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Re: [TowerTalk] Splicing LDF4

Subject: Re: [TowerTalk] Splicing LDF4
From: David Gilbert <>
Date: Wed, 27 Aug 2008 18:24:40 -0700
List-post: <">>
Steve, I've played around quite a bit with ways to make inexpensive 
connectors for 1/2 inch Heliax, and here is my suggestion for a splice.

The inner conductor of 1/2 Heliax is 0.190 inch in diameter and made 
from copper plated aluminum.  7/32 inch thinwall brass tubing makes an 
excellent slip fit over the Heliax center conductor to make a solderable 

For the shield, rigid copper tubing for plumbing applications comes in 
three wall thicknesses ... types K, L, and M (type M has the thinnest 
wall).  Type L copper tubing is commonly available at all building 
supply stores (including Home Depot, Lowes, etc) and makes a nice fit 
over the corrugated copper shield of 1/2 Heliax with just a very slight 
amount of deformation of the copper shield ... you can twist the 
plumbing over the shield with your bare hands.  You could make a splice 
from one piece of tubing by sliding it down the Heliax before splicing 
the center conductor, and then sliding the 1/2 inch plumbing back over 
the joint and carefully soldering both ends with a propane torch.  I try 
to use a long enough piece that the solder joint is well back from the 
exposed foam.  Or you can use two pieces of 1/2 inch plumbing and join 
them at the middle with a normal plumbing solderable sleeve coupling.  
Either way, if you are careful with the heat and use non-corrosive flux 
you should be able to make a water-tight joint.

I've never tried it, but you could even backfill the finished splice by 
pre-drilling two small holes (one at each end of the 1/2 copper tube) 
and squirting polyurethane foam in one hole until it comes out the 
other.  That might help improve high voltage performance.

If you also want to try making your own connectors for the ends of 1/2 
inch Heliax, check out the 7/16 by 3/8 brass plumbing adapter made by 
Watts and sold by Home Depot ... the designator is A-200 and it has a 
green stripe across top of the bag.  A light touch with a file will 
allow the male end to be soldered inside of 1/2 inch Type L copper 
tubing, and the female end has exactly the right threads to accept a UHF 
barrel.  For the center conductor, you need to transition from the 7/32 
brass thinwall tubing over the center conductor of the Heliax to 3/16 
and then to 5/32, the latter being the right diameter to make a nice 
surrogate pin to go through the plumbing fitting and into the UHF barrel.

ACE Hardware's online outlet store carries a really wide selection of 
brass tubing (made by K&S Engineering) at much better prices than I 
could find anywhere else.  I bought several feet of it ... if you need 
just a couple of inches, let me know and I'll send some to you.

One of these days I'll take some pictures of my experiments and upload 
them to my web site.

Dave   AB7E

Steve London wrote:
> Anyone have some good tricks for splicing Andrew LDF4 1/2" Heliax ?
> This would be for HF use, only - not worried about a small impedance bump.
> 73,
> Steve, N2IC
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