With a 2-port VNA, I would like to characterize the performance of a single coax connector, specifically to obtain VSWR.

The specific situation is an SMA connector at the end of a short section of semi-rigid 50 ohm coax that has no connector on the other end.

The two techniques I can think of are:

  1. Add the same connector to the coax, creating a short double ended cable, and using the known electrical length, obtain the S parameters for one connector. I do not know the exact formula.

  2. Try to terminate the coax with a 50 ohm load. I think that soldering a resistor would not work properly at these frequencies (>9 GHz).

Are there techniques I am missing?

  • \$\begingroup\$ If you put a connector on each of of a transmission line of essentially zero length and measure the 2-port s parameters, you can mathematically calculate the s parameters of the individual connectors, the assumption being that they are identical. \$\endgroup\$ – scorpdaddy Jul 18 '19 at 19:16
  • \$\begingroup\$ You can get a SMA 50 ohm terminator that will work at that frequency. \$\endgroup\$ – EE_socal Jul 18 '19 at 19:50
  • \$\begingroup\$ @EE_socal, but you will need to find a terminator with much lower VSWR than the connector you are trying to measure. Depending on the quality of the connector and the amount you're willing to spend, that might not be possible. \$\endgroup\$ – The Photon Jul 18 '19 at 20:49

Put the connector on a long piece of coax instead of a short one. I'd use 1 m or more, given your 9 GHz bandwidth.

Then use time domain gating (a feature available on many VNAs) to measure only the reflections coming from the connector and not the reflections from the open far end of the coax. Time domain gating means the VNA will transform the response to time domain, apply a window around a part of the time-domain response you specify, then transform the result back to frequency domain before displaying the results.

This would be a one-port measurement. The second port of the VNA would not be used.

  • \$\begingroup\$ Very interesting, this is intuitive. I need to figure out how this works on my hardware and how it interacts with the SOL calibration on the test lead. \$\endgroup\$ – ararat Jul 18 '19 at 23:07
  • \$\begingroup\$ Update: this technique was effective at distinguishing between a sample "good" and "bad" connector without modification of either assembly. Thank you. \$\endgroup\$ – ararat Jul 22 '19 at 20:40

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