# Re: [SI-LIST] : Question on propogation of high freq. signal in conductors

From: Scott McMorrow ([email protected])
Date: Thu Sep 21 2000 - 06:14:32 PDT

Prasanna,

There are two ways to clear your doubts:

1) Read a classical text on electromagnetics.

2) Make measurements at various frequencies on a trace that
is placed on low dielectric loss material.

prasanna kumar wrote:

> Hi all,
> Thanks for the answers.But i still dont have my doubts
> cleared.
> I have learnt about skin effect but had the idea that
> it is only concerned with EM waves travelling from one
> medium(say air) to another(a conductor).I am unable to
> digest the concept of using skin effect here.Please
> clarify.
> And why is it that more flux is linked with inner part
> of a conductor so that by Lenz's law the opposing
> force for current is high and hence no current flows
> in the inner parts of the conductor?
> As quoted by Mr.Clewell,
> "'First slice a wire lengthwise to make concentric
> tubes, like growth rings on a tree trunk. Now if you
> consider the inductance of each ring separately
> the inner rings would be long and skinny and have more
> inductance than the outer rings which are fatter.'
> and what here is meant by sayin that the inner rings
> are long and skinny?Why should the inductance be more
> for such rings?"
> Thanks,
> Prasanna.
>
> --- "Jian X. Zheng" <[email protected]> wrote:
> > Hi,Prasanna:
> >
> > In short, the skin effect causes the current to be
> > flowing on the surface of
> > a conductor. In fact, you can demonstrate such a
> > phenomenon on a plane wave
> > incident onto a big sheet of metal:
> >
> > \
> > \ plane wave
> > \
> > ---------------------------------------- top surface
> > of the metal sheet
> > |
> > | plane wave in the metal
> > |
> > ---------------------------------------- bottom
> > surface of the metal sheet
> > \
> > \ transmitted plane wave
> > \
> >
> > When a plane wave is incident onto the surface, you
> > will see the electric
> > field is exponentially decaying in the -z direction.
> > J = Sigma * E, where E
> > is the electric field, J is the current and Sigma is
> > the conductivity. In
> > the other word, the J is exponentially decaying and
> > the decaying factor is
> > very big. It is just like the current only exists on
> > the surface of the
> > metal sheet. Thanks!
> >
> > Best regards,
> >
> >
> -----------------------------------------------------------------------
> > Jian-X. Zheng, Ph.D
> > Zeland Software, Inc., 48890 Milmont Drive, 105D,
> > Fremont, CA 94538, U.S.A.
> > Tel: 510-623-7162, Fax: 510-623-7135, Web:
> > http://www.zeland.com
> >
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> ----------------------------------------------------------------------------
> > --
> >
> > > -----Original Message-----
> > > From: [email protected]
> > > [mailto:[email protected]]On Behalf
> > Of prasanna kumar
> > > Sent: Tuesday, September 19, 2000 6:26 AM
> > > To: [email protected]
> > > Subject: [SI-LIST] : Question on propogation of
> > high freq. signal in
> > > conductors
> > >
> > >
> > > Hi,
> > > i have a small doubt.
> > > in high frequencies why does the signal is
> > confined
> > > only to the surface of the conductor?
> > > ie,(if i am putting it right)why does the current
> > > flows only at the surface of the conductor?
> > > thanks.
> > > Prasanna.
> > >
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```--
Scott McMorrow
Principal Engineer
SiQual, Signal Quality Engineering