You can probably model it, Pete. Just put in a wire (or series
of wire segments) in where the coax shield runs, run NEC,
and see how much current shows up on this "coax shield
simulation" wire. You can model the chokes you already
have installed by placing a lumped impedance equivalent
to the common mode impedance of the choke balun in
series between the shield simulation wire and the side of
the lazy vee dipole that gets the physical connection from
the coax shield (presumeably the bottom leg). You can do
the same thing on the other end of the shield simulation wire.
If the coax cable shields are grounded to the tower, connect
the shield simulation wire to the tower in your model. If you
want to see what effect a choke balun would have at this
point, insert a lumped impedance equivalent to the common
mode RF impedance of the choke balun between the shield
simulation wire and the tower. Follow all the NEC rules for
getting good convergence of course.
You can also measure the current on the coax shield, but
that involves climbing the tower in the cold winter weather
with a current probe, or at the very least checking dipole
impedances with and without choke baluns (e.g. lots of
cussing and swearing).
73 de Mike, W4EF......................................
----- Original Message -----
From: "Pete Smith" <firstname.lastname@example.org>
To: "Tom Rauch" <email@example.com>; <Towertalk@contesting.com>
Sent: Monday, November 08, 2004 2:26 PM
Subject: Re: [TowerTalk] Bead chokes on feedlines; K3LR array
> My problem is that I don't know enough to know how to go about making that
> judgment -- can you offer any suggestions?
> Are you suggesting that I might put the chokes at the switchbox end
> instead, or elsewhere? I already have a choke where my feedline enters
> house, so I think this has more to do with the antenna's performance than
> with common mode problems in the house.
> 73, Pete N4ZR
> At 03:31 PM 11/8/2004, Tom Rauch wrote:
> > > When I built this array, I put 50-bead chokes at each
> >feedpoint, thinking
> > > that I would help suppress common mode currenst and
> >thereby avoid any
> > > destructive interaction with the antenna pattern. Now,
> >however, Tom makes
> > > me wonder if this is useful at all, given that the
> >feedlines are in the
> > > very near field of the dipoles anyway. Would I be just as
> >well off to
> > > eliminate the chokes altogether, or do they serve a useful
> > > assuming they are of the right material and have enough
> >inductance to do
> > > the job?
> > >
> >If the feedlines parallel the elements, I'd sure look close
> >at what the baluns do and where they are located.
> 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
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