[Antennas] Re: Removing the inner from coax.

[Barry L. Ornitz]

Martin Sole, HS0ZED G4UQF, asked:

> I want to use the dielectric from RG8/RG213 coax cable as a 
> spacer for some dipole wires and open wire feeder. Removing 
> the outer and braid is easy enough but pulling the inner out 
> is hard work. All in all I will need about 100 pieces 
> approximately 6 inches long each so the effort involved 
> seems a little excessive. There has to be an easier way, any
> ideas?

Several suggestions followed, but I tend to agree with George, 
W5YR, who wrote:

> Yes - do something else.  <:}

Polyethylene, the plastic material used for the coax 
dielectric, has no ultraviolet resistance.  Used outdoors, it 
will quickly break down and eventually crumble.  Long before 
this happens, the surface will become crazed with small cracks 
which hold dirt and moisture and make it quite lossy.

Dave, W9LYA, suggested the use of small diameter PVC 
(polyvinyl chloride) pipe instead.  While PVC is not a 
particularly good dielectric at radio frequencies, it will 
work well in this application if you are talking about 
relatively wide spacings (2.5 cm, 1 inch, or more).  The rigid 
pipe will hold up far better outdoors than the soft PVC clear 
tubing.  There are several reasons for this: first the rigid 
pipe has white or black fillers that will refelct or absorb, 
respectively, the ultraviolet light that does the damage, and 
second, the soft tubing contains considerable amounts of 
plasticizer (often over 50%) to make it soft.  Most 
plasticizers will eventually leach out of the vinyl, and all 
have poor dielectric properties at radio frequencies.

Chlorinated polyvinyl chloride, CPVC, pipe is also available.  
It has a higher heat resistance, better chemical 
resistance, and is somewhat less flammable.  Its low frequency 
electrical properties are similar to rigid PVC but it has less 
ultraviolet resistance.  The dielectric properties of CPVC at 
RF are worse than rigid PVC.

Recently I ran across another item that should work well in 
this application.  I found fiberglass rods at the local farm 
supply store that are used for temporary electric fences.  
These rods are about a half inch in diameter (12 mm), and 
about three feet long (slightly shorter than 1 m).  They are 
very strong and rigid.  Being glass filled, they will wear out 
steel drills in short order, but they can be drilled with a 
carbide bit easily.  They cost about $1 each.  As spreaders 
and insulators, they should last quite a while outdoors.  A 
coat of acrylic paint, such as Krylon, will waterproof them 
and provide even longer ultraviolet protection.

        73,  Dr. Barry L. Ornitz     WA4VZQ     ornitz@tricon.net