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Re: Corroded antennas



At 14:23 01/05/2001 -0400, you wrote:
>Here is one for the experts.
>
>Living near the bay, most antennas are covered with a white almost sandy
>corrosion layer.  As routine maintenance, I occassionally re-do the
>connections and polish the metal-to-metal contact areas (to avoid RF
>mixing products), but I never thought much about the aluminum element.
>Hey, its still a conductor...
>
>Ah, but what about the skin effect?  If the RF is actually flowing only on
>the "outside" of the conductor, then maybe this crud level could be
>affecting my performance through I-squared-R losses in the "skin"..
>
>Has anyone seen an analysis of element corrosion on skin effect?
>
>de WB4APR@amsat.org, Bob
>
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Bob,

it's my turn to give a little back, at last!!!

In a previous life, when I worked at a calibration centre at a Naval Air 
Station, we played around with a network analyzer and some other fancy gear 
(all part of a pleasant dream now I'm afraid!!!) looking at just this 
effect. I'm pretty sure the work wasn't classified, tho' it was in relation 
to some data gathering equipment.

The upshot was, at the frequencies of interest, airband V/UHF and a higher 
sub uwave spot, yes, it does make a difference. The primary effect has to 
do with the dielectric constant of the corrosion film. It can change the J 
portion of the impedence quite markedly depending on it's composition.

(Just looked thru my lab notes... did I really used to do all that stuff?)

Using a protective coating also changes "J" but the results are 
predictable. We used "radome" paint.

Anodizing doesn't help by the looks of things.

The greatest effect is at hi imedence nodes. Stripping the coating of the 
hi Z part of a dipole and "corroding it" has the same effect as corroding 
the whole dipole.

Simply washing with (deionised) water (I guess that was meant to simulate 
rain as we washed at varios "rates" Reduces, but doesnt eliminate the effect.

The conclusion drawn was that painting a protective coating was the best, 
and the change in dielectric constant should be taken into account at the 
design stage. The paint we used was "radome" paint, Which I believe is 
classified. It was a two part mix.


For what its worth.





Norm. Vk2XCI.
QF27wd, Mount Hope, NSW, Australia
Voice of The Edge Of The Outback.

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