On Wed, Jan 27, 2021 at 06:43 AM, dsay2001 wrote:
I've had my H4 for a while now, and I really like it.
I've been trying to get an idea of its SWR accuracy, so I compared it to a
friend's RigXpert.
I wrote an app that reads the RigXpert's AntennaScope files and
NanoVNAServer's Touchstone files, and plots the SWR of each.
The results are not what I expected - pretty different (image attached,
RigXpert-NanoVNA-Comparison.png). I didn't expect the results to match
exactly, but I thought they would be close.
I've looked over a number of posts here, and haven't really found an answer. I
hope these points answer some of the issues I've seen raised for similar
questions:
- Both source files have 101 datapoints.
- The SWR plots I generate match the plots I see in the AntennaScope and
NanoVNASaver software.
- I connect the cable from the antenna directly to the RigXpert, and for the
Nano, I add a 12" SMA-SO239 adapter cable.
- I calibrate the Nano for 144-148 MHz, Open/Short/50 ohm.
- NanoVNAServer v0.3.4
- The H4 version info is in an attached image (NanoVNA-H4-Version.png).
It's certainly possible that the RigXpert is wrong, but that seems like a low
probability.
I see there is a newer firmware version, but it seems to be lacking release
notes. Will that solve the problem?
I own a RigExpert and a NanoVNA-H4. The results are almost identical when I make measurements if I do things correctly.
I think your problem is that you have a common mode issue on your antenna system. If you do not have a proper balun, unun or RF choke at the antenna feedpoint the outer surface of the coax shield will form part of the antenna. Anytime you change the length of the coax or how it is grounded at the radio end you are effectively changing the antenna feedpoint impedance and will get slightly different SWR measurements. For VHF this is very common with antennas like a dipole, J-pole or a whip.
Here is a simple test to see if this is the case. Measure the antenna with the RigExpert in stand-alone mode and then use the Save feature to store the results on the RigExpert analyzer. Then connect it to your computer with a USB cable, measure again and save these results in another file on the Rigexpert. Then start AntScope 2 and connect to the RigExpert. You can import both files into AntScope2 (bottom right corner of AntScope2). These are RigExpert files not Touchstone. Both will plot on the screen and you can see if there is any difference. Now save both files on your PC. Exit AntScope.
Next step is to disconnect the RigExpert from the antenna and POC. Calibrate your NanoVNA for the desired frequency range and connect to the antenna with your pigtail. Connect it to the PC and start AntScope2. Go into settings and you will see an option to connect to your NanoVNA (latest AntScope2 supports the -H4). Connect and then exit settings and do a sweep. It will now plot on the screen. You can now import the 2 files you saved earlier and compare all 4 measurements. I suspect only two will closely match and the standalone RigExpert will be different.
Roger
Roger Need