How to Test CO-AX With a Digital Multi-Tester

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fourthhorseman

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have a bunch of coax im thinking may be damaged,possibly bad bnc to co-ax connection.

so,i know theres an answer out there waiting to make its debut on the world wide web!

thanks all,,
 

fineshot1

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have a bunch of coax im thinking may be damaged,possibly bad bnc to co-ax connection.

so,i know theres an answer out there waiting to make its debut on the world wide web!

thanks all,,
How to test it depends on whether you have access to both ends of each length of coax,
or is this coax installed in a home or vehicle? More info please.

Normally if you have access to both ends you would inspect the entire length and make
sure it is not crushed at any point along each run and then do a continuity test with the
dmm on the center and the shield. If you have continuity and have nothing connected to
either end then somewhere along this run you have a short. Usually the shorts are in a
connector on either end.
 

jim202

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As was said already, it would be good to be able to get to both ends of the coax.

If it is shorted, you should be able to pick out which end has the shorted connector. Take your
Ohm meter and measure from the center conductor or pin on the connector to the shield or
the outer shell of the connector. Write down the reading. It will be very low, so make sure
you use the lowest range of the Ohm scale. Then go to the other end. Measure the same
way from center pin to the shell of the connector. The end with the lowest reading is the shorted end.

Remember the difference will be in the tenths of an Ohm. Like one end may show 0.8 Ohms
and the other may show 0.2 Ohms.


If there is no short on the cable to start with, then short the distant end and measure the close end.
You should see something in the order of a couple of Ohms or so. the actual reading will depend on
the length of the coax cable and the type of cable. This may take a couple of people to do unless
you have some coax connector adapters that will allow the short to be placed at one of the connectors.

Jim
 

jackj

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You can't really do any more than basic continuity and short testing with a multi-meter. Others have told you how to do it and what to look for but again, that is only part of what you need to know about coaxial cable and you can't find any of that out without thousands of dollars worth of test equipment. The shield and the center conductor should show continuity from one end to the other but be open when tested between the center conductor. Good luck.
 

kb2vxa

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DC continuity/short checks will tell you only that much, or in other words whether or not it can be used as battery cable. What you need short of a Q meter and a bunch of RF theory under your belt is an antenna analyzer. Don't let the name fool you, you can also make some fairly complex transmission line measurements as outlined in the instructions. The bottom line here is there are two "Ohm's Laws", one for DC and the other for AC otherwise known as VAR, volt-ampere reactive. When you get up into RF you're in another world entirely.

"...you can't find any of that out without thousands of dollars worth of test equipment."
Oh contraire, I found this very useful and of course there are others similarly priced.
http://www.gigaparts.com/store.php?action=profile&sku=ZMF-259B
 
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jackj

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Hmmmmm......

DC continuity/short checks will tell you only that much, or in other words whether or not it can be used as battery cable. What you need short of a Q meter and a bunch of RF theory under your belt is an antenna analyzer. Don't let the name fool you, you can also make some fairly complex transmission line measurements as outlined in the instructions. The bottom line here is there are two "Ohm's Laws", one for DC and the other for AC otherwise known as VAR, volt-ampere reactive. When you get up into RF you're in another world entirely.

"...you can't find any of that out without thousands of dollars worth of test equipment."
Oh contraire, I found this very useful and of course there are others similarly priced.
http://www.gigaparts.com/store.php?action=profile&sku=ZMF-259B
I stand corrected. I was thinking of a time-domain reflectometer. That appears to do almost the same thing with less precision and at lower frequencies. Also at a much lower cost. For a pretty good one try:

Megger 655510F (CFL510F) Hand-Held Time Domain Reflectometer Megger 655510F 655510-F 655510 F TDR1000/2
 

fourthhorseman

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Yes,i do have availability to the full lengths,im concerned that the BNC connectors are poorly connected.

i picked up a box of misc antennas and some LM cable,hooked up the LM bot it seems,i dont know,not the esame feel i guess,not sure how to articulate the difference vs. the pre fab cable i had there already,

so im thinking test the cables,move em around a bit and see if the reading drops or peaks,then ill have a better idea if its the coax,if no i can move on to the BNCs..

just not sure what to set the mulit meter to..sorry for the vagueness,,i guess i needed to catch the replies to focus my intent,,thanks gents!
 

ramal121

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How many people pull their hair out trying to troubleshoot radio problems without doing a simple check of the coax.

A simple continuity check as stated above and a visual inspection of the coax to make sure it is round all the way down will eliminate catastrophic failure as a problem. Suspect water in the coax? Slap the end on your hand or hang coax vertical for a while. A drip or two is bad news.

If it is a transmit capable radio, a wattmeter (yes even a cheap one will do) can measure power at both ends and loss can be determined using - Loss Db=10log powerin/powerout.

Now if you want to get the into the fine details of transmission line you can go with a TDR or a Return Loss Bridge if you have $$$$. The above is just a go/no-go way of testing.
 

petnrdx

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Continuity checks.

I am lucky I guess.
I have a TDR and Spec An with Tracking Generator. (I do contract radio work, so I make money with the equipment)
Checking suspected coax in this order: ( I stop when a fault is found & problem solved )
DC continuity (check your cable center connector & shield end to end with an Ohm meter)
DC short (shield to center conductor)
If the cable passes that and the visual inspections:,
then I use the trusty Bird 43. (any good wattmeter will do) and dummy load.
Next would be the Tracking Generator to verify loss to what would be expected for the cable involved.
I virtually never use the TDR for cables that I have easy access to each end.
Other people may do it differently.
The above suggestion of an "antenna analyzer" is a good suggestion too.
If you don't have any of the expensive stuff, the DC ohm meter will find a lot of problems.
And a decent wattmeter is next.
In about 40 years of radio experience I am certain I have found more failures (and misteaks) with the basic VOM (Ohm meter).
Not a bad idea to start with that.
One suggestion to all: When you find a cable that SEEMS to have a problem, CUT THE SUSPECTED CONNECTOR OFF. Otherwise you will forget and try to use it again, then run into the same problem again.
 

fineshot1

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Yes,i do have availability to the full lengths,im concerned that the BNC connectors are poorly connected.

i picked up a box of misc antennas and some LM cable,hooked up the LM bot it seems,i dont know,not the esame feel i guess,not sure how to articulate the difference vs. the pre fab cable i had there already,

so im thinking test the cables,move em around a bit and see if the reading drops or peaks,then ill have a better idea if its the coax,if no i can move on to the BNCs..

just not sure what to set the mulit meter to..sorry for the vagueness,,i guess i needed to catch the replies to focus my intent,,thanks gents!
If your dmm does not have a continuity test selection then just use the lowest ohm
scale selection(ie:0 - 2k ohm). Most dmm's have a continuity selection position that
emits a beep when a short is detected across the probes.
 

Kennrth

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Other than continuity and short checking


Can not measure RF characteristics with VOM or DVM. Measuring quality of BNC cable requires sophisticated RF equipment preferably an HF network analyzer. And specifications sheet from manufacturer on cable in question where real data is measured against. Network analyzer Smith Chart results show all the characteristics of interest about the cable being measured.
Relativistic measurements are used by amateur radio operators with fair to good results based on experience in the field using inline power meters and VSWR meters . Time Delays – Phasing V to I, Phase relflection Vforward to Vreflective figures not always of merit which are very important cannot be measured with typical rf equipment in field use. Again practical measurements can be made using field strength meters vs VSWR measurements and calculated phase cut measurements to yield optimum results in a transmitter ,conductive line and antenna system.
Practical measurements on a bnc type cable can be made by using inline power meters and resistive loads at a specific frequency in question.

When in question replace cable with new one cut to your required length with same or better specs. Do not recommend you try to replace connectors unless you are experienced. A zero ohm connection is no garrantee for good rf connection.

If you are using uhf Styrofoam type cable and it is over 1 year old replace it. Styrofoam type cable degrades rapidly as humidly penetrates the cable. For durability use hard line for uhf and above.
 
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fineshot1

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Styrofoam?
i think he means the foam dielectric cables(did i spell that right?),
but i never heard of the moisture problem unless the cable was
penetrated by water leakage as in during a rain storm or such.

when attempting to solder connectors on cables with foam dielectric
it tends to deform and damage the cable if not done properly. That
is why it is an industry standard to crimp conn's on these types of
cables with the proper crimp conn's and tools.
 

petnrdx

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Styrofoam.

I was just attempting to be funny.
I didn't think the poster really meant styrofoam.
Over 40 years I have put thousands of connectors on hundreds of types of cable, but never have seen styrofoam, so I was pretty sure he means one of the many types of poly-foams.
I don't think I would throw one year old cable away tho. Unless it had been out in the environment. (rain)
And humidity alone should not infiltrate a cable to the extent that it causes any problems.
I often have reels of cable in storage (dry) for a couple of years before using it all.
I would agree that installing a cable that is from an unknown origin, and therefore unknown condition, is likely not worth doing.
Unless the cable checks out OK, and the installation is a simple one.
 

DPD1

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How many people pull their hair out trying to troubleshoot radio problems without doing a simple check of the coax.
Many... Connector connections are usually the problem 90% of the time, but often the last thing checked or even considered. I've talked people into buying $5 continuity checkers at the hardware store and they finally discovered a problem they had for months.
 

dd364

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Unless I missed it,you didn't say what you might use these cables for.A cable that is "bad" for one application might work perfectly well for another.As was already said,an ohmmeter will tell you most of what you need to know not considering power and frequency requirements.
 
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