Difference in SWR readings between nanoVNA and SWR meter

[shadowless]

I have been doing some testing with my nanovna for 140/440mhz.  I keep getting crazy swr for the 140mhz band.

I then tried to connect my nanovna to the pc in home of updating the firmware.  That's when I found out that if I have the usb cable plug in while I use the device, I get more accurate result that is consistent with my swr meter. The antenna dip shift back to the 140mhz.  If the nanovna is ran in the floating mode the 140 band is off as the antenna dip is shifted to 150 or even 160mhz. 

The usb cable connection probably provide the ground that is required for a more accurate testing for antennas.  So far I have only tested the HT antennas, not done any testing on the larger antennas yet.

I am not sure if that is the solution but I don't really trust using these device for antenna swr test over dedicated swr meters.

[radiolistener]

I then tried to connect my nanovna to the pc in home of updating the firmware.  That's when I found out that if I have the usb cable plug in while I use the device, I get more accurate result that is consistent with my swr meter. The antenna dip shift back to the 140mhz.  If the nanovna is ran in the floating mode the 140 band is off as the antenna dip is shifted to 150 or even 160mhz. 

This is indication that your antenna has common mode current and it affects your measurements. This is why you see difference.

Such result cannot be used to compare swr meter with nanovna. Just a wrong measurement method.