[TowerTalk] Question for common mode choke balun experts.

Jim Thomson jim.thom at telus.net
Sat Aug 10 09:27:38 EDT 2013

On your typ  common mode choke balun, wound with  4 turns of RG-393 coax
through 4 x stacked 2.4 inch OD, type 31 cores.   I have two of these made up, but
with coax connectors on each end.   All silver soldered.  More cores should have been
used, but too late now. 

It occurred to me that IF used on a single band  or perhaps 2 x adjacent bands, a simple
method to increase the Z  might be to parallel resonate the choke assy on the band of
interest.    This could be done with a NPO cap between the shields of the input and 
outputs.   In theory,  the Z  should skyrocket in value  over a large chunk of spectrum.

I don’t have room for any more cores inside my nema box..nor turns.   This is for 80 + 40m only.
Resonating the existing assy for say 5.5 mhz....and trying to cover 80 + 40m  would
be great.   If it only covers 1 band, no big deal, as I have two line isolators, and 
can resonate one box  for each band. 

Looking  at K9YC  various charts etc,   by using various amounts of turns + cores,
the assy  can be optimized for either a given band... or a wider range of bands.
It appears that  by adding  either more cores, and or more turns, the choke assy
is being resonated  either lower or higher in freq.   More cores = higher Z
More turns will shift the resonant point down  in freq.   The C between turns
is whats used to parallel resonate the choke. 

I should be able to do the same thing  with a NPO cap across the in and out braids. 
What Im looking for here is a quick fix, so I can optimize an existing 4 turn-4 core
choke balun on 80 and 40m.   Has this been done b4 ?   Each choke measures 128 uh,
but that is at 1 khz.... not 4/7 mhz.   Rough calcs show 15-35 pf might work. 

Jim  VE7RF 

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