After my earlier post which considered use of the MiniVNAPro Extender using a USB connection, I wanted to give the Extender a go with the BlueVNA program.
Using BlueVNA with the extender
When running BlueVNA, a picture of the device will show. This could be a a MiniVNA, a MiniVNAPro or a MiniVNAPro with the extender. I swiped left, which went through these three devices. As the pictures suggest, use of the extender requires the MiniVNAPro with extender to be shown on the screen.
As I discussed in my earlier post, I have found that I need to use a specific startup sequence to get useful data from the extender. When using USB, the USB cable needs to be connected before connecting the CAT5 cable between the MiniVNAPro and the extender. In the case of bluetooth, the CAT5 cable should not be connected when turning the MiniVNAPro on. I wait for the Bluetooth blue light to come on on the MiniVNAPro, and then connect the CAT5 cable. The extender’s sole orange light then comes on.
After that, the calibration sequence is as per usual. After calibrating with a open, short and 50ohm standard, I ran a test on a Diamond RH771 2/70 dual band whip antenna. Side note: This thing is not crash hot on 2m, it is better on 70cm.
Here’s a pic of me holding the MiniVNAPro connected to the extender with the Diamond on the RX port:
Here’s a look at a screenshot:
With a return loss of over 16dB at 439MHz, this antenna will work well at the 70cm FM calling frequency. It is still respectable at 432MHz.
An interesting thing about this data is the presence of a few artifacts in the data. They are the “square wave” type formations that can be seen. These arise from the calibration process. BlueVNA does not allow the number of steps to be set during calibration, unlike VNA/J. With VNA/J, I set the calibration steps to the max value, which is 25000, rather than the default 2000 steps. It makes the calibration process take longer, but then actual use of the device is not any slower. What it does do is reduce the size of any “square wave” artifacts.
Suspect Bluetooth interference
If I calibrate in VNA/J using a lower, say 500, calibration steps, using USB, I also notice the presence of artefacts, but they are much less than what we see here. We are seeing the Return Loss being moved up to a full 1dB in these results shown above. This then flows through to everything else, SWR, resistance, reactance, because all of these things are calculated by the software using the return loss and phase (I have the phase switched off on the screen shot).
When using the MiniVNAPro with the extender in USB mode, things are getting a little “hairy” around 1.3GHz and above, I don’t think the dynamic range of 50dB is true at 1.5GHz. It’s still not bad, and certainly still useful in doing antenna analysis for 23cm.
I’m not sure I could say that about Bluetooth mode. I’ll do some 23cm analysis in the future, but here at 70cm, we are seeing these 1dB artefacts that are affecting the results. We can still look through them, and we can still draw conclusions about the antenna.
Perhaps there may be some value in BlueVNA allowing several calibrations and averaging the results. Same goes for the scans. Doing an average scan would slow things down, but it may help to mitigate some of these effects. Perhaps there also might be ways to better shield the Extender from Bluetooth activity.
Even with all of this, it is still better to have the flexibility of the bluetooth mode. Also it looks like the artefacts have a predictable behaviour (either adding or subtracting from the return loss an amount up to 0.5dB). It might be possible to correct for this in software.
Regards, Wayne VK3WAM
ADDENDUM: I used the extender to do some testing on a 70cm colinear I have been building. I performed a calibration outside, and the results I obtained did not show the effects that are seen above. So, there is another noise source on my desk at home that affects the extender/MiniVNAPro combination in bluetooth mode, rather than it being bluetooth itself.