[Antennas] Re: Old Coax (Plasticizer)
WD8 O KN
[email protected]
2002年9月13日 23:16:21 -0400
Excellent response! You are rather correct about polymers and their
compounds. They can also add other agents if the cable manufacturer
spends the money. These agents can allow the outside materials to age
better with the elements.
Sean
On 2002年9月13日 22:15:43 -0400 "Barry L. Ornitz" <[email protected]>
writes:
> 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]
>> - - -
>> Your moderator for this list is:
> Larry Wilson KE1HZ [email protected]
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