> 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
balance.
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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