[TowerTalk] UV and WX deterioration of THHN insulation, and effects

[jimlux]

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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