Calcul/measurement of coax impedance ? 1

[toulouse]

Hi,

I wonder how I can simply find the carracteristic impedance of a coax cable ?

I wanted to calculate it knowing that Z0= SQRT(L/C) with SQRT=> square root ; L,C: lumped elements...

But I've another problem... it's that I don't know the dielectric material so I can't find Epsilon_r !!

Can I get some help ?

Thks.

Thomas K.

http://www.genius.org.br

http://www.thomask.fr.st

 

[nbucska]

Every HF coax has the Z specified.

<nbucska@pcperipherals.com>

 

[toulouse]

I'm sorry Nbucska but not &quot;every HF coax&quot;... as the one I have doesn't !

It comes from recuperation, and I would like to use it to feed a microstrip antenna with 50 Ohms impedance...

Thomas K.

http://www.genius.org.br

http://www.thomask.fr.st

 

[IRstuff]

The classical theory that the a cable of specific impedance should be terminated by the same impedance should be applicable here. You ought to be able to tell if a 50 ohm termination results in:

> no reflections
> maximum power transfer

TTFN

 

[hacksaw]

if you are familiar with electronics, then you should already be familiar with identifying the various insulations used in the industry.

if your cable is not properly documented then you should obtain a better grade.

dealing with microstrip implies that you need high performance cable, one that is properly tested and doc'd per IRStuff take the advice and report back what you find.

 

[buding]

Hi!

The most of cables nowdays are 50ohm or 75ohm. Just check the matching.


Buding.

 

[Mattman206]

Hi,

I don't know if you have access to an HP8510C vector network analyzer (or something similar.)

But if you do, I would attach the cable to one of the ports and look at the Smith chart of S11 (or S22). If you see a dot in the middle of the chart, then you know that the coax is 50ohms, because the port impedance of the analyzer is 50ohms. You could even go to Time Domain mode and check out the reflection coefficient. (Time Domain mode simply does an FFT on the frequency response.) If the reflection is near 0, then the coax is 50ohms. If the Smith chart or the Time Domain modes give you other information, you could figure out the exact impedance from that.

Z_coax = Zo * ( ( 1 + |gamma| ) / ( 1 - |gamma| ) )

where Zo = 50 ohms.

HTH,
Matt

 

[GregChenco]

Measure the diameter of the dielectric (D) and the diameter of the inner conductor (d) and use the formula:

Zo=138*Log10(D/d) and divide this by the square root of the dielectric constant of the dielectric.

Most coaxial cables have a polythyene dielectric which has a dielectric constant of 2.25

Greg Chenco

 

[toulouse]

Thanks Greg for your reply, it's exactly what I was waiting for !!!

I found this formulla in an electrical school book...



Thomas K.

http://www.genius.org.br

http://www.thomask.fr.st

 

[divebuddydave]

Just be careful about your assumption that the dielectric constant is 2.25. Look at the dielectric, if it is white, then it is likely not polyethylene, but either teflon, or foamed teflon. If solid teflon, then E=2. If it is soft and white, it is most likely a fomaed dielectric and E can range from 1.3-1.6.