[TowerTalk] Dipole puzzle

[ersmar@comcast.net]

Bill:

     Before you start pulling your hair out and dismantling all the work on
your antenna, answer this:  Is there an AM broadcast station serving your
area?  The MFJ259's are notoriously finicky in the presence of strong BCB
stations.

     MFJ make an extra-cost optional filter for BCB energy.  (
http://www.hy-gain.com/man/mfjpdf/MFJ-731.pdf#search='mfj%20filter' .)   But
it does work.  I got very strange results when I first installed my 160M
shunt-fed system for my tower.  Then I borrowed an MFJ-731 and got results
that made sense to me.

73 de
Gene Smar  AD3F


----- Original Message -----
From: "William Q Meeker" <wqmeeker at iastate.edu>
To: <towertalk at contesting.com>
Sent: Saturday, October 15, 2005 12:49 PM
Subject: [TowerTalk] Dipole puzzle


>
>
> 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
>
> _______________________________________________
>
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>
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