# characteristics impedence

• posted

hi all,

can anybody plz tell how do we measure cahracteristic impedence of a cable, may it be coaxial, stp, utp or power cable.it looks impedence matching and trying to get the maximum power. i s there any other way?

• posted

If you really want to do this look here: http://72.14.205.104/search?q=cache:-K9ty8myB2YJ:sist.fnal.gov/archive/2000-topics/Owusu/owusu.doc+measuring+characteristic+impedance+of+cable&hl=en&ct=clnk&cd=8&gl=us

Tom

• posted

Well, you can safely forget about the concept for an equipment power cable.

Graham

• posted

Just measure it.

1] Measure the "L" with the lead ends short circuited. 2] Measure the open circuit "C" 3] Then Zo= square root of L/C

Some I did last year ...

2mtr of "75ohm" coax L=1.48uH C=146pF ... Zo=100ohm* 2mtr of 50ohm coax L=0.83uH C=168pF ... Zo=70ohm 2mtr of 'twin flex' L=1.23uH C=70pF ... Zo=132ohm 2mtr of 'mains cable' L=2.84uH C=281pF ... Zo=100ohm 3mtr CAT5 patch cable L=2.51uH C=157pF ... Zo=126ohm

(measured twice, still same result!)

• posted

"john jardine"

** Likely the frequency you used for the L and C measurement was not in the normal RF range.

Zo for co-ax tends to rise at low RF and audio frequencies.

....... Phil

• posted

I would either look it up in the cable spec, calculate it, or measure it with a TDR.

Good Luck! Rich

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