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Re: [TowerTalk] 9913 coax

Subject: Re: [TowerTalk] 9913 coax
From: Ian White GM3SEK <>
Reply-to: Ian White GM3SEK <>
Date: Wed, 20 Sep 2006 22:56:20 +0100
List-post: <>
Craig Clark wrote:
>It bothers me that 9913 coax gets a bum rap in the amateur radio
>field. I personally have used a 130' run for over 15 years to feed
>one tower and to date, there is no moisture in the cable. The key to
>use is proper waterproofing which is in the realm of all amateur's
>capabilities. The same holds true of Davis's Buryflex which has a
>foam dielectric. (Or any other coax for that matter.) You need to
>properly weather seal the connection to make sure water stays where
>it belongs, outside the coax.
>FWIW, I have never had a customer either complain or return a 9913
>order for or due to water ingress.
>Any combination of tape, liquid tape, self vulcanizing tape, duct
>seal, coax seal or Scotch-kote, to name a few products, will ensure
>water stays out of your coax.

Like K6BR, I've been around the block with airspaced cables like 9913 in 
a wet climate, with 1500W at 432MHz to punish any mistakes.

If the end of an airspaced cable like 9913 is not completely sealed, the 
space inside the cable acts as an expansion/contraction chamber which 
can pull in water vapor. When the temperature drops at night, the vapor 
condenses, and then the water runs away down the inside where it can't 
get out.

A very effective solution is to plug the entry into the airspace by 
pumping in a shot of hot-melt glue. Then connect the cable to the 
antenna in the usual way, and finish off with the waterproofing methods 
that Craig suggests.

If the cable comes all the way into the shack with no intermediate 
connectors, it is good to make a vent hole in the connector so that the 
airspace can follow changes in atmospheric pressure. If the airspace 
ends at another connector outside, then plug that end too.

Another benefit of fixing the ends is that it helps prevent the center 
conductor from slipping downward, which can be a problem in a long 
vertical run.

73 from Ian GM3SEK         'In Practice' columnist for RadCom (RSGB)


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