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Take long section of cable. Attach it to the bridge and load opposite side of cable section by some known resistor (let say 30 Ohm), the same resistor attach to the other sholder of bridge. Tune frequency of generator (from lowest frequency to up) to balance bridge. You will find the frequency where your cable has 1/2 length of wavelength.
Then tune your generator at 1/2 of that frequency, thus your cable will have 1/4 of wavelength here. Balance bridge by variable resistor at other sholder of bridge and find impedance Z.
Find characteristic impedance of cable by this formula:
Zcable=SQRT(R*Z)
To avoid some errors make measuring at short wave frequencies, so take long enough section of cable.
characteristic impedance of cable is not depending of frequency - it will be the same on 1 MHz, 30 MHz or 900 MHz. So, you can measure it on low frequency and then use on high.
Anyway, better use classic method suggested by springf2000 - you will need only LC meter.
by the way of calculate c and l and r of the line in this way we calculate aproximation of main Zc but when my cable lenght is about 1 meter and freq=900Mhz we have disturbuted c,l in all of line and we have some difference between calculations.I accept your opinion but I dont know how much error will be in this calculations.
thank you
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