I looked at the original question again and it seems I answered incorrectly.
I don't have a NanoVNA on hand, so I can't reproduce it. Let's see what other Nano users say.
In summary, NanoVNA is a very low-cost VNA that performs reasonably well. However, it should not be expected to have excellent performance at high frequencies. 430 MHz is already considered...HighIt's over.
Many amateur radio operators do not consider frequencies above 430 MHz when building their own antennas, so they may not even notice problems in the 900 MHz range.
Regarding why Mag S11/S21 suddenly increases near the 1/3 position, here's aPossible explanationsAs follows:
Edy555's original NanoVNA was designed to operate over a range of 50 kHz -300MHz. Hugen's design extended the measurement frequency up to 900MHz by utilizing harmonics of the internal frequency synthesizer
The native 300 MHz and the 900 MHz achieved through harmonics definitely have different effects.
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