Determining Zo for coax cable
Some of us may already know this; for those that don't, it could well be useful for you.
If you have test equipment for measuring inductance and capacitance and you need to know the characteristic impedance of a length of coax (which, for the procedure below, should be at least 3m. long or more), you can use the fundamental equation:
Zo = √(L/C) ohms,
where C is in farads and L in henries.
Procedure.
1. Make sure test equipment has warmed up, etc.
2. With one end of the cable open-circuit, measure the capacitance of the cable at the other end: call it C.
3. With one end of the cable short-circuit (can be same O/C end as in step 2), measure the inductance of the cable at the other end: call it L.
4. Then use the equation above.
Example.
Today, in my workshop store room, I found about 40 feet of cable of unknown Zo.
The above procedure produced C = 1800 pF; L = 10 µH.
Calculation result: Zo = 74.5 Ω.
To me, that's Zo = 75 Ω. (Experimental error, etc.)
And finally, yes: I do know that there are several other methods for determining cable Zo. I might discuss them in another thread, which I - or maybe another member - may choose to open.
Al.
|