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The first electronic
circuit I remember building as a budding teenage tinkerer was a "whistler" detector,
aka a "sferic."
Instructions and a schematic came from a project book I bought at Radio Shack. A
whistler is a time-varying electromagnetic signal that decays in both frequency
and volume over a short time - like sounds made by the eponymous fireworks genre.
Having always had an interest in weather phenomena as part of my
flying hobbies,
it seemed like an apt learning endeavor. To my recollection, the whistler detector
was a simple diode circuit with a couple Rs, Ls, and Cs strewn around in a particular
configuration, and a long wire antenna. I can't honestly say whether or not any
whistlers were ever heard with it. My interest was a layman's curiosity, but elsewhere
in the world, professional scientists were expending a lot of effort in their attempts
to analyze and quantify a whitler's particulars.
Thunderbolts and Whistlers
By Don Gleason
The why and wherefore of eerie-sounding radio signals generated by certain
types of lightning
The theoretical path taken by a whistler is demonstrated by Harold
E. Dinger with a loop of wire. Because it follows the earth's magnetic lines of
force, the path extends into intersolar space.
Pen-and-ink spectrograms are analyzed to determine how the sound
changes in frequency as time elapses. The whistler below is represented by the dark
band running from left to right.
Bonk!! ... Sheeeeeooooooooo ... ... Bonk! ............ Sheeeeeooooooooo ... Bonk!
............ Sheeeeeooooooooo ...
These odd sounds were emitted over and over by a loudspeaker in The Naval Research
Laboratory, Washington, D. C., during a stormy afternoon last summer. For hours
the popping, crackling sounds of "sferics," the usual very-low-frequency radio static,
had been monotonously pouring out. Then, about midday, a thunderstorm with strong
lightning strokes had combined with the proper condition of the atmosphere. "Whistlers"
- strange, drawn out, eerie-sounding radio signals - were being generated by lightning
stroke after stroke in great profusion.
What, you might ask, could be sillier than a bunch of people listening to static
instead of turning the radio off during the storm? But it's not ordinary static
they are listening to. The "Bonk!" of the lightning crash is followed a second or
two later by a swooping whistle; a weird, downsliding tone like a sigh from the
ether. The mysterious cause of these "whistlers" is what our scientists want to
fathom.
According to theory, the sighing whistle means that part of the radio wave generated
by the lightning flash has zoomed thousands of miles through the upper reaches of
the atmosphere and out into space, turning downward again to the Southern Hemisphere.
Way down near the Straits of Magellan, it bounces back off the earth and dogs its
own track back to its point of origin. With uncanny precision, this errant radio
wave seeks out the small storm area where it was "born" from all the immensity of
the earth and sky. No homing pigeon could be more faithful than this mysterious,
short-lived, v.l.f. radio wave.
For years, scientists at The Naval Research Laboratory have been studying these
curious radio waves. From a long, low building on the Laboratory grounds, a coaxial
cable runs up to the roof and connects to a long-wire antenna. This is 200 feet
long and ends at the top of a 120' radio tower. Radio signals from thousands of
sources fall on this receiving antenna. Local broadcast stations "plaster" it, and
on 16 kc., the dahdahdit-dahditditdit-ditdahdit of GBR, Rugby, England, can be heard.
To Harold E. Dinger, the NRL expert on v.I.f. radio waves, these man-made signals
are simply interference, requiring careful filtering for their elimination. "Whistling
atmospherics" presently hold Mr. Dinger's concentrated attention, and he and his
associates have surrounded themselves with special electronic apparatus for their
study. The long omnidirectional antenna feeds an audio amplifier which covers the
frequency range from 800 to 14,000 cycles per second, and can amplify signal voltages
by a factor of one million. A high-fidelity tape unit is fed by the amplifier and
automatically records whistler activity.
Since a whistler tone is never completely pure and often very ragged, a careful
analysis of the taped spectrograms is necessary to determine the dominant frequency.
Mr. Dinger and his staff have studied thousands of whistlers, corroborating previous
results by other workers, and have discovered new evidence which as yet is not completely
explained.
What is the precise mechanism of propagation of these waves? Any attempt to explain
even the common types of whistlers must show how the single sharp "Bonk!" of electromagnetic
energy from a thunderbolt is changed into a delayed echo many seconds long. Where
did the energy go during the one or two seconds between lightning flash and whistler?
The delay strongly suggests an echo or reflection from a remote point.
The German scientist, Heinrich Barkhausen, is generally credited with discovering
whistlers while intercepting Allied landline telephone conversations in World War
I. Later, both Barkhausen and the English scientist T. L. Eckersley offered the
theory that the pulse from the lightning flash traveled through the ionosphere.
Eckersley further proposed that the earth's magnetic field would cause the energy
from a flash to be split and was responsible for the generation of a whistler.
In 1950, in the famous Cavendish Laboratory, University of Cambridge, England,
L.R.O. Storey took up the study of whistlers. By 1953 he had calculated that the
path length of whistlers was a minimum of fifteen thousand miles. With the help
of data obtained from British thunderstorm locating stations, he deduced that the
waves were focused into a huge curved beam, closely following a line of force of
the earth's magnetic field out into space and down into the Southern Hemisphere,
and that they returned after reflection to an area around the initial flash. He
proposed that the whistlers sometimes heard without a preceding "Bonk!" or "click"
had originated in the Southern Hemisphere during thunderstorms there.
It seems virtually certain that whistlers will provide a valuable tool to assist
in determining the nature of the "outer" ionosphere. These studies may also throw
new light on partially unexplained geophysical phenomena such as "radio blackouts,"
magnetic storms, and the aurora.
Posted February 14, 2022
(updated from original post on 4/13/2015)
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