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What are the 50 ohm scattering parameters for a piece of 75 ohm coax? (I just need two of them of course). For simplicity assume that the length of 75 ohm coax is 2 electrical wavelengths at the frequency of interest and that the line is lossless.

I am trying find discrepancies in measurements from a VNA for a two port DUT. I suspect that 75 ohm coax was used to connect to a DUT on at least 1 port with equipment set to a 50 ohm system.

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    \$\begingroup\$ IIRC, the electrical length is exactly two wavelengths, then the mismatched coax has no effect. But if you're measuring at more than one frequency, the cable length won't be 2 wavelengths at every frequency. \$\endgroup\$ – The Photon Mar 5 at 19:47
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If you want to know the reflection coefficient (which is the scattering parameter S11), you can calculate S11=(Zload-Zw)/(Zload+Zw). For 75 ohm cable and 50 ohms load this is S11=-1/5

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