[Moon-net] Array Deterioration

Sun Jan 7 18:56:59 CST 2007

Hi All.

This thread brings up a couple of interesting points.
The first one is that the result of a copper and aluminum junction
in an antenna is so obvious that it is surprizing to me that anyone
would use it in an EME array. Aluminum and copper are very
far apart in terms of battery action and make for a very good battery
with the result being a leaching of one material to the other in the 
presents
of even slight moisture. I use a "T" match made of Brass and it is soldered
to the "N" connectors in the case of the horizontal elements and 1/2"
hardline and it is soldered directly to LMR400 in the case of the vertical
elements. I use large aluminum clamps with electrical antioxidant at the 
junctions.
This brings me to the other point. I try to avoid air spaces inside RF 
connections
when they are outside. The problem of getting a good seal is similar to 
trying
to patch a tire under pressure. It really does have to be completely air 
tight
or water will be inside your airspace. So, my first rule is not to have air 
spaces,
and after that, I use rubber tape followed by nylon tape. Sometimes I will 
follow
that with Krylon spray paint or ScotchKote. If there is an airspace inside 
your
RF connection, you can be sure that moist air will enter it and condense to 
become
water. Water gets in, but not out. Here's a tip. I KNOW water will get into 
my
preamp boxes. I have a hole in the bottom to let water drain out. This has 
protected
my relays pretty well. I just should have stayed off the air one night when 
it was
very humid, foggy, and rainy (burned the contacts).

In my climate, corrosion is not generally a problem with the radiating 
surface of the  elements, but I DO
like the idea of anidized elements. Some experienced moonbouncers on 2M
have stated here before that the elements really DO have to become pretty
corroded before there is degredation.  I use Krylon spray paint on my 
elements, but
it will wear off in a few years. I didn't hear any mention as to the RF 
connectors
used, but I have seen aluminum connectors badly corroded after a few short 
years.
One real possiblility is that when your rf connections become corroded, they 
become
diodes and IMD will degrade your reception. Another reality is that as your 
neighbors
get more toys, Rf  pollution is greater.

I think the best solution is to keep water out as best you can and just plan 
for
water being present inside the RF connection. This makes choice of feedlines
and connectors important. A good plan is critical. Does your feedline carry 
moisture
from end to end? Do your RF connectors use internal seals?

73, Jim Shaffer, WB9UWA.
Array details at:
What may not be evident is my dependance on Krylon spray paint !
http://www.gm4jjj.co.uk/WB9UWA/details/index.html

----- Original Message ----- 
From: "Lance Collister" <w7gj at accessoutwest.com>
To: "Bernhard E. Keiser" <keiser at ieee.org>
Cc: "moon net" <Moon-net at list-serv.davidv.net>; "Magic Band EME Group" 
<magic at 6meme.com>; "John Fridenstine" <w8pat at eriewave.com>
Sent: Sunday, January 07, 2007 2:26 PM
Subject: Re: [Moon-net] Array Deterioration

> Bernhard E. Keiser wrote:
>> John, I've experienced the same problem (not hearing well), and am 
>> looking
>> for the solution.  However, all my coax connectors are covered with
>> Coax-Seal.  Were yours?
>> 73,  Bernie. W4SW
>>
> Hello Bernie,
>
> I think a critical question to ask at the beginning of the investigation
> is what kind of antennas are exhibiting these problems.  For tropo or Es
> or strong local signals (which is for most antenna users), this hearing
> loss would probably not be so apparent.  However, when it comes to EME,
> the slightest degredation is immediately apparent to the operator.
>
> Some antennas (at least the older M2 models) created a connection by
> crimping aluminum rod down on copper wire inside the connector block on
> the driven element.  This connection is very prone to corrosion by
> virtue of the contact between the dissimilar metals, and the result is a
> buildup of resistance between the two metals.  As you can find from a
> simple internet search, this phenomenon takes place regardless of
> whether the connection is sealed against the air and water or not,
> although it is accelerated if exposed to moist air and/or water.
>
> This corrosion is not easily noticeable on transmit, since with high
> power, the resistance is easily overcome.  However, the result on
> receive is that weak signals simply disappear entirely.  I don't know if
> this manufacturing procedure is still currently used at M2 or not since
> it has been a couple years since I looked into this  (a simple solution
> is to crimp the aluminum over copper tinned with lead bearing solder),
> but the driven element connections on many dozens of M2 yagis that have
> been used on EME have had to be rebuilt to correct this problem.  W5UN
> discusses one solution on his website, which was to use a special
> brazing alloy to braze the copper wire to the aluminum.  W1JJ replaced
> the lower T match bars with copper tubing so they could be soldered to
> the copper wire inside the connector blocks (be sure, however, to tin
> the copper tubing where the aluminum T match shorting bars contact them).
>
> If it is some other type of antenna (commercial or homebrew) which is
> causing the problem, the concept is still something to be carefully
> addressed. Good luck and 73, Lance
>
> -- 
> Lance Collister, W7GJ (ex: WN3GPL, WA3GPL, WA1JXN, WA1JXN/C6A, ZF2OC/ZF8)
> P.O. Box 73
> Frenchtown, MT  59834  USA
> QTH: DN27UB
> TEL: (406) 626-5728
> URL: http://www.bigskyspaces.com/w7gj
> 2m DXCC #11/6m DXCC #815
>
>
>
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