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

[jimlux]

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 at earthlink.net> To:
> towertalk <towertalk at 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)
>
>
>
> _______________________________________________
>
>
>
> _______________________________________________ TowerTalk mailing
> list TowerTalk at contesting.com
> http://lists.contesting.com/mailman/listinfo/towertalk
> _______________________________________________
>
>
>
> _______________________________________________ TowerTalk mailing
> list TowerTalk at contesting.com
> http://lists.contesting.com/mailman/listinfo/towertalk
>