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
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
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
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.
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
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 April 13, 2015