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Author Topic: Can you check coax impedence w/ just a meter?  (Read 368 times)
KAISERSOUSE
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« on: January 16, 2009, 12:25:02 PM »

I have a fancy-pants digital meter and a rat's nest of coax to test (before I give it the Caesar thumbs-up or down).

Continuity checks are brainless, but how to I make sure its 75-ohm? Do I need some sort of load or short-circuit or something? Can I homebrew something together or do I need a special....whatever?

Thanks, and googling this yielded ALLL kinds of stuff on coaxial networks, continuity how-to, etc.

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W5DXP
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« Reply #1 on: January 16, 2009, 12:36:05 PM »

1/8WL of any short-circuited stub is +jZ0. 1/8WL of any open-circuited stub is -jZ0. An MFJ-259B will make those measurements.
--
73, Cecil, w5dxp.com
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N3OX
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« Reply #2 on: January 16, 2009, 12:39:07 PM »

You really need an RF source to check coax impedance.

You can do it with a dummy load and your transceiver if nothing else... terminating 75 ohm coax in a 50 ohm dummy load will lead to a ***variation in SWR indication*** on a SWR meter as you tune in frequency.

50 ohm coax to a 50 ohm dummy load will show a SWR of 1:1 and ***no variation***.

75 ohm coax and a 75 ohm dummy load will show a SWR of 1.5:1 and ***no variation***.

But all of these require a measurement *at RF frequencies*



If you can turn your RF power down to 5W and your SWR meter still indicates well, you can build a dummy load out of resistors you have laying around the house.

If you can't do a RF measurement, measuring the inner conductor and shield diameter or looking for part numbers on manufacturers websites could do the trick.

A lot of coax cables are stamped with a manufacturer number that doesn't mean anything to Google because it doesn't index deep enough, but if your cable says "CAROL C1785" on the side, and you go to the Carol cable website and put in C1785, you can find it.

73
Dan
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73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
K1BXI
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« Reply #3 on: January 16, 2009, 01:40:40 PM »

Perhaps not the best method, but a visual inspection will show a smaller diameter center wire with a given shield diameter for 75 ohm than 50 ohm cable.

John
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KZ1X
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« Reply #4 on: January 16, 2009, 01:53:21 PM »

... and after all that, I'd want to test longer pieces with a time domain reflectometer ... ideally, a good hunk of coax has only the end reflection, and not any 'in-betweenies' that could indicate mechanical damage.  I do this on surplus Heliax all the time, to see if I am interested.  So many times, I find that during removal the cable got pinched or twisted.  Same with RG-11 and aluminum hardline from CATV re-purposing.  Fortunately, these surplus cables are often long and cheap, so it's worthwhile to do the homework, and cut the bad spot out and re-terminate, or just use the resulting two shorter pieces.
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KK7KZ
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« Reply #5 on: January 16, 2009, 02:20:24 PM »

How about a magnifier and check out the markings on the jackets, then look up the information on the mfrs. data sheet?

R.
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VK1OD
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« Reply #6 on: January 16, 2009, 02:40:13 PM »


You said "I have a fancy-pants digital meter".

I am sorry, that means nothing to me.

If you have a meter that measures capacitance at low frequency, AND you know the dielectric constant or velocity factor, you can determine Zo. The formula is probably in the ARRL handbook, or ARRL Antenna Handbook, but it is easy to work out from first principles.

For example, coax with a solid polythene dielectric with capacitance of 100pF/m (or 30pF/' is Zo=50 ohms.

Owen
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VK1OD
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« Reply #7 on: January 16, 2009, 03:24:30 PM »


I was asked offline for the formula.

The formula is:

Zo=1/(C*vf*c0)

where c0 is the wave speed in a vacuum, C is the capacitance per unit length (in F/m), and vf is the velocity factor.

vf=1/er^0.5 where er is the relative permittivity of the dielectric (er for solid polythene is 2.25, therefore vf=0.67.)

Zo=1/(C*vf*3e8)

So, working that through, common solid polythene dielectric has capacitance of 67pF/m for 75 ohms, and 100pF/m for 50 ohms.

Easy!

Owen



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AA4PB
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« Reply #8 on: January 17, 2009, 06:53:03 AM »

If you are asking if you can use an ohmmeter to measure coax impedance, the answer is NO. An ohmmeter uses DC to measure resistance.

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WB6BYU
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« Reply #9 on: January 18, 2009, 12:20:41 PM »

I've used my DVM with capacitance meter several times to check the
impedance of an unknown cable.  If you check the standard coax tables
you'll see that 50 ohm coax is generally around 28 pf/ft and 75 ohm
cables are around 20 pf/ft.  If you cut off a 10' piece and measure it
you meter should be able to differentiate between 200 or 280 pf.

(And if you have a spool of known cable you can estimate the length
using by simply measuring the capacitance across the available end,
as long as the far end isn't shorted.)
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VK1OD
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« Reply #10 on: January 18, 2009, 08:51:24 PM »


Dale,

Velocity Factor figures in the equation (as I quoted), so whereas RG-58C/U should be around 100pF/m, RG8X should be around 81pF/m.

Likewise foam 75 ohm cables will have lower C/m than solid poly dielectric.

All the above can be converted to /ft figures for those who design in imperial units by multiplying by 0.3048. (Gee, even when we used imperial units, and I learned imperial units at school, scientists and engineers tended to design in metric, either cgs or ISO... it *is* easier.)

Owen
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