Man-made VLF emissions
The AC Power
The most noteworthy VLF signal comes
from the AC (alternating current) power grid. Practically every
home in the most industrialized nations is connected to an AC power
source. The grid in the United States is based on a 60 Hertz system.
Other countries use the same frequency or sometimes 50 Hertz, as in
the United Kingdom and parts of Europe. Main transmission lines of
the AC grid use unshielded open wires suspended in the air tens of
feet above the ground using tower structures. Extremely high
voltages exist on these wires, up to 250,000 volts and possibly
more. This is a recipe for proliferation of VLF
noise in the general vicinity of these lines. Wooden power poles, usually called
"telephone poles" are seen in many places. They route power from the substations to
houses and businesses. The AC grid is a major nuisance to VLF radio enthusiasts; they
must be far from any type of power line.
The image here is a spectrogram, it shows the many harmonic
signals of the AC grid system The vertical scale is frequency, the
horizontal scale is time. The harmonics extend beyond a hundred
times above the base frequency (60 hertz).The sample was taken 75
feet away from 12,000 volt lines.
Here is a close-up of power line emissions recorded
from a VLF receiver located about two miles from a 48,000 volt
service line. The harmonic signals are clearly visible at 180 and 300
Hertz. The dark vertical columns are lightning sferics.
Click here to hear a recording of the AC power grid
The Alpha System
Alpha consists of several transmitting
stations operating at frequencies from 11.9 to 14.8 Kilohertz. Alpha
is used for navigation purposes, primarily for submarines as VLF
radio waves can propagate through water with good efficiency.
Click here to hear a recording of Alpha transmissions
MSK or Minimum Shift Keying is a form of
radioteletype (RTTY). Transmissions are generally heard on
frequencies from 16 to 160 Kilohertz and are usually encrypted for
security measures. These stations are very powerful, several
megawatts are used to propel the signals into the noisy radio
environment. Military vessels are the primary users of MSK
Click here to hear a recording of a MSK station broadcasting at 55 Kilohertz
The United States had a system similar to Alpha, called Omega. Because of the
new GPS navigation system, Omega was dismantled in September of 1997. The Alpha
system signals are seen in addition to the Omega positioning system; Omega signals
are the longer dashes. the vertical lines are lightning sferics (or strikes).
Click here to hear a recording of Omega transmissions
VLF radio enthusiasts are not limited to listening, some are sending out signals using home
built equipment. The samples here are from an experiment to find out if VLF can
be used for amateur communications. It was done in the Sierra Nevada Mountains in California.
The transmitter sent out an intermittent CW (continuous wave) signal at
one mile from the receiver, then again at two miles. The frequency used was 1.8
Kilohertz, well within the VLF range.
Sample 1 (650 Kb) Low power (one-half
watt) at one mile
Sample 2 (650 Kb) High power (100 watts)
at one mile
Sample 3 (650 Kb) High power (100 watts) at two miles
More about this experiment can found here:
VLF Transmitting Experiment #2
VLF Emissions from automobiles
Most vehicles are equipped with an alternator or generator that recharges
the battery while the vehicle is running. The samples here are recordings
of natural emissions interrupted by passing vehicles. Due to the quiet
conditions at the VLF receiver site, the passing cars and pick-ups were
first detected from a distance of about a thousand feet.
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