Hi Marco,
your measured file looks like measured directly at the antenna. But running antenna with you rig, you need a cable with connectors. So take a look at that cable and connectors and make a vna-plot with this cable. Maybe there are faults in the cable / connectors.
73, G¨¹nter, DK5DN
Am 04.11.2020 um 18:40 schrieb I0MOM via groups.io:
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Hi,
just received my nanoVNA H4 and after a few satisfactory tests, decided to use it to fix my 3 element 50Mhz yagi which presents a little too high SWR on FT8 frequency (50.313Mhz).
The SWR reading at the radio was around 2.8 and that put in protection the rig.
To have an idea of how work on the antenna I run a scan with NanoVNA (after a calibration from 48 to 54 Mhz) and the results can be seen on the attached NanoVNA saver file..
The SWR at 50.313 Mhz were 2.6 quite similar to the 2.8 measured on the rig , but when I moved to 51.450 Mhz , the predicted lower SWR frequency, SWR jumped to infinite!
Quite shocked I resort to the old usual way of tuning an antenna and sampling with the transmitter a range of frequencies, found that the SWR trend was the opposite way from the predected and the real SWR minimum (1:1) was between 49.6 and 49.7 slowly rising to 2:1 at 50.1 Mhz. and then higher and higher as the frequency increase.
I did the measurements a couple of time before writing and I wont to believe to NanoVNA but instead of helping was misleading, and I have no idea of what I did wrong.
Have a nice evening,
73 de I0MOM Marco in sunny Rome
schweppe