# characteristics impedence

Discussion in 'Electronic Basics' started by mahak, Nov 24, 2007.

1. ### mahakGuest

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?

3. ### EeyoreGuest

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

Graham

4. ### john jardineGuest

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!)

5. ### Phil AllisonGuest

"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

6. ### Rich GriseGuest

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

Good Luck!
Rich