How to improve measurement results with pocketVNA

Question

I use a pocketVNA to measure the impedance of antennas and I also bought a couple of SMA connectors and a few cables to connect the antennas to the device.

I run a 2-point calibration of the device (according to the manual) by using the calibration kit. Please take a look at the screenshot for the calibration results.

I´m not sure if these calibration results are good or bad. What are the critical regions for such a calibration?

After the calibration, I have tried to measure the impedance of a know 868 MHz antenna, but the result doesn´t look good:

I would expect the peak at around 868 MHz and not 900. As I understand it the antenna should have a resistance of around 50 Ohms at 868 MHz. So why does the antenna has an impedance of around 10 Ohms?

Also, the result is getting really worse when the antenna is laying on my table instead of holding it into the air:

I don´t understand the results and I suggest that my handling of the device is wrong. How can I improve it? Are there some tips for using a VNA?

Update:

The calibration process is a simple 2-Port calibration with three different load types:

• Open
• Short
• Load (Don´t know the resistance)

Please take a look at the photo to see my setup. I have done the whole calibration with the cable, because I want to use the cable during the measurements.

Answer 1

calibration looks ok.

• You can calibrate within a smaller frequency region (e.g. 700 MHz to 1000 MHz) for your antenna. This will reduce the risk of interpolation errors.
• If you want to check if your calibration is ok, please measure the calibration standards and check their position on a Smith Chart.
• To improve precision, use the real calibration kit values instead of ideal standard values. This is described here:https://docs.google.com/document/d/1EEMBrsMAF8PqhVpCFavttOTwVPbuzsg1uB_m7AVQ6yQ/edit#bookmark=id.85cf21mzyg2e

Maybe your table has some metal in it, so the antenna response changes when you move it?

Hope this helps, pVNA