[TowerTalk] Question for common mode choke balun experts.

[Jim Brown]

On 8/10/2013 7:34 AM, Steve Hunt wrote:
> You need to remember that there is a large resistive component in the 
> choke impedance, so you'll not see "High-Q" effects.

Right.  Typical circuit Q on the order of 0.4.

>
> 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.

No surprise.  Note that since we're looking for as much choking Z as 
practical, within size and cost limits, so if we want to move the 
resonance down, we are far better off with more turns and/or more cores.

I don't understand your comment that you can't add more turns or cores 
-- with reasonable care, I can nearly always fit five turns through the 
1.4-in i.d. cores with a 83-1SP attached. Are you using a larger connector?

BTW -- as noted in the "Cookbook," it's also possible to have two or 
more chokes in series, either to cover a wider frequency range, or to 
raise the Z without going too low in frequency.

73, Jim K9YC