How to measure impedance in a coax cable

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Steve Rifkin:
Well, I am embarrassed to say, but I have never measured the impedance of a coax cable before.
And, I need to find out how to do it.

I've been having some antenna problems recently, and I wanted to check to make sure that my 50-ohm coax is still at 50 ohms.  Same with my 75-ohm coax (that attaches to a vertical I have).

Anyway, I have the coax cable with PL259 (male) connectors on it.  I have my trusty Fluke 77 multimeter which I've switched over the the Ohms indicator.  Now, what?  What am I measuring?  I've got connector A and connector B with the cable between.  Am I going to test from center conductor to center conductor?  Or center conductor one one side to the other side's shield?  Or am I to short out one side (center to it's shield) and take readings from the other side (probe on other center and probe on other shield)?

Of course, neither connector is hooked up to a device during the test.

So, essentially, I guess I'm trying to figure out how to take a coax cable and show that it's indeed 50 ohms (or 75, depending upon the cable), etc. in an effort to rule out a cable problem with my antenna.  (Or should I be checking resistance as well?  I've been a bit confused as to the difference between impedence and resistance.   Not sure what to check when it comes to cables).

If anyone can offer advice on how to take impedence measurements, please let me know.

Thanks!

Steve Rifkin
KA3ZMN
steve410@erols.com

Steve Katz:
Resistance and impedance are almost entirely unrelated, and you can't use a multimeter to measure impedance of cable.

You can, however, use a simple and inexpensive SWR bridge to closely estimate cable impedance: Connect a sample of the cable under question to a transmitter (signal source) and SWR bridge, with a proper termination (dummy load) at the opposite end of that cable.  For example, for 50 Ohm cable, you'd use a 50 Ohm termination.

Transmit, measure SWR.  If it's 1:1 as it should be, you've got 50 Ohm cable.  This test is best run at the highest frequency you can generate and have any instrumentation for; e.g., if you can, run the test on the two meter (or even higher frequency) band, rather than on a very low frequency like 80 meters.  Reason is, on 80 meters, if you use even a few feet of cable to test, even if the cable were mismatched, the resulting SWR would still be very low if the cable were terminated in a 50 Ohm load.  That's because the cable is too short (in fractional wavelengths) to transform impedance much at such a low frequency.  But even a short random length of cable will cause transformation at VHF, unless by coincidence you just happen to hit an integer of precisely one-half wavelength in the coaxial line.

You can closely estimate coaxial cable impedance by measuring the outside diameter of the center conductor, the inside diamter of the outer conductor, and knowing the dielectric constant of the insulator used between them.  This requires no electronic measurement at all, just knowledge of the cable's construction and an accurate calipers.

WB2WIK/6

Steven E. Matda:
And a 75 ohm line terminated in a 75 ohm load will show a 1.5:1 VSWR when used with the standard 50 ohm transmitter output.

:
Sounds to me like you are having some trouble and are suspicious (at least a little) about the coax. But I have to ask...

What kind of problems are you seeing? Varying SWR?  Generally poor receiving?  Can't work anyone? Noise?

Many of these issues are not the fault of the antenna to be sure.

Coax can certainly be problematic, but generally it's with old installations that have increased loss due to water ingress and/or cuts or other obvious physical damage.  Most coax (other than air dielectrics) is pretty robust.

It's quite doubtful that you would have any measurable change in your coax's impedance - certainly not enough to noticeably affect the antenna system's performance. So while the techniques mentioned could allow you to determine Z, it might not be useful. But it's possible...at higher (>100MHz) frequencies the effects are more severe.

I would use the ohm meter to check for shorts and opens in the coax (be sure to understand what effect any load such as an antenna at the far end would have - best to remove/disconnect it). The coax should measure an open between the shield and the center (easily measured at either connector), and measure a short from shield-to-shield and center-to-center (easily measured from one connector to the other).

Also, you may wish to measure the cable's loss - you may have higher than normal loss in the cable/connectors due to corrosion or water ingress. You could place a dummy load at the far end, and put some RF into the cable, measure the power at the near end, then compare it to the power measured at the far end calculate the loss and compare it to the spec for the coax.

Impedance is essentially the AC (or RF) equivalent of resistance and must be measured at the frequency of interest. Coax is somehwat unique in that its impedance is somewhat constant across a wide frequency span. Now, your ohmmeter measures DC resistance - it cannot be made to measure impedance.

Think of a power transformer - at its most simple, it's two independent pieces of wire, each wrapped on the same core - one is a primary the other a secondary. Now... a piece of wire will measure close to zero ohms at DC, right? Yes. Then why does it not present a (near) short to your 60Hz wall power and blow the fuse? Because it has some impedance at 60Hz (we're talking AC now) - perhaps a few dozen to several hundred ohms.

Mike N2MG  
       

Jean Giesler:
Don't forget to check all of the solder joints. Check both visually and mechanically for good joints. They could have worked loose or even never made a good joint. Check for moisture inside the connectors. You may even want to remove the connectors and check that the central conductor and shield is bright. Darkness indicates that moisture has penetrated the coax. When you reinstall the coax take pains to weather proof the connectors.

Ole man JEAN

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