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I'm testing my own made interdigital capacitor. It is for 1.5 T MRI (63.8 MHz). Right now I have soldered a SMA connector to a custom capacitor board and connected the SMA connector with a cable to the TG source of the VNA. I calibrated already successfully with the open/short/load kit. As of right now I have a capacitance of 4.01 pF (which I'm not really sure about is right). On top of that I get this weird loop. I have never seen it before.

I calculated a capacitance of 27 pF, but I only get 4.01pF, and I see this weird loop which I can't put my finger on. Does somebody know why this could happen? When do such things happen?

weird loop on VNA

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  • \$\begingroup\$ I'm not clear on the circuit arrangement you are describing. Is the SMA connector connected to one side of your capacitor, and the other side of your capacitor to ground? \$\endgroup\$ Commented May 1 at 9:59
  • \$\begingroup\$ The circuit is as followed: the capacitor is soldered to a board, which is soldered to a sma connector, which is connected with a cable, which is connected with the VNA. \$\endgroup\$ Commented May 1 at 18:46

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If there is a resonance in your wiring , you will see loops like that.

And because the resonance is likely caused by an inductance in series with your capacitor, it can tend to cancel the capacitor's reactance: the VNA indicates the impedance seen at the S11 port, not at the end of the 50 ohm coax test cable unless you perform open/short/50 ohm calibration at the cable end.

How physically large is your capacitor, how long is the 50 ohm impedance test cable ?

Did you calibrate at the capacitor end of the test cable you mention or at the VNA end of the test cable ..

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  • \$\begingroup\$ I calibrated at the capacitor end of the test cable. \$\endgroup\$ Commented May 2 at 13:12

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