From: rohre (email@example.com)
Cleve raised a question of what antenna he should put up with 125 foot of space,
and essentially asked "a Windom or a G5RV?"
Now there are discussions of the Windom as practiced today and originally in a
number of texts, one is in a RSGB, (U.K.) antenna publication.
It points out as do the others that the original Windom antenna used a single
wire feed , off center , and of course that vertical part radiated as did the
flat top. The original was I believe based on the half wave dipole length for
the flat top.
Now if you examine the curves for the half wave dipole voltage and current in
the RSGB handbook, ARRL Handbook, Antenna Book, the W6SAI antenna books, etc.,
all agree the maximum current is at the center, and therefore, that is the ONLY
place the impedance is low enough to use normal low impedance coax to feed a
dipole flat top. There are some "Windom's" advertised that must use some
matching device, (lossy) to do otherwise.
Three hundred ohm twin lead that Cleve asked about as an alternative feed on the
Windom, is designed, as is ladder line, to feed a balanced point on an antenna,
where it would see equal halves of the antenna for each of the feeders. This a
"Windom" does not have, when off center fed. Analysis by computer programs that
were not available in Windom's day show any attempt to use this design with coax
or twin lead feeder is less than optimum to say the least! Remember, even
Windom himself was using a single wire feed, against ground to achieve energy
transfer to what is still a higher impedance point on the antenna than the
center. There is then, no advantage to the Windom that cannot also be fulfilled
by some of the G5RV varients. Or even a balanced line fed dipole fed thru a
balanced tuner for all bands
I have had good success with one G5RV varient by a ZS6 that W6SAI published in
his CQ series; but later analysis by him shows I perhaps was lucky, and actually
invented a varient that made my "G5RV" work even at only 20 foot elevation of
the 92 foot flat top. (This length was picked to also cover WARC bands.) What
I had to do with such a low support, was to bring the 40 foot ladder line that
feeds the center off at right angles to the dipole portion, at the same height,
to a support off to the side. This was the 450 ohm window line. From the end
of it, I spliced RG 8X without a balun, onto the ladder end, and went on to the
rig with 50 or 60 feet of coax. This antenna worked very well on bands 40
meters and up, especially on 20 Meters, where the classic G5RV is a gain
antenna. It even loaded on all bands I tried, without a tuner. 15 Meters had
3:1 SWR or more that reduced my power, but it communicated almost as well as the
others. Only a severe storm and excessive growth of the trees at one end has
ended its use for the present. (And my desire to experiment with the all band
Gap Titan Vertical.)
However, given the available 125 foot flat top space, I advised Cleve that a
dipole fed with ladder line and a tuner might be the best all band compromise.
This has been called the McCoy dipole after Lew McCoy who often advocates it.
It works, is efficient with a good tuner, and uses the space well for the
purpose. What of course would also work, is to parallel other half wave dipoles
onto a slightly short 80 meter dipole of 125 foot length. If you hung 5 foot
tuning wires vertically off the ends, you actually would resonate the 125 foot
flat top into the 80 meter band, depending upon the frequency desired. Gary
Breed in RF Design magazine, recently revived the parallel dipole concept,
where you do not even have to attach the parasitic, higher band dipoles to the
feed coax, but just have them spaced a few inches away from the lowest frequency
dipole to which the coax attaches. Plastic or fiberglass, or even PVC pipe
spacers would work for such a use. If one coated wood dowels with shellac, they
would serve as well for the spacers.
Perhaps someone who has one of the "Carolina Windoms" will counter what negative
reports I read about the use of modern Windoms, it would be interesting for
those with access to antenna analysis programs to tell us how they could work.
In the mean time, to answer Cleves' desire for a good efficient low loss antenna
for 125 foot space, a center fed solution looks most desireable. And ladder
line is lower loss than coax. If bringing it to an interior shack, not on an
outside wall, is a concern, I suggest reading Fred Bonivita's past articles on
using ladder line, and mounting the balun under the eves of the house, etc.
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