Contrary current flow within a radiator
 

JB wrote:


It is interesting to note though, that it takes a lot of PhD's to design
something so that impoverished people (often children) can have a bowl of
rice every day to build the technology. Also, the environmental disaster
that is created by throwing away "gadgets" every 6 months to implement the
latest technology. I wonder if the Liberal academic establishment is in
touch with the environmental disaster they are creating so that we can all
be enslaved to the global neural network.

Respectfully JB, is this liberals created the disposable society rant
something you can lay hands or links to citations for, or is it just
truthiness?

- 73 de Mike N3LI -

Contrary current flow within a radiator
 

Ed Cregger wrote:

What is truly saddening to me is the amount of philosphy/religion that
creeps into these so called scientific discussions. No wonder antenna
modeling is still mostly an art form and not a science.

So right, Ed. Last time I checked, antennas did not have either a
liberal or conservative bias.

- 73 de Mike N3LI -

Contrary current flow within a radiator
 

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.

Roy Lewallen, W7EL

Contrary current flow within a radiator
 

"Jim Kelley" wrote in message
...
Dave wrote:

"Art Unwin" wrote in message
...

( I am assuming that skin depthg is not limitless.)

of course it is limitless, it is an exponential function so it never goes
to zero. the so called 'skin depth' is only the point where the current
has dropped to 1/e or about 37% of the surface value, still a significant
current.

The plots at the link Frank provided show current going rather abruptly to
zero - even negative ("contrary"?) in some cases. I wouldn't presume to
know whether it is modeled correctly.

ac6xg

the 'contrary' current makes sense because of 2 different effects. the main
one is the phase delay of propagation through the conductor. the velocity
of propagation of the wave within the wire is MUCH slower than through space
outside the wire. this produces a phase delay such that when you take a
snapshot, as those plots most likely are, you can get opposing currents. if
you were to take a time average or peak value you would get the plain
exponential as predicted by the simple equations that are normally taught.
The other confounding factor on those examples is the discontinuity between
the copper and steel, again the velocity changes across that boundary
shifting the phase in a step.

Contrary current flow within a radiator
 

Jim Kelley wrote:

Hi Ian,

Please know that my comment was never intended as a slight of anyone's
work. 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.

73, ac6xg

that's because the usual discussion of "skin depth" is cribbed from a
physics textbook, where the (not always explicitly said) assumption is
"in an infinite uniform plane of infinite depth with no other magnetic
fields"

In that restricted (but useful) case, you can model the current (for the
purposes of things like resistivity) as if it were uniform from the
surface to the skin depth.

In cases where the thickness of the conductor is "large" relative to the
skin depth, the error in using the rectangular layer of current
assumption is "small".

In cases where this assumption isn't valid (or, if you need higher
precision), then a more complete analytical formulation is needed. If
the conductor happens to be circular, then Bessel functions are surely
involved (differential equations in circular things almost always
involve Bessel functions and/or Hankel transforms). Since most of us
don't do Bessel functions in our heads, we use tables or lookups.

There's two sets of tables and graphs for round conductors: one is for
solid conductors; the other is for tubular conductors. Different
boundary conditions on solving the differential equations, so different
analytical solutions.

A 1998 paper by Gaba and Abou-Dakka gives all the equations and
background, and adds the details needed for stranded wires and cables
made of multiple substances (e.g. ACSR power lines).

There's also some analytical solutions for square and rectangular cross
sections, but they're pretty ugly, compared to the round conductors.

once you start talking multiple materials and dielectrics, it becomes
easier to do FEM (following the dictum of my father's differential
equations professor: useful differential equations should be solved
numerically, because the analytical solution is often harder and more
computation than the numerical one). (another good example of this is
calculating the field between two spherical electrodes)

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.)

Contrary current flow within a radiator
 

On Jan 14, 3:49*pm, 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.

Roy Lewallen, W7EL

Exactly.
And every phase change support current /charge flow in the opposite
direction
in accordance with Newtons laws. Use a vector drawing to prove it for
yourself !
Or provide same as proof of my errors.
While you are at it do the same for a full wave radiator which IS in
equilibrium per Maxwells law
when the inner vector is now non existant since radiation occurs on
the surface at all times in accordance with the "tank circuit"
abilities And where the center path is only resistive in the case of a
fractional wave antenna. This is very, very basic physics to which I
know no challedge in the physics world. For you it is no difference
when you were affiliated with QST and supported the
commercial publishing of rediculas specifications to oppose change.
Sooner or later you will again have to change your tune to one that
does not include opposition to the truth.
The book that Richard is quoting is available on the web for $1.99
which will allow you to confront all the authors of their "rediculus"
errors at the same time together with all the Universities that use
the book as part of their physics curriculum
I await your appearance on CNN where it will undoubtably push aside
the viewing of the president

Cecil,
This is how you defrock the self perceived pompous expert. He
ofcourse does make errors which he will not own up to.
Art Unwin......KB9MZ....(xg)

Contrary current flow within a radiator
 

Jim Kelley wrote:
Decrease of RF current with depth below the surface of a conductor
is only a true exponential if the available conductor depth is
infinite. In the modeled situations where there is 'competition' from
a skin effect on the opposite side of the conductor, the solution is
a Bessel function which does pass through zero and reverse direction
at certain depths.
In other words, the model is behaving as expected.
Programs such as NEC and Maxwell are not released until they have
gone through a very detailed process of checking and validation. The
first step is to check against special cases that can be
independently solved by analytical methods (in other words, pure
math). The work isn't complete until all the results agree within
close margins, and the reasons for any differences are fully understood.
By the time we amateurs come to hear about these programs, they have
already been thoroughly validated by developers and professional
users. That doesn't make them immune from further criticism... but
only by people who have done the work to earn that right.

Hi Ian,

Please know that my comment was never intended as a slight of anyone's
work.

I didn't think that for a moment, Jim.

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.

As noted above, that commonly-held assertion is only true for conductors
of limitless depth.


--

73 from Ian GM3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek

Contrary current flow within a radiator
 

On Jan 14, 4:18*pm, Jim Lux wrote:
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.)

Why not insert a wafer of the same material parallel to the axis and
apply
a non destuctive test on the material as a whole. When the wafer is
withdrawn
would it not be possible to observe the actual effective skin depth.
Of course, the slot for the wafer must not enter the radial surface
of the
radiator other wise circular flow would be interupted thus destroying
the
datum apearance. Obviously I have not utelised a non destructive test
first hand.
From my point of view as long as there is an eddy current on the
surface to eject a resting particle
there is not the requirement for endles depth and decay would be the
condition of the particle alone
and not that of the conductor. The particle still has nuclear content
when it emerges from the Sun's
arbritary field which is obviously subject to decay
Regards
Art

Contrary current flow within a radiator
 

On Wed, 14 Jan 2009 16:29:33 -0500, Michael Coslo
wrote:

Ed Cregger wrote:

What is truly saddening to me is the amount of philosphy/religion that
creeps into these so called scientific discussions. No wonder antenna
modeling is still mostly an art form and not a science.

So right, Ed. Last time I checked, antennas did not have either a
liberal or conservative bias.

*** observing nothing technically redeeming in this thread ***

Hi Mike,

Circular polarity is not political? You can have a left hand screw or
a right hand screw.

*** Fulfilling the entertainment mandate of RRAA for 14 years ***

73's
Richard Clark, KB7QHC