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Re: [TowerTalk] Vacuum variable / inductor ratings for shunt fed tower

To: "K4SAV" <>
Subject: Re: [TowerTalk] Vacuum variable / inductor ratings for shunt fed tower
From: "Rick Karlquist" <>
Date: Fri, 12 Jun 2009 11:52:48 -0700 (PDT)
List-post: <">>
K4SAV wrote:
> The balun is an unusual design.  It's a combo 22 to 50 ohm unun plus a
> common mode choke.  Most people say that won't work.  However if you
> look at what they did it almost makes sense.  The unun part is tri-filar
> wound which produces very little core flux.  They only used about half
> the core for this.  So if it produces little core flux then it shouldn't
> affect the other part of the core.  So they took the the coax that hooks
> to the unun and used the other part of the core to wind a choke.  That
> is the part that is questionable in my mind.  The choke will produce
> core flux.  So shouldn't that couple back the the unun?  Apparently this
> arrangement works or so they claim.  I just don't know how well it
> works.  I was hoping you had measured it.
> Jerry, K4SAV

It's that old gag: it depends on what you mean by "works".

It will work in terms of operating the SteppIR whether it is a voltage
balun or a current balun.  So that fact that it works for that
purpose doesn't shed any light on whether it will work for top loading
a shunt fed tower.

I don't agree with some of your analysis.  The unun certainly produces
core flux at the lower frequencies.  Below 7 MHz, the MonstIR balun
goes south due to insufficient magnetizing inductance, according
to my measurements.  On 160 meters, the capacitive reactance of
a 40 meter element as a top hat is about 300 ohms (measured).  For
a SteppIR 20 meter element, double this.  Even if the SteppIR balun
acted as a common mode choke on 20 meters and up, it would have
relatively low impedance on 160 meters, and shouldn't amount to
much in series with 600 ohms.

I can't measure this now because all 3 of my SteppIR band antennas
are on the tops of towers :-).

Rick N6RK


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