Re: Contrary current flow within a radiator

Roy Lewallen wrote:
I don't want to add ammunition to support Art's gibberish, but it's 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 (about 37%) of the surface density is the "skin depth". This is why a hollow tube is just as good a conductor as a solid one, providing only that the tube wall is at least several skin depths thick.

But the *phase* of the current changes with depth, too, quite dramatically. As you go each skin depth deeper below the surface, the phase becomes one radian (about 57 degrees) more lagging. So at pi skin depths below the surface, the current is completely out of phase with the surface current, i.e., it's flowing in the opposite direction. Of course, the current density at this point is very small, only 1/e^pi ~ 4% of the surface density, so only a small fraction of the total current flows completely backward. (Good thing!) At two pi skin depths, the current is again in phase with the surface current, but its magnitude is only 1/e^(2*pi) ~ 1/500 of the surface density. And so forth.

This isn't of much immediate practical use, and it's certainly not offered as supporting in any way Art's fanciful theories (whatever they might be). But it is an interesting fact.


I wonder if one could set up some sort of interesting demonstration of this. If you could, for instance, have a 1 foot diameter conductor with skin depth of an inch or so, and some (probably not feasible) way to indicate current flow. (yes, in order for this to happen it has to be AC, etc.)
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Relevant Pages

  • 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
    ... opposite direction to the main flow, at certain depths in a homogeneous ... 37%) of the surface density is the "skin depth". ... As you go each skin depth deeper below the surface, ... the surface current, i.e., it's flowing in the opposite direction. ...
    (rec.radio.amateur.antenna)
  • Re: Part 2 Is it possible to ask questions here?
    ... by the reverse contain current flow to the inside. ... or practice. ... The 1.8 MHz skin depth in a round, copper conductor is about 0.06 mm, which means that a tubular conductor with a wall thickness at least 3 times that can be used in place of a solid conductor of the same outer diameter, with no practical change in performance at that frequency. ...
    (rec.radio.amateur.antenna)
  • Re: using coax shield to create a loading coil ?
    ... the use of coax cable with braided outer Cu ... conductor can cause problems. ... RF charge flow in the braid experiences a non-linear circuit ...
    (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)
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