[TowerTalk] Bead chokes on feedlines; K3LR array

[Pete Smith]

Thanks, Mike -- I hadn't thought through the modeling systematically.  I 
see no reason why this shouldn't work.

What really motivated me to try to get this right was having 2 of the four 
dipoles fail right at the beginning of SS.

73, Pete N4ZR

At 11:26 PM 11/8/2004, Michael Tope wrote:

>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" <n4zr at contesting.com>
>To: "Tom Rauch" <w8ji at contesting.com>; <Towertalk at 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
>the
> > 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
> > >purpose,
> > > > 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.
> >
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