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how to measure characteristic impedance of wire?

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icekingdom

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sorry guys..i'm newbie..
may i know the method how to measure characteristic impedance of wire?
when I online searching,there have a easy method for low frequencies,
"measure the capacitance along the cable,C;then short the other end of the cable,measure the induce,L then Z0=sqrt(L/C)"
but how to measure the capacitance along the cable? is it any circuit can provide for reference ? thanks a lot !
 

A single wire has to have a very specific matching network in order for you to actually launch a wave along it. But once launched, you can then talk about its characteristic impedance.
 

The characteristic impedance shows up its role usually at higher frequencies when the two way transit time along the line becomes comparable to the wavelength. Otherwise it does not get noticed.
 

If you drive a shorted wire with a raise time faster then the reflected wave comes back, then the step function you see in the output is a result of the driver impance in relation to the wire impedance.
 

I think instead of putting it in these terms better to attempt an explanation based on the approach shown in T Peters post #2. This (Pdf file article) gives clear idea of how you look at a conductor or a transmission line and ascribe attributes to it.
Figures in the article show inductance using a different symbol but serves the purpose.
Raoof
 

if i want to find the characteristic impedance of coax cable?
it is have any simple way or experiment to measure it which can construct in my Uni?
even thought I know the value...

---------- Post added at 17:19 ---------- Previous post was at 17:18 ----------
tpetar said:
Cable impedance

**broken link removed**
Click to expand...

ark5230 said:
I think instead of putting it in these terms better to attempt an explanation based on the approach shown in T Peters post #2. This (Pdf file article) gives clear idea of how you look at a conductor or a transmission line and ascribe attributes to it.
Figures in the article show inductance using a different symbol but serves the purpose.
Raoof
Click to expand...

if i want to find the characteristic impedance of coax cable?
it is have any simple way or experiment to measure it which can construct in my Uni?
even thought I know the value...
 

My personal favourite for an visual demonstration (and measurement) of characteristic impedance is a TDR (time domain reflectometry) setup. A pulse generator with ns rise time (< 2 ns), an oscilloscope with > 200 MHz band width and some kind of coupler, in the most simple case a BNC T-connector placed at the high impedance oscilloscope input, BNC cables and a 50 ohm termination.
 

If you try to see a fast rising step or pulse using a fart oscilloscope (as described in post by FvM) you will find the reflections in addition to the actual pulse. The PDF file in T Peters post (#2) will provide information on how to correlate magnitude of the reflections with the characteristic impedance of the cable.
If the characteristic impedance of the cable ( say 50 or 60 ohms) is equal to the input impedance of the oscilloscope or it is connected by a matching load there will not be a reflection.
 

Maybe there is a confusion:
@ icekingdom: in post #1, are you speaking about a coaxial cable or a single wire?

Z
 

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