[Antennas] Old Coax

2002年9月12日 12:31:46 -0400

I would agree that some coax used in some applications will last a
lifetime...
And.. some coax used in other applications will last MUCH shorter.
As an example:
I have in-house coax of WWII vintage (dated 1945 and acquired on a spool in
1965) used for low voltage apps (intercom & alarm) that's been installed
since 1971 and is doing fine. The outer covering is a bit brown and dried
where it's exposed in the basement and attic, but even the front door
intercom is still clear as a bell after 30+ years.
I've had coax used up the inside of a sailboat mast that needed to be
replaced after four years. This was primarily due to the inability of
securing it inside the 35' mast. The cable would slap around inside the
mast (although not noisily) almost 7x24. The entry and exit points were
well sealed and secured. After the one event of having to drop the mast
during the year to replace it, I decided to replace it every two years.
I have some (equiv of 213) coax used for medium power (less than 50W) that
powers my two attic sat loops. The wire has been in place for over 15 years
and is fine.
Our cable TV coax that comes from the pole into the house has been in place
for almost 20 years and looks as good as the day it was installed. This
cable runs quite a ways from the road to our house, and has unsupported
sections of about 100' each. Cable TV companies must use a coax that has
excellent UV protection and sheds ice very well.
A coax that runs from house to barn (actually a multi-conductor cable inside
a coax like shield/sheath) has been in place and working fine for intercom
and telephone (both relatively low voltage apps) for 30 years and shows no
sign of aging. It does have an unsupported span of about 75'. We do plan
to put the cables underground when the power line is buried next spring - a
long overdue project.
Outdoor coax sections (from where they leave the attic to get to the short
masts on roof and chimneys) are replaced about every 4 years. Again, this
from experience of having to replace one piece during a Thanksgiving
holiday.. Brrrrrrrr.... never again. All maint is now done during late
spring or early fall.
I don't have a rotor anymore - but when I did the last 9 feet ( three
sections for three antennas) that took the flex was replaced almost EVERY
year. Again, this based on the advice and experience of others.
Coax that goes to an outdoor dipole has been up about 5 years now and still
functions AOK. Although I very rarely put in any more than 100W - more
typically 10-20W. High power does breakdown the dielectric properties
somewhat in most cable.... I don't push the 1KW+ of a few of my friends,
but if I did, I would definitely be thinking about replacement every time
the antenna came down for maintenance.
We also have a sea-side winter home... I take the antennas inside when we
pack up to leave in late March every year. So far -- 4 years on the same
coax, but pushing less than 100W on all applications. This winter will be
replacement time for all the outdoor sections.
-------
The very last of the oil-fired steam-boiler carriers built was recently
decommissioned. Installed on the ship (both inside and out) were many,many
miles of coax for a number of applications - including high power HF and VHF
to mast mounts -- most of it there from the day the ship was built well over
40 years ago. The secret to it's longevity may be the fact that the outdoor
sections were well secured AND painted over during most refits.
-----
So, as a previous poster noted so aptly: Your mileage may vary.
UV protection, casing resistance to moisture, keeping the wire from flexing,
keeping power well under max spec for the specific cable (many people ignore
this - not even knowing what max power is for their particular cable),
sealing the connections, and application ALL have an effect on useful coax
life.
Picking the right coax for the application and staying within it's specs
will give you max cable life. I'm not a cable engineer - so I get my advice
from those that are. And the best advice I ever got was to buy the RIGHT
cable for the job and very best you can afford..
73
Frank
 ._._.
----- Original Message -----
From: "Lofstead, Jerry" <[email protected]>
To: "'Lefczik, Larry D'" <[email protected]>;
<[email protected]>
Sent: Thursday, September 12, 2002 9:48 AM
Subject: RE: [Antennas] Old Coax
>> Larry,
>> Coax does age and become more lossy with age. Their especially shows
> y=up when used at VHF and UHF frequencies. The plasticizers used in the
> plastic to make it flexible, contaminate the center insulator. There
> are non-contaminating types of coax available. The useful life is
> determined by measuring power into the coax terminated in a dummy load
> and the power measured at the dummy load. If you are satisfied with the
> loss, it OK for your application. Most coax cable today are good for at
> least 5 years. You may be interested in getting "the wiermans coax book"
> or look at www.wireman.com .The www.davisrf.com site also has good
> info on coax losses.
>> Your mileage will vary compared to anyone else's...
>> Jerry
> W3CDE
>> -----Original Message-----
> From: Lefczik, Larry D [ mailto:[email protected]
> <mailto:[email protected]> ]
> Sent: Thursday, September 12, 2002 9:13 AM
> To: [email protected]
> Subject: [Antennas] Old Coax
>>> I've seen several times that coax cable should be replaced after a few
> years. Why is this? Is it just because of UV exposure to the outer
> jacket?
> If so, is coax which is still on a spool good forever? Is coax used
> indoors
> subject to aging? What about MIL-STD coax, is that less susceptible to
> aging?
>> A lot of questions, but the main question is, why is it recommended to
> replace coax periodically?
>> Thanks de WA2EHZ
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