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How to measure the impedance of coax

Question asked by drkirkby on Jun 18, 2013
Latest reply on Sep 6, 2013 by drkirkby
Despite owning a couple of HP VNAs, and *not* owning a low-cost VWNA from SDR-kits, I do read the VNWA mailing list. A question arose on there about how to measure the characteristic impedance of cable. As someone pointed out, you can use the time domain and look at the impedance some point along the line.  the impedance of the line, but not the load on the end. I was wondering if you could

Assuming the line was of constant impedance, and you did *not* have a TDR facility, how could one measure the line impedance? I was thinking there might be some way based on the reverse of a sliding load. In a sliding load you know use the line impedance, but not the load impedance. I wonder if you could do the reverse, if you know the load, but not the line. A small frequency change should give a circle on a Smith Chart, and I wondered if the centre of that would be Zo of the line. 

I'm just intersted in ways other than the time-domain to do this. Luckily both my VNAs have the TDR facility, and I don't personally need to do this measurement, but the question on the VNWA mailing list got me thinking. 

Dave.  

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