It is likely 75ohm if it is CATV distribution coax, so also test with
that value resistor. There is also a lot of 50ohm cell base station
hardline in the market. I picked up N connectors for hardline I have on
ebay for $10 ea, My cable is Commscope labeled FXL-540 and is smooth Al
shield and Cu clad Al smooth center conductor, 50 ohm, and uses Andrew
540EZNM N Male connector for 1/2" hardline, check the Andrew specs to
see if these fit your cable.
Another way to test cable Z is to use a pulse generator and scope and a
variable resistor (not wirewound) at the far end. At about 3 nsec per
foot round trip, the reflection can be seen at the drive end if the
termination R is wrong. Adjust the R for no/least reflection and then
measure it with your ohm-meter, that is the cable Z. You don't need to
know the length or cut any off. This method also works for open wire
feeder, flat(3M)cables, twisted pairs, etc. but the line shouldn't be
coiled, otherwise adjacent layers will influence the Z.
Grant, KZ1W.
On 11/3/2010 1:48 PM, TexasRF@aol.com wrote:
> Guy, Bob and all, the 50 vs 75 ohm question can be solved with a hand
> calculator after taking a couple of measurements; the coax center conductor
> and
> shield diameters. It most likely has a foam dielectric and you can assume a
> v.f. of 80% without introducing much error.
>
> Get your handbook out and look up the formula for coax characteristic
> impedance and get after it.
>
>
> The methods of terminating the coax with various resistors and making
> measurements does also work but you can't expect meaningful data measuring 75
> ohm coax with a 50 ohm measuring system. The coax electrical length will
> cause many different readings depending on the frequency.
>
> One method that would remove that uncertainty is to connect the coax in
> parallel with a known good load and sweep the frequency until a vswr peak
> shows up. You can then take that frequency as one of an odd quarter wave
> multiple. Then the known good load can be moved to the end of the coax and
> vswr
> remeasured. If the coax and load are close in impedance, the vswr will be
> low. If they are not close in impedance, the vswr will be elevated by the
> ratio of the two impedances squared.
>
> A variation of this test is to simply connect a good load to the coax and
> measure the vswr while sweeping the frequency. If the load and coax are
> close in impedance, the vswr will be low at all frequencies. If not, the vswr
> will vary from 1 to 2.25 and back as the frequency changes. This is assuming
> 75 ohm coax and 50 ohm load.
>
> While set up, you can also estimated the loss of the cable by leaving it
> unterminated and measuring the vswr. The lower the loss, the higher the vswr.
> Using VHF for this test accentuates the loss, making it easier to
> estimate. Your handbook will have info on how to interpret the test results.
>
> The first method is by far the easiest way to determine the coax impedance!
>
> 73,
> Gerald K5GW
>
>
>
>
>snip
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