I recently installed an off-center-fed dipole, cut for 1/2 WL on
1.824 MHz (233 feet), using #12 Davis RF flexweave. I am feeding off
center so that the feed point and transmission line will be in the
backyard instead of the front yard! The front yard support is about
50 feet high in a tree. The backyard support is about 70 feet high at
the top of my tower. The distance between the supports is about 280
feet and the dipole is centered between them with 4-inch insulators
and rope on the ends.
I choose the feed point to be about 36.82 feet from the tower end of
the dipole, where EZNEC predicted Z=196+1.5j, a good match for my 4:1
balun and 50 ohm coax. EZNEC also predicted other interesting points
where the antenna would be resonant.
After pulling up the antenna, I used my MFJ259B to check things out,
expecting the need to fine-tune the location of the feed-point. I was
surprised to see that I was measuring something close to the
predicted results above 3MHz or so (e.g., a nice resonant point in
the middle of the 80-meter band), but totally weird results in the
160 meter band---R values on the order of 1 or 2 ohms (where EZNEC
predictes R in the 100s'). I then brought my radio outside to confirm
the results. Infinite SWR in the 160 meter band, but sensible things
below 80 meters.
I also pulled the balun out to see what would happen. Again,
predictable results below 80 meters (above 3.5MHz) and weird results
(R close to 0) in the 160 meter band.
I came across the following in a recent TT posting, and am beginning
to wonder if the flex weave might, for some reason, be a problem at
low frequencies? It is working fine for inverted vees that I have up
for 40 and 80 meters.
Braiding has significantly higher impedance than solid conductors.
A textbook I have on transmission lines says it is 3 to 4
times worse than a solid
conductor but that is for a transmission line lay that is
clean and tightly pressed.
Lays used in transmission lines are generally not at sharp angles
and are not densely woven, so they have less resistance per
unit length
than a rapid weave that is at more at right angles to the current path.
I am contemplating two alternatives:
1. Replacing the OCF dipole with an inverted vee with the apex on the
tower at 65 feet or so (but EZNEC predicts the OCF dipole will be 3
or 4 dB better that the inverted vee in the right directions and at
the right angles).
2. Replacing the flexweave with copperweld (the flexweave is a joy to
work with, relative to copperweld, however---but I did note recent
posting suggesting potential lack of longevity with some flexweaves).
I would rather not have too many more failed experiments, so am
seeking knowledge and advice.
Bill
K0KT
William Q. Meeker
Department of Statistics
304C Snedecor Hall
Iowa State University
Ames, Iowa 50011
Phone: 515-294-5336
Fax: 515-294-4040
Home Fax: 515-232-1323
www.public.iastate.edu/~wqmeeker
_______________________________________________
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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