Re: NEC-LIST: Coax, Skin effect or Proximity effect ?

> -----Original Message-----
> From: nec-list-bounces+p.s.excell=bradford.ac.uk_at_robomod.net
> [mailto:nec-list-bounces+p.s.excell=bradford.ac.uk_at_robomod.net
> ] On Behalf Of hideho.yamamura.wj_at_hitachi.com
> Sent: 26 May 2007 01:14
> To: nec-list_at_robomod.net
> Subject: Re: NEC-LIST: Coax, Skin effect or Proximity effect ?
>
>
> Thank you for the responces.
>
> Is that that the definition of skin effect and prosimity
> effect is not very strict ? Perhaps, I am just puzzled with that.

Skin effect tells us about the penetration of current density into a
conductor as a result of field incident on its surface. As I see it, it is
'strictly' defined.

Proximity effect tells us about (arguably unexpected) distributions of the
incident field due to the effect of proximate conductors. For coaxial
cables, the system is axisymmetric and so nothing 'unexpected' occurs, hence
we take the view that there is no 'proximity effect' worth talking about,
IMHO.

>
> Is the Glenn Smith's analysis for parallel wire, or coax ?
> (I could not find info on the web)

Parallel.

>
> Is there any article available with strict solution of
> current distribution, and/or frequency dependency, for coax
> and parallel wires ? (Wikipedia says : The effect was first
> described in a paper by Horace Lamb in 1883
> for the case of spherical conductors, and was generalized to
> conductors
> of any shape by Oliver Heaviside in 1885. )

Hard to see how it could have been done for the spherical case, since EM
waves cannot obey spherical symmetry...

>
> Also, I am interested on resistance of rectangular conductors.

I think a numerical solver would be essential.

>
>
> I believe, we usually think that,
> proximity effect is caused on one conductor, by current(s) on
> other conductor(s). skin effect is for one conductor alone.

Correct, but maybe "one conductor alone" is a confusing phrase. Skin effect
is 'local' to each surface point on any conductor, no matter how many are
present.

>
> But, following the suggessions,
> it seems that, in broad sense, both are skin effect,

I don't really agree, although skin effect needs to be taken into account in
working out the exact proximity effect.

> and geometries should be considered carefully.

Certainly true.

>
> We learn in textbook that,
> a) for a round conductor, skin effect causes the current to
> diminish exponentially, the skin depth is proportional to the
> inverse of square-root of frequency, and resistanc increases
> following the square-root of frequency.

The skin depth statement is true. The *overall* resistance of the wire will
be affected by the frequency dependence of the skin effect and the proximity
effect, which I think are effectively independent for good conductors.

> b) for infinite plane conductor, with uniform radiation of
> electromagnetic wave, the skin depth is also proportional to
> inverse of square-root of frequency,
>
> On the other hand, I hear people say, when wires are very
> close, like in transformers for AC, audio or switching power
> supplies, or Litz wire, wire spacing is smaller than the wire
> radius, resistance increases a lot, proportional to frequency
> or to the square of, explained as proximity effect.

Sounds plausible.

>
> When I think of "trapped radio wave" inside a coax,
> I feel I easily agree that currents have exponential
> distribution, on both conductors,
> because of axisymmetricity and orthogonal H, E and surfaces.
>
> But when I think the proximity-way,
> I am not sure if the "skin depth" of the outer conductor
> varies proportional to the inverse of square-root of frequency.
>
> If there is no current in the inner conductor,
> the current on the outer conductor will flow on the outer
> surface. When inner conductor current exists, it "pulls" the
> outer conductor current to the inner surface. Hence, this is
> proximity effect. The "pulling" should depend on inner
> current amount. (for usual coax usage, inner-current =
> outer-current, opposite direction). This was what I thought.

For a solid-shield coax (e.g. semi-rigid) the inside and outside of the
outer conductor are effectively completely decoupled and independent of each
other. For a braided shield, I am sure the same applies, but you could spend
many weeks proving it!

>
> It seems to me that coax and parallel wires has different
> current distribution and different frequency dependency
> (geometry dependedent though).

Yes, because there is no 'proximity effect' in a coax, in the sense of
causing unexpected, frequency-dependent, redistributions of incident field.

IMHO.
Peter Excell

>
> any comment highly appreciated,
>
> best regards,
> Hideho YAMAMURA
>

-- 
The NEC-List mailing list
NEC-List_at_robomod.net
http://www.robomod.net/mailman/listinfo/nec-list