On 12/30/16 10:47 AM, Hans Hammarquist via TowerTalk wrote:
That equation is valid for a homogeneous conductor. When you have a
sandwich of several layers with different conductivity you have to
calculate it more thoroughly by using the formula for each of the
sections (which makes it harder).
yes, the microwaves101.com reference from a couple days gives the
multilayer equation.
The point still is that a layer of (very) bad conductivity over a
good conductor will only have a small (if any) influence. Like was
pointed out earlier, it's like two resistors in parallel, the
resistor with a very high resistance will not have any major
influence on the current in the resistor with low resistance.
I think one question, which I haven't bothered to try an analyze yet, is
whether there is particularly "bad" resistivity that one could put over
a good conductor that would be a worst case.
If the material is fairly high resistance (most all metal salts, wood,
etc.) then as you say, very little current will flow in the resistive
layer, so the loss will be small. And if the material is very low
resistance, then it won't have much loss.
But things like nickel (and iron) present an interesting point: they're
fairly conductive - they're still metal, and they're magnetic: so the
skin depth is small, more of the current density distribution will be in
the coating, not the core. More than one microwave designer has been
bitten by this because nickel is a popular strike/flash plating between
layers to prevent things like migration, or just to make a coating that
actually sticks. But there, the trick is having the gold/silver surface
coating thick enough that very little of the field penetrates to the nickel.
That said, I don't think any of the corrosion products that folks are
talking about fall in the bucket of "fairly conductive and magnetic".
ANother factor that might lead to "degraded performance" of an antenna
is that the insulation will cause the resonant frequency to change, and
it could change substantially if the insulation is hygroscopic. The
degradation wouldn't be because of dissipative losses, but because the
antenna is now mistuned.
I do like your suggestion of making a coil and measure the resistive
value of the coil at different phases of the deterioration.
Hans - N2JFS
-----Original Message----- From: jimlux <jimlux@earthlink.net> To:
towertalk <towertalk@contesting.com> Sent: Wed, Dec 28, 2016 1:13 pm
Subject: Re: [TowerTalk] UV and WX deterioration of THHN insulation,
and effects
On 12/28/16 7:49 AM, jimlux wrote:
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.
Oops.. resistance = length*resistivity/(skin depth * width)
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