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

Good day,

I think the general effect could be called "current crowding", and "skin
effect" and "proximity effect" are two special cases. Your thoughts about
the current distributions are in the right direction. The parallel-wire
case assumes the currents are all going in the same direction, as they would
be in a solenoid winding. The two special cases of an isolated round wire
and a group of parallel round wires are the only ones I know of with
approximately analytical solutions. Other geometries would, in general,
have to be investigated numerically. You can read an update of Smith's work
in the book "Small Antenna Design". The web is getting better all the time,
but I believe it is still true that the best technical information is
available in books and archived journals.

Regards,
Doug Miron

<hideho.yamamura.wj_at_hitachi.com> writes:

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.

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

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. )

Also, I am interested on resistance of rectangular conductors.

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.

But, following the suggessions,
it seems that, in broad sense, both are skin effect,
and geometries should be considered carefully.

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.
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.

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.

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

any comment highly appreciated,

best regards,
Hideho YAMAMURA

>This is somewhat of a question of definitions. Skin effect and proximity
>effect are both cause by current crowding due to the interior magnetic
>field
>in the conductors. Skin effect is symmetrical and all there is in the
>isolated coaxial cable you hypothesize. In a bank of parallel
>current-carrying wires, the current-crowding causes an unsymmetrical
>distribution in each wire, or, at least, an uneven distribution. If I
>recall correctly, Glenn Smith's analysis of proximity effect assumes first
>a
>skin effect distribution, that is, the current is effectively crowded into
>an outer layer on each conductor. Then he calculates the redistribution
>due
>to proximity of the other conductors.
>
>Regards,
>Doug Miron
>
><hideho.yamamura.wj_at_hitachi.com> writes:
>
>Hello all,
>
>Excuse me if this is a bit off topic.
>I have a very basic question.
>
>It is said that Loss of coaxial cables has frequency dependency
>due to skin effect.
>The inner conductor has current concentrated on the outer surface,
>and outer conductor on the inner surface.
>
>Here, I encounter a basic question:
>"when the current is concentrated on the INNER surface,
> is it due to skin effect ?"
>
>Obviously, the current is on the INNER surface,
>because there is current in the inner conductor.
>So, isn't that due to Proximity effect ?
>
>How about the inner conductor ?
>Skin effect and Proximity effect mixed ?
>
>If so, is the loss proportional to the square-root of the frequency ?
>(I believe resistance change due to Proximity effect is different
> and stronger than the square-root of the frequency, but not sure.)
>If so, the fact that actual coax cables show square-root dependency
>is because Proximity effect is masked and dominated by Skin effect ?
>
>I would appreciate any related info.
>
>Hideho YAMAMURA. //
>
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