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I am using the Siglent SSA3032A-R spectrum analyser which has a Vector Network Analyser (VNA).

Before measuring the impedance of an antenna under test so that I can match this antenna, I have to calibrate the VNA. I perform a 1-port calibration, doing an Open, Short and Load calibration.

The method for calibrating is as follows:

  • Open, connect the 50Ohm cable, soldered to the PCB to the VNA and disconnect the antenna from the 50Ohm cable.

Open calibration

  • Short, using a 0Ohm resistor, connect the antenna feed line to ground.

Short calibration

  • Load, using a 50Ohm resistor, solder the 50Ohm load between the antenna feed line and ground.

Load calibration

When I perform the calibrations, the open, short and load is plotted on the smith chart in the expected location (open plotted on the right, short plotted on the left, load potted in the centre).

I then remove the 50Ohm load and connect the antenna to the antenna feed line using a 0Ohm resistor (as shown below). Connecting this to the VNA results in a negative resistance reading off of the smith chart plot. A negative reactive component would indicate a capacitive load but a negative resistive component should not be possible.

DUT testing

Here is the smith chart plot seen when the antenna under test is connected to the VNA after calibration.

Smith chart plot seen on VNA after connecting antenna under test

Can anyone advise what I might need to do differently?

I have been following the 'How to do VNA calibration on PCB' https://www.baseapp.com/iot/antenna-tuning-for-beginners/

Image of test setup below. Test setup

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  • \$\begingroup\$ The open and short must be at the same place, and define your 'reference plane'. It's simplest if this is where you connect the device under test. You've used the terms '50 ohm cable' and 'antenna feed line'. Are these synonyms, or are they the same thing? Post a picture and a schematic of your measurement setup. The VNA is only as accurate as your cals, your open doesn't sound very 'open', and I've found a 50 ohm resistor doesn't often make a good load, two 100s in parallel tend to be better, depnding on frequency. \$\endgroup\$
    – Neil_UK
    Feb 23 at 14:15
  • \$\begingroup\$ I perform the open and short at the same place. I say antenna feed line, referring to the cable inside the coaxial cable. I said 50Ohm cable to confirm I was using a 50Ohm cable and the cable was not the source of the issue. We are actually using two 100Ohm resistors in parallel as these were readily available. \$\endgroup\$
    – MRB
    Feb 23 at 14:33
  • \$\begingroup\$ Could you add a plot of the smith chart to see the location of the antenna on it? \$\endgroup\$
    – Big6
    Feb 23 at 14:50
  • \$\begingroup\$ @Big6 this has been added. The negative resistive component I cannot make sense of. I read that a negative resistive component suggests incorrect calibration, however I have tried calibration countless times over several days, testing the PCB is soldered correctly (i.e. there is a short or open when performing those calibraitons) \$\endgroup\$
    – MRB
    Feb 23 at 14:57
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    \$\begingroup\$ Your open standard actually has some capacitance, and your short standard has some inductance. Since you don't know what these are, you (or the instrument's software) assumed they are 0. The error caused by this assumption can make it appear as if your DUT has negative resistance. Your options are, don't worry about it, do the measurement without correction, or obtain a fancier VNA with well characterized cal standards. \$\endgroup\$
    – The Photon
    Feb 23 at 15:51

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