Why i got the test different S11 with different ground?

Hi all,
I measured 5.8G helix antenna(S11 parameter) soldered with different gounds as photoes.And I got the different results,i do not know what cause the different results,anyone can help me?

Thank you

At 5.8 GHz, your feeding cable to PCB interface can be expected to have already a considerable reflection coefficient. It can't be identical for the three cases.

The cable position also affects the antenna impedance to some extent.

Generally, the effects could be better understood from a S11 frequency sweep, it would reveal e. g. resonances.

Thank you for reply.And any criteria to obey?and how to select the correct point ?Any theoretics to explain the differnce?