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The
"Galaxy"
dipole
by S. C.
"Chuck" Smith, WA7RAI
This
is likely the most unusual antenna design you'll find anywhere.
It's a product of my experimentations with loop loading that I
was conducting approximately a decade
ago. In this particular case, I was looking to see if it was possible
to have a very short dipole demonstrate good radiation efficiently
plus a respectable bandwidth. The "Galaxy"
dipole not only met that criteria, it
provided a most useful bonus as well.
THE PERFECT ATTIC
ANTENNA
I'm not
sure whether to say this dipole is omni-polarized or it just
simply has no definitive polarization. Whatever the case, one of the most interesting
aspects of the "galaxy" dipole is that one can tune it, say, on a pair of sawhorses on
the ground, and the tuning will hardly change when it is elevated
high above ground. Similarly, one can place it adjacent to wires
and metal poles with equal results. In other words, it does
not-- to any significant degree-- interact with the ground, or nearby
metallic objects or wires. I suspect this is so because the antenna
has no straight wires, lumped inductances, or a well defined polarity with which to
induce mutual coupling or strong ground currents.
Since
the radiation from this antenna is unlikely to load, by any
great amount, into the network of wires, electrical
conduits, etc., that are usually
found in one's attic, this could prove to be the best little attic
dipole ever, as very little of its radiated energy should
get absorbed by its environment.
DESCRIPTION
The "galaxy"
dipole consists of approximately 50½ ft. of copper wire that is formed into
a series of sinusoidal waves that become progressively smaller
(or larger, depending upon the direction). A greater portion of
the wire is twice folded back on itself in each half, with its waves occurring in both horizontal and vertical
orientations
to form a shape that is somewhat reminiscent of a series of spheres
that have progressively smaller circumferences away from center. The end result is an
electrically resonant dipole that is
about ½ normal length that employs no lumped L or C or LC
constants.
CONSTRUCTION
A prototype was made of #12 solid copper wire
using two joined 8'
lengths of 1½" PVC tubing as a support structure. Holes,
in groups of four, are drilled through the PVC tubing (two
vertical and two horizontal) at the specific points along its
length where the wire waves will cross (and pass through). These
holes are necessary in order to maintain wire position and wave-shape,
and must be drilled slightly offset to prevent shorting between the
crossing wires as they pass through.
The tubing sections
are joined in the center with a 4-way
sleeve in which short vertical sections of PVC tubing are inserted to
support the top and bottom of the large center loop and feed-point. In retrospect, square fiberglass tubing would likely
be a much better choice of material.
The
numbers shown in the drawing below describes the length of each
wave, and wire directions are color coded in an effort
to illustrate the continuity. Short (3 to 6 inch) stubs are
added at points A and B to allow some adjustment of the resonant frequency and feed-point impedance.
This is accomplished by modifying the stub lengths as suggested.
It is highly recommended that a 1:1 balun be employed at the
feed-point (FP) to suppress common-mode feed-line radiation.
PERFORMANCE
Although no calibrated
field-strength comparisons were made, on-air transmit/receive
tests suggested the VSWR bandwidth and overall performance was consistent
with a full-sized dipole. It seemed he "Galaxy"
dipole gave slightly
better performance off the ends than was expected.
For
other bands, simply scale the dimensions accordingly.
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