[TowerTalk] Dipole puzzle

Sat Oct 15 13:24:47 EDT 2005

Gene:

Thanks for the response.

There is a moderate-power station on 1430kHZ about 6 miles away, but 
it has never caused problems when I used the MFJ to adjust the 160 
meter loading coil on my butternut vertical.

I had heard about such problems. That is why I confirmed the results 
with my rig (which, as I said, showed infinite SWR in the 160 meter 
band where it was predicted to be near 1:1 by EZNEC).

Bill
K0KT

At 12:08 PM 10/15/2005, you wrote:
>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
> >
> > _______________________________________________
> >
> > 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.
> >
> > _______________________________________________
> > TowerTalk mailing list
> > TowerTalk at contesting.com
> > http://lists.contesting.com/mailman/listinfo/towertalk

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 