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How best to determine impedance of a few metres of coax.

Question asked by drkirkby on Jun 18, 2015
Latest reply on Jun 24, 2015 by Dr_joel
Someone at my amateur radio club asked me if I could measure the impedance of some coax. which he purchased from a retailer of amateur radio equipment. It was sold as 50 Ohm, but looks like cheap 75 Ohm TV coax to us both. Personally I would chuck it in the bin, but I said I will measure it for him. I have a few metres of the stuff. 

What is the best way to measure the impedance of this? 

I know its possible to do things with the time-domain option on a VNA, but I'm not sure what frequency range to use. Two issues make me think that going from DC to light is not ideal

1) I know the impedance of coax rises above the high frequency value, so would collecting data at less than 10 MHz result in me giving him the wrong nominal value. 

2) I also know that this guy will not use this above 432 MHz. 

So is there any point sweeping it over a frequency range below 10 MHz, or above 432 MHz? The fact the stuff looks pretty cheap and horrible, makes me think it is unwise to test it at a high frequency. If it  is 1 Ohm or 1000 Ohms at 10 GHz, it is going to make no difference to him. 

But I only have a sample that is a few metres long. 

I have at my disposal. 

* 8720D with time domain option. But the lower frequency limit of the 8720D (50 MHz) is higher than the frequency of interest to him.  

* 8753ES. The frequency range of this (300 kHz to 3 GHz) more closely matches the frequency range he would use this stuff at. But the 8753ES lacks the time-domain option. But of course I could do the inverse Fourier Transform outside the VNA - the software for the low-cost VWNA will do the IFT from a Touchstone file. 

Dave  

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