What does a VNA tell us about this antenna ?
It is a bit misleading label for this tread.
Antenna mounted directly at VNA is a very poor mounting place if it not is by purpose that VNA should be a part of the antenna measurement. Actual measurements curves shows dips at 400 and 1200 MHz. Antenna in it self is too short to provide resonances at these frequencies.
What is measured is to a degree the antenna and its nearfield such as the VNA. It is neither an antenna designed to be placed parallel and close to a big metallic surface.
Measurement setup is less optimal and VNA probably not correct calibrated.
Normally when measuring this type of antenna do we try to avoid that measurement cable is included in actual antenna measurement.
A free space with nothing that resonated near antenna and ferrite tubes around measurement cable is much the standard setup.
Connecting antenna directly at VNA and VNA is a bit hard to hide from actual measurement.
There is also some kind of problem with instrument calibration. For actual setup and type of antenna and frequency range, it is not even close to realistic impedance in last Smith Chart above. Guess that correct port forwarding not is performed.
I repeated a similar measurement below as a comparison. Measured a D-Link router antenna.
Connecting cable have several ferrite tubes and nearfield is not a part of the measurement. Note that Smith Chart is stable and not rotates a lot of turns. No low frequency anomalies are seen.
Similar result for most of my collection of old router antennas.
It is an interesting tool and price but very hard to judge what it is able to measure. Is it a network analyzer or single port analyzer (can it measure S12)?
Is it maybe a problem with its calibration? I have searched in the data sheet that could be found but could not find such information.