[Elecraft] K3 6m pre-amp

[Alan Bloom]

When you wire two chokes of different value in series, you almost always
get a series-resonant "hole" in the attenuation somewhere between the
parallel-resonant frequencies of the two chokes.  It's not hard to see
why that's true if you model each choke as an ideal inductor in parallel
with a capacitor.  So you need to design the combination so that the
series-resonant frequency falls somewhere unimportant.

Al N1AL


On Thu, 2008-07-31 at 09:14, Jack Smith wrote:
> Jim:
> 
> Thank you for the reference.
> 
> Indeed, the self-resonant frequency of  the  typical small (FT50 size) 
> chokes I've wound are in the 5-10 MHz range, but at 100 MHz some 
> (depending upon the  core material) still show enough Z to be useful. 
> For truly wideband 10 KHz - 100 MHz choke action, it's necessary to 
> series two wound with different core material and turns, e.g., 35 turns 
> on Steward 40 material for the 2.5 mH, followed by, e.g., 10 turns on 
> FairRite 43 material for > 50 MHz.
> 
> You have to pay particular attention to the u' and u'' values of the 
> ferrite material and how they change with frequency as this is a case 
> where core loss at high frequencies can be good.
> 
> Jack K8ZOA
> 
> 
> Jim Brown wrote:
> > On Wed, 30 Jul 2008 20:33:53 -0400, Jack Smith wrote:
> >
> >   
> >> Yes, I have several ferrite core 2.5 mH chokes here, including the 
> >> Hammond one you mention. There's a significant difference in high 
> >> frequency performance of the pi wound on ceramic form versus the smaller 
> >> pi-wound over ferrite and that's one of the things I'm hoping to 
> >> illustrate in the article. 
> >>     
> >
> > The RFI and Ferrite tutorial on my website includes clear explanations of 
> > the nature of ferrite materials from a circuit point of view. You may find 
> > it helpful in explaining why your very correct in your analysis. You are 
> > welcome to cite it as a reference. 
> >
> > http://audiosystemsgroup.com/RFI-Ham.pdf
> >
> > However -- ANY 2.5 mH choke has a good chance of looking capacitive at 50 
> > MHz. 
> >
> > 73,
> >
> > Jim Brown K9YC
> >
> >
> >
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