Most antenna aluminum is 6061 or 6063, an alloy that is not the best
choice for corrosion resistance. It is chosen for yield strength.
From my sailing experience -
Powder coatings on a mast work on Al for a while, it is very important
that the appropriate chemical conversion of the Al surface be done right
before the coating. This is true for any paint as well. What I have
seen is once the polymer film is disturbed, corrosion tends to wick
under the film, with characteristic bubbling of the film surface from
the corrosion underneath.
I've found that hard anodizing can last a long time in salt air
exposure, particularly if it is cleaned and waxed/sealed/impregnated
from time to time. Antenna alloys take anodizing very well, but finding
an anodizer for long tubing might be a challenge. The overlap sections
probably need to be masked so the electrical conductivity is preserved
and then liberal applications of Noalox etc. may keep the joint from
seizing up.
There are "chloride" concentration maps available on the web, that show
the concentration in air from salt spray/mist. I can't find the
concentration map I've previously seen, but here is a link with some
interesting data
http://www.imoa.info/moly_uses/moly_grade_stainless_steels/architecture/coastal_salt.php
This link would indicate that the proximity of antennas to seawater
needed for superior propagation (<< 1/4wl) will have maximum corrosion
exposure. It also indicates that to get away from high chloride
concentrations, it might take a distance of 15km to the beach. So, your
local topography and antenna installation may have more impact. An
elevated site with a near and clear shot to the ocean behaves as a high
tower, so think of your site as adding 20' or 200' to whatever tower you
plan. The "high bank" sites around the Pacific Northwest can add
significant effective height to a tower. Then there is some improvement
in the gain with the sea out to a few thousand feet. You can use HFTA
to get a feel for this, it has a uniform ground. I don't know if there
is software that can simulate arbitrary ground conductivity patterns
that would be the case with the antenna several hundred feet from the ocean.
We are planning a DXpedition with the vertical antennas about 10'-20'
from sea water, that is what is reported it takes to get the propagation
magic, besides the rare call.
Grant KZ1W
-----Original Message----- From: Mike & Coreen Smith VE9AA
Sent: Friday, May 17, 2013 10:17 AM
To: towertalk@contesting.com
Subject: [TowerTalk] How close to salt water is close enough?
Wife and I are looking at waterfront properties in VE1/9/VY2/VO1/2.
Ideally
I would like to locate my antennas inland (in the trees)
a little ways to (hopefully?) minimize(??) salt water corrosion and/or
'potential' problems from neighbours.
If I was to locate a yagi up 40' of tower, (or say a Butternut
vertical 10'
agl) or even a 6m yagi on a short tower inland does it matter?
Let's say the rocky land itself was 20' ASL and 400-500' away from
shore, is
there any benefit seen in being "close" to salt water or is all the
benefit
(whatever that may be) seen only if the salt water is nearly
underneath the
antenna?
Currently, I work all bands from 160m thru 6m.
Comments on corrosion prevention appreciated also. I've always lived
inland
and my only experience with salt water was 3 weeks in 1996 as CY0AA, in
which the salt air corroded all of our aluminum antennas making them look
very old.
Thanks much,
Mike VE9AA
Mike, Coreen & Corey
Keswick Ridge, NB
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