How to use „port extension“
If you calibrate pocketVNA directly at the device but then connect your DUT with an additional cable you need to tell the device that you are using this cable in order to compensate for the phase shift / delay caused by the cable.
This example shows a measurement of the open standard measured directly at the device where it was also used for calibration:
Here what happens if you measure the open standard behind a cable:
The added delay can be seen by the saw tooth wave of the measured phase. This can be removed mathematically using the port extension function which can be found on the bottom left part of the live measurement window:
In order to use it activate the check box:
Click on the gear wheel to open the port extension settings:
There are three ways you can find the right values for length and velocity factor:
1. You know them e.g. from the data sheet.
2. You can try to find it by changing the length until the phase becomes flat again. In this case velocity factor does not need to be changed and 0.66 can be used.
3. You can measure the distance between two jumps in the saw tooth: In our example the distance between two of the jumps is around deltaf = 116.7 MHz. Using the formula:
l=Vf*c / (2* deltaf); where Vf is the velocity factor 0.66; c is the speed of light and deltaf is the frequency difference between the jumps.
we receive l=0,8450 m for the length. Also in this case velocity factor does not need to be changed and 0.66 can be used.
When we enter this into the port extension settings we can see the phase corrected:
The „open“ measurement is only to determine the cable delay/length and to demonstrate/check that it is working. For your measurement of e.g. an antenna behind the cable you should then use the same port extension settings as for the "open" measurement.