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Re: [TowerTalk] Balun question

To: <>, <>
Subject: Re: [TowerTalk] Balun question
From: "Tom Rauch" <>
Reply-to: Tom Rauch <>
Date: Thu, 14 Jun 2007 16:15:58 -0400
List-post: <>
> The book shows a bead balun  attributed to M. Walter 
> Maxwell, W2DU. If I
> understand the theory  correctly a bead âchokeâ balun 
> would have most of
> the
> advantages of a wound  toroid-type balun without the 
> inductance and stray
> capacitance that Jim, K9YC,  describes since there is no 
> âcoax outside the
> coreâ except at the far ends  where the coax enters and 
> leaves.
> He does point out in his  tutorial that you do end up 
> using a lot
> of ferrite (cost) for this  (luxury?).

I can't imagine the need for multiple turns through a core 
on six meters.

First, stray C can quickly become a problem at VHF...even on 
low VHF. The core would have to be small and that would mean 
small coax.

Second, even a short string of beads would give more than 
enough impedance.

One thing people forget on six meters and higher (or 
actually ALL bands) is when we ground a coaxial cable 1/4 
wave away from the balanced termination end, if forms an 
almost perfect balun. One person who actually wrote a 
well-known reference book on baluns  concluded his test 
dipole, which was almost exactly 1/4 wave above ground, 
didn't need a balun because the terminals were close 
together!!! The real reason it didn't need a balun is 
because his feedline was 1/4 wave long from ground, and 
suspended in air.

As a matter of fact if we have a dipole and use a 1/4 wave 
feeder to a point where the feeder is grounded, adding a 
choke balun can increase common mode current and reduce 

So for VHF, where a 1/4 wave is easy to get, a simple way to 
create a very effective balun is to space the feedline away 
from everything else and ground it to something with a low 
RF impedance 1/4 wave from the feedpoint.

There are other misplaced schemes people swear by, like 
folding the braid back over the coax or sliding it in a 
metallic tube to form a decoupling sleeve. The problem there 
is the outside of the cable with its poor dielectric jacket 
forms a stub with the sleeve. The open end impedance of the 
sleeve is what does all the choking, and if you have high 
loss or a low surge impedance in that cable, or if you  miss 
resonance because you forgot to include the dielectric 
velocity factor of the jacket of the cable, you have a very 
poor very lossy balun.

A good sleeve (or bazooka)  balun requires a moderately 
large D1/D2 ratio  (outside of the coax to inside of the 
sleeve ratio). This ensures a high impedance and  reasonably 
low loss as a transmission line, and it means the 1/4 wave 
is close to freespace if most of the dialectic is air so the 
sleeve is close to a physical 1/4 wave.

73 Tom


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