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Author Topic: Coax question  (Read 424 times)
KC0SHZ
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Posts: 372




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« on: April 11, 2006, 12:10:32 PM »

This may be a dumb question, but I do need to get an answer.

I have bought a section of coax with BNC adapters at each end that came out of a cell phone tower.  How could I test the actual impedence of the coax (50 ohm vs 75 ohm) with a multimeter?
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AA4PB
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Posts: 12899




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« Reply #1 on: April 11, 2006, 12:59:59 PM »

How could I test the actual impedence of the coax (50 ohm vs 75 ohm) with a multimeter?
-----------------------------------------------------
You can't. The easiest way is probably to connect a 50 ohm dummy load to one end, apply RF to the other end and measure the SWR.

If it came out of the cellular service then it is very likely 50 ohm.
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G3RZP
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Posts: 4731




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« Reply #2 on: April 12, 2006, 02:39:42 AM »

If you have a multimeter that reads inductance and capacitance.........

Measure the capacitance of a length with the far end open circuit.

measure the inductance of the same length with the far end short circuit

impedance = square root of inductance divided by capacitance

this isn't exact when measured at low frequencies, but is hopefully near enough to get you into the ball park.

You also need to be sure that at the frequency you're measuring at, the length is less than a quarter wavelength.

Around 50+ years ago, TV was a 'new' thing. Bicycle shops had sold radios and charged batteries, and so it was natural for them to move into TV. The technical level demanded was too much, though.....so in a small town, the one guy who really did know what TV was about would get queries from his 'competitors'.  One ham (the late G3BTU) I knew had a guy from one of the other shops in town come round with some coax in his hand. 'Harold, what's this 75 ohms? I've measured this cable inner to inner and it's zero: I've measured it inner to outer and it's open. I've even cut a bit up, but I can't find 75 ohms anywhere!'
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WB6BYU
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Posts: 13343




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« Reply #3 on: April 13, 2006, 12:49:55 PM »

You could check to see if there are any markings on the cable
that would be of help...

My DVM includes a capacitance range.  If you check the
specs on 50 and 75 ohm cables you will see that the 50 ohm
cables tend to be around 30pf/foot and the 75 ohm cables
are closer to 20pf/foot.  So a capacitance meter and a
measuring tape will give you a quick estimate.  (This isn't
foolproof, as there are some low capacitance 50 ohm cables,
but it works pretty well with most cables.)
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