Editing Very low frequency (section)

===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.