[Elecraft] K3 6m pre-amp

[Jack Smith]

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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