A local (Washngton, DC) ham uses the C-shape for his vertical wire
antenna on 160M and claims he does pretty well with it. He cut the dipole
for 160M as usual. The center section he hung vertically next to a rocket
launcher push-up mast, suspended off the mast by about four or five feet.
The rest of the dipole wires he ran out to a single tree. So his antenna
looks like a rectangular letter C, with the bottom wire about ten feet off
the ground, the top wire at about fifty feet, and the rest of the two wires
parallel to each other out to the distant tree. The antenna is fed half way
up the forty-foot vertical portion, through a balun, I believe. He claims
it demonstrates pretty good low-take-off angle performance (empirically
determined, not modeled), possibly due to the cancellation of horizontal
radiation from the parallel horizontal wires.
I did a quick (and very inexperienced) EZNEC model for this
contraption. For a total dipole length of 135 feet, and forty feet of the
center wires (twenty feet either side of feedpoint) hung vertically and the
rest horizontally as described above, I got a resonant point (min SWR) of
3.61 MHz and an impedance of 34.4 -j 1.2 Ohms. 2:1 BW was from 3.56 to 3.67
MHz. Max gain was negligible (appr 0.24 dBi at 30deg elev.), but there was
still good performance below 10 deg elev (gain = -2.2 dBi.) But the model
showed a supression of radiation at high elevation angles (gain = -4.7 dBi
at 90 deg elev.)
Sounds easy and cheap enough to try on 80M.
Gene Smar AD3F
From: Van Fair <email@example.com>
To: firstname.lastname@example.org <email@example.com>
Date: Thursday, March 06, 2003 1:09 PM
Subject: [TowerTalk] vertical dipole for 3.8 MHZ
Anyone have any experience with a vertical dipole for 75 meters. I can hang
a vertical wire off my tower from 66 feet and feed it at any point. The
problem is that this is too short on 3.8 MHZ. I can put an extra 28 or so
feet horizontally at the top and bottom in a Z or C arrangement to make it a
full half wave but is there a better way? I would prefer to feed with coax
and a balun. Ground wires are out of the question so if I am to have a
vertical I think it probably must be a dipole. Would a short dipole with
loading coils in each side suspended vertically be any better than one with
partially horizontal elements. ANY THOUGHTS??
See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless
Weather Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any
questions and ask for Sherman, W2FLA.
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