Does coax have a shelf life?

I was rummaging around my father's garage the other day and came across a
roll of URM43 that I know to be over 20 years old. It looks OK and the
copper is shiny. However, can I be confident that it will not have lost any
of its electrical or physical properties? I intend to take it on holiday
with me soon and don't want it to fail.

Advice please!!

Thanks
John/G4IRN

"John" wrote in message
...
Does coax have a shelf life?

I was rummaging around my father's garage the other day and came across a
roll of URM43 that I know to be over 20 years old. It looks OK and the
copper is shiny. However, can I be confident that it will not have lost
any
of its electrical or physical properties? I intend to take it on holiday
with me soon and don't want it to fail.

Advice please!!

Thanks
John/G4IRN


as long as it was kept dry and not exposed to excessive heat or chemicals
(oil or gasoline in a garage) it should be ok. give it a good flexing and
check end to end to be sure the jacket isn't cracked. a quick electrical
test is to hook it up to a dummy load, transmit on the highest frequency
band you can and measure the power in and out (if you only have one meter
you can make two separate measurements). as long as you aren't loosing too
much it should be fine.

John wrote:

Does coax have a shelf life?

I was rummaging around my father's garage the other day and came across a
roll of URM43 that I know to be over 20 years old. It looks OK and the
copper is shiny. However, can I be confident that it will not have lost any
of its electrical or physical properties? I intend to take it on holiday
with me soon and don't want it to fail.

Advice please!!

Thanks
John/G4IRN

I've never had new coax in fourty five years of hamming! I get old
stuff at flea markets or wherever and have used it with great success!
--
--------------------------------------
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Beating it with diet and exercise!
297/215/210 (to be revised lower)
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--------------------
Irv Finkleman,
Grampa/Ex-Navy/Old Fart/Ham Radio VE6BP
Calgary, Alberta, Canada

"Roy Lewallen" wrote
You can also determine the loss by measuring the input impedance or SWR
of an open or short circuited piece of cable -- the better the SWR, the
higher the loss. Loss in dB = 10 * log[(SWR + 1) / (SWR - 1)].

===============================

The above formula may be correct but it's not of any practical use.

(1) It applies only when SWR is measured by running alongside a slotted
line with a voltmeter, stopping to make a note of the voltmeter readings at
the max and min points, and then calculating Vmax/Vmin. It is of academic
interest and is entirely unrelated to line Zo.

(2) The error in application arises from the confusion caused by the
old-wives incorrect assumption that the TLI (Transmitter Loading Indicator)
actually measures SWR. It doesn't.

(3) If the ordinary TLI did measure SWR then the measurement would apply
only to 50-ohm lines. On any other line, such as old coax when Zo is
unknown, the measurement would be meaningless.

(4) If the cable to be checked was really 50 ohms and the so-called SWR
meter was standardised on 75 ohms then the measurement would be equally
meaningless.

(5) If the shack 50-ohm SWR meter can be relied upon (and most of them
can't) then the ONLY information to be gleaned is to measure both the
short-circuit and open-circuit SWRs. If they are in the same ball park then
the unknown cable is probably not far from 50 ohms.

(6) Line attenuation is a very small quantity. It is the most difficult of
all line parameters to measure accurately. The last instrument to choose is
the SWR meter in the shack. The most attrocious mechanical damage can be
caused over a short length without hardly any effect on attenuation over a
longer length. To detect the presence of other than catastrophic cable
faults is not possible by attenuation tests. There are far more sensitive
ways of detecting and locating faults.

(7) To check an old length of line just test for DC continuity and look at
the ends. If all is bright and clean there's nothing wrong with it.
----
Reg, G4FGQ

"Dick Carroll;" wrote in message
...
One of the worst mistakes I ever made was a new roll of 9913! :-)

( Took it a couple years but it did fill with water)

A friend of mine saw a puddle of water under his rig. Closer examination
showed it was coming out of the 9913. I think the flexible version is OK
though. Still, I bought LMR400 at work. I use 9913 indoors only.

Tam/WB2TT

Chop off 3 or 4 feet from both the ends and throw the ends out. Also
check the outer jacket for nicks and cracks. Any nick or crack will
let moisture in and cause problems. Most problems with older cable
will be from moisture seeping into the cable from the ends or through
nicks and cracks in the outer jacket. Do the cutting even if it has
connectors on it because most connectors especially PL-259 type are
not moisture resistant. Also look for places the cable has been
crushed, kinked or flattened. These will be bad spots. I've
successfully used 20 year old cable many times.

On Sat, 9 Aug 2003 11:11:45 +0100, "John" wrote:

Does coax have a shelf life?

I was rummaging around my father's garage the other day and came across a
roll of URM43 that I know to be over 20 years old. It looks OK and the
copper is shiny. However, can I be confident that it will not have lost any
of its electrical or physical properties? I intend to take it on holiday
with me soon and don't want it to fail.

Advice please!!

Thanks
John/G4IRN

"John" wrote in message ...
Does coax have a shelf life?

I was rummaging around my father's garage the other day and came across a
roll of URM43 that I know to be over 20 years old. It looks OK and the
copper is shiny. However, can I be confident that it will not have lost any
of its electrical or physical properties?

1) Build, beg, borrow or buy a decent wattmeter and dummy load of the
characteristic Z of the cable.

2) Connect the output of your rig to the dummy load through the cable
to be tested. Put the wattmeter between the rig and cable to be
tested. Measure power on the highest band.

3) Move the wattmeter so that it is between the cable to be tested and
the dummy load. Measure power on the highest band (don't change
anything else!)

Difference is loss at the highest band when matched. A little math
will give you the answer in dBs.

73 de Jim, N2EY

I disagree that it's not of any practical use. The MFJ antenna analyzer
uses essentially this method, and on every occasion where I've used it
(mostly on RG-58 cable, occasionally on RG-213), it agrees within a
tenth of a dB or so with measurements made with a Bird wattmeter. Agreed
that the Bird isn't terribly accurate, but it's not too bad for
comparative readings where the differences aren't great.

Assuming that the measurement is made with an SWR meter, I agree that
the result won't be accurate if the line impedance is different from the
SWR meter's. However, for the normal variations one sees in "50 ohm"
cable, the results are accurate enough to be quite useful. The method
certainly shouldn't be used for lines whose nominal characteristic
impedance isn't the same as the meter's. I apologize for not pointing
this out, and thank Reg for doing so.

Contrary to (1) below, no slotted line or voltmeter is necessary to use
this equation.

I do agree that this wouldn't be a good method to measure very small
amounts of loss. But for the original purpose of deciding whether old
cable is usable, it should be fine. If the loss is that small, there's
no reason not to use the line.

There are many occasions where a casual measurement is better than no
measurement at all -- always with the caveat that you realize the
limitations of the measurement's accuracy and don't try to draw
conclusions from it that aren't justified. This is certainly a method
that gives results which, under a number of circumstances, is quite useful.

Roy Lewallen
Certifed Reg's Old Wife, Nit-picker, and Busy-body

Reg Edwards wrote:
"Roy Lewallen" wrote

You can also determine the loss by measuring the input impedance or SWR
of an open or short circuited piece of cable -- the better the SWR, the
higher the loss. Loss in dB = 10 * log[(SWR + 1) / (SWR - 1)].


===============================

The above formula may be correct but it's not of any practical use.

(1) It applies only when SWR is measured by running alongside a slotted
line with a voltmeter, stopping to make a note of the voltmeter readings at
the max and min points, and then calculating Vmax/Vmin. It is of academic
interest and is entirely unrelated to line Zo.

(2) The error in application arises from the confusion caused by the
old-wives incorrect assumption that the TLI (Transmitter Loading Indicator)
actually measures SWR. It doesn't.

(3) If the ordinary TLI did measure SWR then the measurement would apply
only to 50-ohm lines. On any other line, such as old coax when Zo is
unknown, the measurement would be meaningless.

(4) If the cable to be checked was really 50 ohms and the so-called SWR
meter was standardised on 75 ohms then the measurement would be equally
meaningless.

(5) If the shack 50-ohm SWR meter can be relied upon (and most of them
can't) then the ONLY information to be gleaned is to measure both the
short-circuit and open-circuit SWRs. If they are in the same ball park then
the unknown cable is probably not far from 50 ohms.

(6) Line attenuation is a very small quantity. It is the most difficult of
all line parameters to measure accurately. The last instrument to choose is
the SWR meter in the shack. The most attrocious mechanical damage can be
caused over a short length without hardly any effect on attenuation over a
longer length. To detect the presence of other than catastrophic cable
faults is not possible by attenuation tests. There are far more sensitive
ways of detecting and locating faults.

(7) To check an old length of line just test for DC continuity and look at
the ends. If all is bright and clean there's nothing wrong with it.
----
Reg, G4FGQ