Re: Skin depth question

I'm not sure I understand 'zactly what the question is here...but
perhaps if you step back a bit from the formulas, it will become
clearer.

"Skin depth" is just talking about the fact that at high frequencies,
current tends to flow only in the outer layer of a conductor. The
effective thickness of that layer is the skin depth. If you have a
conductor that's several skin depths thick (at the frequency of
interest), the inside of that conductor isn't doing anything for the
conduction--though it may be very significant with respect to
mechanical properties--how well an antenna element supports itself or
holds up in the wind.

Qualitatively, the skin depth:
--is GREATER (thicker skin) for poorer conductors
--is LESS as frequency increases
--causes INCREASED resistance as frequency increases
These effects go as the square root of DC conductivity and frequency,
so for example aluminum has about 1.53 times the resistance of copper
(depending on alloys...), but that results in a skin depth in aluminum
that's about 1.24 times THICKER in the aluminum, so the RF resistance
of aluminum is not 1.53 times the RF resistance of copper, but only
1.24 times as great. And for a given RF current in a wire (like in
coax), if you double the frequency, the I^2*R loss goes up only 1.41
times, not 2 times. All that assumes that you're at frequencies high
enough that the wire or conductor is much thicker than one skin depth.

For visualization, it's also useful to think of the current tapering
off exponentially as you go into the conductor from the surface, so at
one skin depth, the current density is 1/e = 37% as much as it is at
the surface.

If you happen to have a very thin conductor, like your aluminization,
much less than one skin depth thick, then the current density in it
will be essentially independent of the depth into the conductor. Other
effects may (very likely will) cause the current density to not be
uniform over the width of that very thin surface, but that's another
matter. For example, current in a wire in a coil, or in a
parallel-conductor transmission line, is not distributed uniformly
around the wire because of interaction with current in the nearby
wires. See "proximity effect."

Does that help, or is there more to your "personal puzzle"?

Cheers,
Tom

.

Relevant Pages

  • Re: Contrary current flow within a radiator
    ... interesting and not widely known that current *does* flow in the opposite direction to the main flow, at certain depths in a homogeneous conductor. ... In a solid conductor, the current density does, of course, decrease exponentially with depth. ... The depth at which it's decayed to 1/e of the surface density is the "skin depth". ... As you go each skin depth deeper below the surface, the phase becomes one radian more lagging. ...
    (rec.radio.amateur.antenna)
  • Re: Contrary current flow within a radiator
    ... it is an exponential function so it never goes to zero. ... The plots at the link Frank provided show current going rather abruptly to zero - even negative in some cases. ... Decrease of RF current with depth below the surface of a conductor is only a true exponential if the available conductor depth is infinite. ... I simply don't presume to know anything about it other than to observe that the citation appears to contradict the assertion that skin depth is limitless and exponential in real conductors. ...
    (rec.radio.amateur.antenna)
  • Re: Contrary current flow within a radiator
    ... opposite direction to the main flow, at certain depths in a homogeneous ... 37%) of the surface density is the "skin depth". ... tube is just as good a conductor as a solid one, ... As you go each skin depth deeper below the surface, ...
    (rec.radio.amateur.antenna)
  • Re: Contrary current flow within a radiator
    ... goes to zero. ... Decrease of RF current with depth below the surface of a conductor is ... already been thoroughly validated by developers and professional users. ... Present thinking, I thought, suggest that the skin depth is quite thin ...
    (rec.radio.amateur.antenna)
  • Re: a Big Pump
    ... some smooth bore hose for it and some oversized wire to power it. ... copper for aluminum and aluminum for tinned copper. ... "310.14 Aluminum Conductor Material. ...
    (rec.boats)
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