Having spent 11 years working on radar systems in the Marshall Islands, I'm
a bit of a expert of issues of maintaining antennas in tropical salt
conditions. In the case of the Marshall's, you are setting 8 degrees north
of the equator. The temperature is a constant 80 degrees F (plus/minus 5
degrees) year round and there are constant trade winds blowing 10 months of
the year coating all exterior surfaces with warm salt spray. Keeping
anything outdoors in this environment is a real challenge. For example, a
new bicycle will last a couple of years and either the frame will rust in to
or the spokes will rust out.
On the antenna side, the real issue is the corrosion due to the dissimilar
metals. Anywhere there is a dissimilar metal, there is a galvanic reaction
and the stronger material will eat away the weaker material. For the
amateur antennas, you traditionally have stainless steel bolts holding
together aluminum elements. The aluminum will simply erode away where it is
in contact with the stainless. Here is a picture of a less than six-month
old Mosley antenna, http://www.ab5k.net/images/kwajant2.jpg. You can see
on the aluminum phasing lines where the stainless bolts are attacking it.
You can see the Mosley boom at http://www.ab5k.net/images/kwajant1.jpg.
The stainless steel u-bolts are very rusty. They were treated with
never-seize before they went up. I believe the use of the never-seize
probably increased the rusting on the bolts due to adding more dissimilar
metals. That is not really bad as a good coat of rust provides a protective
coating which helps slow down the corrosion a bit.
The corrosion issue is serious and is very hard to stop. Any antenna taken
into a environment like this needs to be hardened. For example when
assembling aluminum elements use a aluminum pop rivet rather that a
stainless steel bolt or hose clamp. You will probably still have some
dissimilar metals due to different alloys in the aluminum but it will slow
down the corrosion.
On areas where you have to have dissimilar metals, due to structural
strength requirements, the use of a sacrificial washer works well. Rather
than have the stainless bolt erode away the aluminum element, you place a
sacrificial aluminum washer between the stainless bolt and the aluminum
element. That way most of the corrosion will take place on the sacrificial
aluminum washer. The sacrificial aluminum washer can be replaced as
needed, assuming you can get the rusted bolt out.
At work at the TRADEX radar, dissimilar metals also caused both major and
minor repairs.
http://www.ab5k.net/images/tradex.jpg.
http://www.ab5k.net/images/TradexFeed.jpg
http://www.ab5k.net/images/TradexFeed2.jpg.
73 Terry AB5K, ex KX6OI and V73AQ
www.ab5k.net
Mike describes the effects of the severe Aruba ocean atmosphere on an
aluminum vertical over 2 years. I wonder if an initial assembly using
thorough greasing of all joints with Noalox or equiv, and periodic touch
ups
could preserve the connection integrity for a significant period, or
whether
even then, the attack by the salty air is not significantly slowed.
73, DX, de Pat Barthelow AA6EG aa6eg@hotmail.com
From: "MIKE GREENWAY" <K4PI@peoplepc.com>
To: "Jim Lux" <jimlux@earthlink.net>, <TOWERTALK@CONTESTING.COM>
Subject: Re: [TowerTalk] RADIAL WIRE
Snip;
On the subject of
aluminum oxidizing , we had a Gap Challenger that was put up new in Aruba
using the supplied hardware. In 2 years the aluminum started turning to
white oxide powder where the screws go through the tubing. The holes for
the screws go so big and the area around them got so brittle it had to be
taken down 73 Mike K4PI
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