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LF: VLF Antenna

To: [email protected]
Subject: LF: VLF Antenna
From: "Walter Blanchard" <[email protected].uk>
Date: Fri, 12 Mar 2004 20:44:38 -0000
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This'll make you all jealous!!!!  It's an extract from the "Memoirs of Dr. J
A Pierce" that I happened to come across recently.
He is describing the then-new US Navy VLF antenna at Cutler, Maine when it
was built in 1957.

"Watching the VLF transmitter grow at Cutler, Maine, was very interesting.
Everything about it was of unprecedented size.  The antenna consisted of two
halves, each a mile and a half or so in diameter. Metal cables extended from
a thousand-foot tower in the center somewhat fanwise to cover the spaces
between six slightly lower
towers half way to the outer edge, and then continued to come toward points
at six more towers some eight hundred feet high at the edge itself. Thus
each of the two antennae consisted of six diamond-shaped panels of cables.
All were insulated from the towers by trains of porcelain insulators about
thirty feet long.  At each supporting point at a tower, there was a
counterweight weighing, I believe I remember, 4O,OOO pounds, that permitted
the antenna to sag somewhat if it were under unusual strain from wind or ice
loading. I have mentioned above the 2,200 miles of wire buried under the
antennae (and extending into the Gulf of Maine on both sides) to provide the
earth connection.
I was especially interested in the two loading coils required to resonate
with the capacitance of the antennae at the operating frequency of 14.7 kHz.
Each of these was housed in a shielding "helix house" of aluminum in the
form of a cube about a hundred feet on a side. In the center, the coil
itself was a helix of a dozen or more turns
about twenty feet in diameter and much the same in length. These were wound
of the largest Litzendraht "wire" I ever expect to see.  Each strand
consisted of a few thousand fine wires braided around the surface of a rope
half an inch in diameter.  A large number of these strands were then formed
upon the surface of another rope about three inches in diameter.  This sort
of construction was necessary because at radio frequencies the current in a
wire is insistent on traveling only in a thin layer at the surface.  By
making each individual wire as thin as this surface layer a vast amount of
copper can be saved; but then it must follow that hundreds or (in the case
of Cutler) tens of thousands of individual wires had to be used to achieve
the low resistance that is vital in the final coil. I have been told that,
when in operation, the heat losses in these loading coils are adequate to
maintain a pleasant temperature in the shielded helix house.  On the only
occasion when I was in one of these hundred-foot cubes, the station was not
yet operating.  The date was in March and it was a sunny but windy day. I
was amazed at the way this big aluminum cube sucked the heat out of my body,
my winter clothing notwithstanding. It was certainly far less comfortable
inside than outdoors, even on a windy day.
These twin antennae at Cutler are driven by currents of 4,000 amperes or so
at about a quarter of a million volts.  The two, operated together as is
usual, have an input of two megawatts and radiate about half as much power.
This  represents a phenomenal efficiency for a radio station operating at
such a low frequency. It is achieved
only by extreme care in controlling the losses in every part of the antenna,
loading coil, and grounding system."

Goes for 137 kHz, too.  Get that "2200 miles" of earth wire!

Walter G3JKV.


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