[Elecraft] K3 6m pre-amp

[Alan Bloom]

Good point.  That's why ferrite beads make good parasitic suppressors -
at VHF frequencies they act more like resistors than inductors.

Where I ran into this problem was trying to come up with a plate choke
for a kilowatt amplifier that would work from 1.8 to 19.7 MHz.  The high
inductance needed for the 160 meter band pretty much guaranteed poor
performance at 10 meters.  I couldn't make the choke lossy because it
would burn up at those power levels.  The solution was to use two chokes
in series and select values that resulted in a series resonance that
fell safely in between amateur bands.

Al N1AL


On Thu, 2008-07-31 at 10:34, Jack Smith wrote:
> Indeed such is the case, and it is quite pronounced with high Q inductors.
> 
> You may wish, however, to model and/or measure two series chokes when 
> both are wound on lossy ferrite material. The response looks quite 
> different. The resistance in parallel with the inductors radically 
> modifies the response when the material causes the inductor Q to be < 1.
> 
> I'll E-mail you some early measurements and a quick simulation plots.
> 
> Jack K8ZOA
> 
> Alan Bloom wrote:
> > 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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