On 12/28/16 5:51 AM, Patrick Greenlee wrote:
Skin effect... If skin effect can force conduction into the outer limit
of the wire (the chemically altered part with poor conductivity) then
why doesn't the skin effect force conduction out into the insulation and
really have poor conduction? (or in bare wire out into the surrounding air)
My friend and guru (who refuses to post here) has been a ham for several
decades, is a retired EE, and has 35+ years antenna design experience
(his specialty) agrees with the concept that RF conductivity can be
characterized as a collection of parallel impedances, a continuum
actually. The depth of penetration of RF in a conductor does not have a
"magic" cut-off point but instead has an exponential extinction. That
is, the deeper into the conductor the less RF but there is no magic
barrier preventing RF from penetrating to any arbitrary depth, although
at rapidly reduced values.
Exactly this..
Skin depth is a convenient way to measure the exponential fall off: it
is the depth at which if you had a uniform slab of that thickness and
uniform current density it would have the same resistance as an
infinitely thick slab..
That is: you can calculate the resistance by Skin depth* width * length
* resistivity.
It works exactly the same as the RC time constant - after one time
constant, there's still 37% of the voltage on the cap.
there were several links published to papers and calculators yesterday
that let you figure out what the effective AC resistance of a multiple
layer material is.
By the way, most of formulas are for flat slabs. A round wire is
different, but if the flat slab formula says "no problem", then the
round wire is also likely "no problem".
Some folks (myself included) often talk about electrical current like it
was a sentient being saying things like current takes the path of least
resistance like it worked it out. Well, the poorly conducting layer of
corrosion on the surface of wire isn't the path of least resistance is
it? That layer is some of the infinite number of parallel paths and will
get a portion of the current flow in inverse proportion to its
conductivity compared to the uncorroded conductor.
A question:
Are we considering corrosion so advanced as to reduce the cross
sectional area of the wire, effectively reducing the surface area of
good copper available for skin effect conduction so increasing the wires
impedance?
My apologies to anyone offended by my getting too close to calculus.
Patrick NJ5G
On 12/28/2016 12:28 AM, Guy Olinger wrote:
Current betting money in my neck of the woods is on some actual
deterioration of the surface of the copper caused by compounds in the
degrading insulation. Skin effect forces some degree of current into
the degraded copper which has a higher resistance than the non
deteriorated good stuff in the middle of the wire.
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