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Re: [TowerTalk] Molded coax junctions

To: Pete Stark K4OM <>, Towertalk Reflector <>
Subject: Re: [TowerTalk] Molded coax junctions
From: jimlux <>
Date: Sun, 18 Jul 2010 17:58:18 -0700
List-post: <">>
Pete Stark K4OM wrote:
> Does anyone know the proceedures and materials needed to form molded 
> junctions  (Tee's generally) from rg11 and rg8 size cable.  I have a harness 
> to re-do for an 8 pole 2 meter repeater antenna and I do not trust coax 
> connectors or coax Tee's at 900 feet.  The last harness lasted 25 years and 
> it was of the molded type.  Thanks

Are you talking about potting, say in silicone?  You get the appropriate 
two part RTV (Newark, etc. carry it) and get some suitable forms and 
cast it.  The key is being able to pump a vacuum on it.. you need to put 
the cable into the chamber and pump it down first (to remove the gases 
in the tiny space in the shield and between dielectric and conductors), 
then you add the potting compound, and let it cure.

Your RTV needs to be a very slow cure type (because you have to mix it 
up before you pump down, so it can degas before you pour).

If you use a thermally cured plastic, rather than an RTV, you can pump 
down, then heat it up to cure.  I think that's what the commercial cable 
  companies use, but they've also invested in the tooling and process 
experimentation to get the right material and temperature profiles.  You 
could go through a few dozen shots before you get it right.

Actually, if you're not worried about high voltage/high power breakdown, 
you could probably skip the degassing by pumping down part, and just 
cast the joints in epoxy, urethane, or RTV silicone.  The key, I 
suspect, will be getting a good bond between the potting material and 
the jacket material of the coax, which is more a matter of choosing an 
appropriate cable and/or surface preparation (e.g. if you have a 
polyethylene jacket, it's hard to get stuff to stick to it).

  In the past when I've done this sort of thing, I've spent some time at 
the store (Sil-Pak) that supplies the potting compound talking to them 
about the appropriate thing to use, given what I was potting, the cure 
time, and so forth.  This is generally not something where you want to 
go to Home Depot and trust in the guy/gal with the orange vest.

I've seen people use plastic plumbing fittings as molds.. they're cheap, 
etc.  It's not as nice looking as a neatly tapered mold, but it's 
certainly easier.

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