[RFI] Interfacing to sound card RFI

EDWARDS, EDDIE J eedwards@oppd.com
Mon, 26 Nov 2001 11:33:29 -0600

I have a question on this.  I'm sort of thinking out loud here.  

Are you trying to reduce distributed capacitance of the coil or increase

First, by winding the coil "closewound" you would be increasing the
distributed capacitance of the coil (less distance between the plates
equals more capacitance).  But by winding the coil with fewer turns
you're decreasing the distributed capacitance of the coil (less surface
area of the capacitor "plates").  These two recommendations seem at odds
to me.  They offset each other to some extent as does inductive and
capacitive reactance.  Then again the distributed capacitance is very
small and usually negligible in a coil, but a cable wrapped around a
ferrite core isn't an ordinary coil either.  Boy, this is complex!

Seems to me to increase overall impedance regardless of frequency you'd
want to increase inductive reactance and reduce capacitive reactance
which in this case should be very small or negligible.  You'd also want
to avoid self-resonance in the ferrite coil, or at least be on the
inductive side of that curve.  The impedance goes down on the capacitive
side of the curve.  This curve is shown in the 1987 ARRL handbook (fig
35) in Electrical Funamentals chapter and might be in newer editions as

So this final question is this: Can the distributed capacitance become a
factor (become large enough?) causing the self-resonance to shift to a
lower frequency and thus reduce the highest possible impedance and
frequency range of the coil or choke?  

I would guess it could, but then I'd think you'd also want, in addition
to winding fewer turns on the core, you'd want to wind the windings
further apart to reduce distributed capacitance.  

Just some thoughts...

de ed -K0iL

> -----Original Message-----
> From:	Jon Ogden [SMTP:jon.ogden@cain-forlaw.com]
> Sent:	Monday, 26 November, 2001 9:29 AM
> To:	'Palomar Engineers'; owner-rfi
> Subject:	RE: [RFI] Interfacing to sound card RFI
> WOW!
> That's a new one.  I never thought you could have too many turns on
> the
> torroid.  I guess it makes sense, but I've always been told that the
> more
> turns you can get around one of those, the better since the inductance
> (and
> thereby RF resistance) goes up with every turn through the torroid.
> I'll have to look into that.
> Does anyone have a "practical" guide for winding torroids printed up?
> It
> would be helpful because I think the common assumption is the more
> turns the
> better..........
> 73,
> Jon
> NA9D
> -------------------------------------
> Jon Ogden
> Sales Engineer
> Cain-Forlaw Company
> 847-202-9898 (Voice)
> 847-202-9896 (Fax)
> jon.ogden@cain-forlaw.com 
> > -----Original Message-----
> > From: Palomar Engineers [mailto:Palomar@compuserve.com]
> > Sent: Tuesday, November 20, 2001 5:17 PM
> > To: owner-rfi
> > Subject: [RFI] Interfacing to sound card RFI
> > 
> > 
> > One possibility is too many turns on the -77 toroids. 
> > Capacitance across
> > winding can leak the signal. For 20 meters try 5 turns 
> > closewound. Keep
> > winding ends apart. Worth a try anyway -- Jack, K6NY, Palomar 
> > Engineers
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