[amsat-bb] Exciting Coax

Donald Olson olsonda1 at yahoo.com
Wed Jan 24 16:27:50 UTC 2018

This has been an interesting chain of posts and I have copied it for distribution at our club.  Thanks. I will continue to follow it.

However, to those out there that think that you can eliminate water in coax with proper sealing, you are only delaying the inevitable, especially if you live in a humid climate like New Orleans.  As an engineer familiar with the psychometric chart, unless you can make your connection "air" proof, anything except DRY air or DRY nitrogen positive pressure will allow water to accumulate.  The theory is this:  at "relatively" high temperatures, air in the coax holds more water as a vapor...invisible.  Normal atmospheric pressure changes moves air in and out of the air spaces in the coax.  As the day or season cools the air inside can hold less water vapor and some condenses out as liquid.  (Think of the liquid condensation on the outside of a glass filled with ice water.)  The liquid deposits itself in the air spaces within the coax and "essentially" never leaves.  Continued cycles of air pressure and humid air condense more and more liquid water within the coax.  It is unavoidable.  It may be possible to drain it out, but you won't get it all because of capillary forces. 

Without positive internal pressure, the enemy turns out to be the void (air) space in the coax.  Eliminate the void space and there is no where for the (humid) air to get into with changes in atmospheric pressure.  

I use LMR-400 with N connectors and "shelter" my connections with three layers of waterproofing.  I say "shelter" because a true seal is impossible.  I wrap from the bottom up to get "shingling" to repel liquid water.  Water vapor (humidity) is impossible to avoid at a reasonable cost.  When SWR's go up, the coax cable is over, and trashed.  Don't buy used coax!  

Keep the "Exciting" comments coming.   Don Olson, PE, WA5WTreasurer and VEC Chairman
Jefferson Amateur Radio Club, Inc. W5GAD, WW2NO
P.S.:  Our Ham Radio Club has suffered some damage recently and Amazon Smile will help us raise money to fix things.  When you #StartWithaSmile, Amazon donates 0.5% of your purchase price to Jefferson Amateur Radio Club Inc (501c3) without increasing the cost to you. Bookmark the link
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Amateur Radio:  Helping protect America with continued communications in times of crisis and having fun while learning and practicing our skills.

      From: Edward R Cole <kl7uw at acsalaska.net>
 To: AMSAT-BB at amsat.org 
 Sent: Tuesday, January 23, 2018 7:30 PM
 Subject: Re: [amsat-bb] Exciting Coax

Important to point out that the F7 variant of 9913 has no air 
dielectric which got the original 9913 into trouble.  Also LMR cables 
use aluminum foil shield covered with wire braid shield.  If exposed 
to water the aluminum corrodes and gets quite lossy.  So wx sealing 
is still necessary (two wraps of good electrical tape at 
minimum).  Good electrical tape = Scotch 33+

I also seal with heat shrink tubing but often cover that with electrical tape.

LMR-400 is generally good to 432.  It is not anymore flexible than 
Belden 9913.  For better flex consider LMR-400UF 
(ultra-flex).  Slightly higher loss but better to go around az-el joints.

For long runs at 432-MHz one should consider LDF4-50 Heliax hardline 
(1/2-inch) or lLDF5-50 up to LDF7-50.  Loss is higher at 1268.  I 
have 15-foot run of LDF4-50 from my tower-mounted DEMI 144/1268 Tx 
converter.  That gets the most power to the antenna and only needs 
12vdc, PTT, and 144-MHz coax to input.

Some archive photos: http://www.kl7uw.com/sat.htm
Drawing shows current satellite antenna installation.

73, Ed - KL7uW

From: Bob- W7LRD <w7lrd at comcast.net>
To: amsat-bb at amsat.org, anderson58625 <anderson58625 at gmail.com>
Subject: Re: [amsat-bb] Exciting Coax
Message-ID: <992329878.579633.1516693575413 at connect.xfinity.com>
Content-Type: text/plain; charset=UTF-8

For 2 & 70cm I use LMR400.  LMR is better (in my opinion), in that 
it's more flexable, 9913 can get water inside.  Unless coax 
connectors are super sealed.
73 Bob W7LRD

73, Ed - KL7UW
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