[Antennas] Re: Old Coax (Plasticizer)

2002年9月13日 22:15:43 -0400

Larry, WA2EHZ, asked:
> I've seen several times that coax cable should be 
> replaced after a few years. Why is this? 

Increased RF losses.
> Is it just because of UV exposure to the outer jacket?

This is generally a secondary issue. By the time the 
jacket has dried and cracked, allowing moisture to enter, 
you would have usually seen high losses many years earlier.
> If so, is coax which is still on a spool good forever? 

No, but certain types can last quite a while.
> Is coax used indoors subject to aging? 

Yes.
> What about MIL-STD coax, is that less susceptible to 
> aging? 

Somewhat, but better materials are available.
Lots of short answers but now the details...
PVC, polyvinyl chloride, is the jacket material for most 
coax. This is a rigid, brittle material unless it is 
plasticized. Plasticizer is blended into the vinyl when it 
is compounded to soften it and make it supple. 
Plasticizers are high molecular weight liquids that act 
much as an internal lubricant to allow the long-chain vinyl 
molecules to slide past each other easier. Think about a 
bowl of cooked spaghetti that has dried sufficiently that 
the strands stick together. Now consider that same 
spaghetti covered in olive oil after cooking and then 
allowed to stand. The oil covered material is much more 
flexible as the individual strands no longer stick 
together.
[Bowls of spaghetti are commonly used as analogies by 
polymer scientists!]
Many materials are used as plasticizers such as dioctyl 
phthalate, dioctyl adipate, trioctyl-trimellitate, di-
undecyl phthalate, etc. My former employer, Eastman 
Chemical Company, made over a dozen different varieties. 
All of these materials have exceptionally low vapor 
pressures at room temperature. Note that I said low, not 
zero. 
In addition to the plasticizers, vinyl plastics also 
contain various stabilizers such as tin, antimony, cadmium 
and lead compounds.
At radio frequencies, polyvinyl chloride is a moderately 
lossy dielectric by itself. It becomes even more lossy 
when plasticizers and stabilizers are added.
As vinyl plastics age, the plasticizers can slowly diffuse 
out of the raw PVC. They do evaporate slowly (which is the 
reason new vinyl in automobile seats "bakes out" during the 
summer to produce an oily, difficult to remove, film on the 
inside of the windows). They also diffuse to the vinyl 
surface. I am sure everyone has seen old vinyl electric 
cords that have become sticky and greasy as the plasticizer 
has migrated to the surface.
Now in a coaxial cable application. the loss of plasticizer 
leads to several problems. First, the jacket becomes 
brittle, shrinks and cracks. But long before this has 
happened, the plasticizer has also migrated through the 
shield braid and into the polyethylene dielectric covering 
the center conductor. This increases the RF loss of the 
cable tremendously. At DC and audio, it doesn't make much 
difference, but at radio frequencies the loss can be 
considerable.
Several things have been done to try to alleviate or 
eliminate the problem. The earliest attempt was to use 
plasticizers that were of higher molecular weight and that 
diffused more slowly. This is the approach taken by the 
military in the so-called "non-contaminating" vinyl 
coverings. The Mil/Std coax lasts quite a while longer but 
it is not perfect.
An alternative approach taken by some other manufacturers 
is to use a jacket material other than vinyl. I have some 
Times T4-50 coax cable that is 25 years old and it still 
has less loss than new RG-213. It has a carbon black 
loaded polyethylene jacket. The surface is pretty 
weathered by now as polyethylene does not hold up to 
ultraviolet light very well but the heavy carbon black 
loading has done its job of protecting the polyethylene. 
Other manufacturers make similar cables. Unfortunately 
these cables are stiffer than vinyl ones and they can 
withstand less flexing such as around a rotor.
The cable TV industry has taken a different approach. The 
main lines are, of course, rigid cable with solid aluminum 
sheaths. But the drop lines into houses are generally 
still flexible coax. Here they wrap the insulated inner 
wire with aluminum foil before the braid is applied. The 
foil is impermeable to the plasticizer, but if simple 
wrapping is used the plasticizer can slowly migrate between 
the foil wraps. To get around this, the foil may be sealed 
with a glue, or in some varieties, a metallized Mylar layer 
is used. The polyester (Mylar) has a very low permeability 
to the plasticizer, and it can be sealed. The foil and 
metallized polyester provide excellent shielding (much 
better than the "window screen braid" seen on cheap coax). 
Since the foil provides such good shielding, the CATV cable 
manufacturers often skimp on the braid. This is 
regrettable for us hams as the foil, and particularly the 
metallized polyester, cannot handle much power in 
transmitting applications.
Several other people mentioned water getting into coax. 
This is a big problem when poor connectors are used. Water 
makes the cable exceptionally lossy, and corrosion on the 
braid ruins the shielding. NEVER let water get into your 
coax. I generally wrap connectors with at least two layers 
of stretchy plastic tape, spray with clear Krylon spray, 
let dry and wrap again with another tape layer. It is a 
little messy to remove, but I have taken apart joints a 
dozen years old to find the connections as bright and shiny 
as the day they were made. In this regard, I like N or BNC 
connectors. Not only do they provide better RF 
performance, they are waterproof and will not let water 
from one section of coax pass through to another section 
like many UHF connectors can.
So to go back to the questions, plasticizer migration can 
occur on the spool or indoors. The migration increases 
with higher temperatures however. Ultraviolet exposure can 
damage a coaxial cable jacket, but the time frame is 
generally much longer than the plasticizer migration 
issue. If you really want exceptionally long life, look to 
cables with jackets other than vinyl. or go to hardline.
Note that the more flexible and supple a vinyl insulated 
wire is, the most plasticizer it contains. I love to hear 
audiophools rant about their multistrand oxygen free copper 
welding cable with the thick, supple vinyl jackets! Once 
the plasticizer migrates to the copper, it become almost 
impossible to solder. And when the stabilizers are carried 
out with the plasticizer and heated, they make nasty 
corrosive compounds too.
 73, Dr. Barry L. Ornitz WA4VZQ [email protected]