[TowerTalk] Question for common mode choke balun experts.

[Steve Hunt]

You need to remember that there is a large resistive component in the 
choke impedance, so you'll not see "High-Q" effects.

I just checked four turns of RG213 on four stacked FT240-31 cores; there 
was a broad peak in impedance between 13MHz and 27MHz, at just over 
3000Ohms.

Then I added an 8.2pF capacitor in parallel; the peak dropped to 5MHz, 
but it was still pretty broad and the peak impedance had actually 
dropped to about 2000 Ohms.

Hope that helps,
Steve G3TXQ



On 10/08/2013 14:27, Jim Thomson wrote:
> 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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