[Moon-net] Array Deterioration

[Edward R. Cole]

At 03:17 PM 1/7/2007 +0000, John Fridenstine wrote:
>
>Hello All,
>
>During the Fall my two meter EME array became increasingly hard of
hearing.  Even the big guns were weak.  Something was wrong but nothing
changed on the transmit side.  SWR was still 1:1, load, tuning and  power
out, all 350 watts, remained perfect.
>
>Fortunately December 28 was a sunny and unusually warm winter day.  I got
out the ladder and started unwrapping N connecters.  I disconnected cables
looking for a problem.   Before reassembling I brushed the internal mating
surfaces with a brass brush and weatherproofed.  I did the same to all
eleven connectors but was disappointed to find no obvious problem.
>
>That evening, the 29th UTC, I worked several larger stations and signals
seemed better.  During the last 10 days I have found I can hear and work
the smaller stations again.  Thirty QSOs,  eleven new initials and three
new DXCC since I cleaned the connectors.
>
>The current array, 4x12, had only been assembled 18 months ago so I didn't
really think there would have been a problem unless water had infiltrated
the connectors.  I wonder if the change in temperature caused condensation
corrosion under the water proofing.  Or perhaps it was just the normal
corrosion of silver surfaces 
>
>I wonder if others have had similar experiences.
>
>73,
>John W8PAT EN81vg

Hi John,

Assuming you are using N-connectors, they are somewhat resistant since they
use a gasket under the compression nut.  But I have been successfull
keeping my coax lines sealed by using heat shrink over the connector body
to the cable (LMR-400), then after connection sliding another heat shrink
over the complete connection, and finally using Scotch-88 tape and some
Scotch-Kote to be sure.  

My challenge is that fall wx is very rainy here (coastal southcentral
Alaska) and that always seems to be when I am doing antenna assembly.  This
year we bought a new home so everything was taken down in Sept. and put
back up at the new QTH (QRV Dec.10).  I had my 4x M2-2mXpol-20 array up
since 1999.  On reassembly, it made sense to measure line-loss and SWR on
each piece as it was installed, to insure all would work well.  Since my
array is four xpol antennas this involves 8 phasing lines (18-foot
LMR-400), two M2 4-way dividers, two 10-foot az-el jumper lines (LMR-400UF)
to my preamp/coax relay box.  That's 20 N-connectors.  There are more in
the Tx and Rx lines to the house.

Interestingly, I discovered two phasing lines with bad SWR.  Upon
disassembly of the connectors I found one with slight moisture and another
with a short from a single shield whisker to the center pin.  How that
worked before is beyond understanding.  I did have a higher SWR (1.25) on
one line in the old array that defeated tuning...hmm.  When repairs were
made and all retested my SWR was 1.06 to 1.08 on all lines (using Bird
meter).  Phasing line loss ~ 0.2 dB.  In the shack with 185w forward the
reflected is 0.3w (of course the Tx line is 170-feet long with 1.7 dB
overall loss so SWR will look better than it is).

I guess what I am wanted to say is with cable assembly, wx proofing is
critical to long trouble free use.  I do this kind of work on commercial
radios at work and have ship antenna lines that survive over five years in
marine conditions (which are terrrible in Alaska).  SWR alone will not tell
the whole story.  Line loss measurements help.  But on-air reception will
be the final test.  Sounds like you may have had some oxidation in some
connectors if nothing visible was discovered (the wire brushing would
suggest that). 

BTW my system seems to be hearing better at the new QTH.  I had to work
into winter wx getting things back up is far from optimum, so I am hoping I
avoided getting moisture into any connections.  Since it is -4.9F today,
all that is frozen.  Thermal changes can really cause ingress of moisture
if sealing is not perfect.  My choice of the M2 xpol-20 was good as this
antenna is not sensitive to rain/ice/snow build-up so SWR is easier to
manage in the Alaska wx.  I can use the array 144-146 MHz, though SWR does
climb at the upper end.

Hope we can work each other in the near future!  PS: I am originally from
Michigan (ex-K8MWA).
73's,
Ed - KL7UW  
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http://www.qsl.net/al7eb - BP40iq 
144-EME: FT-847, mgf-1801, 4xM2-xpol-20, 185w (QRV)
432-EME: Not QRV
USA Rep. for Dubus Magazine: dubususa at hotmail.com
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