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
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