphere producing a rapid change to the amplitude and phase of the received VLF signal.