Hi,
Are you still refering to the use of the ferromagnetic rings-choke on the coax cable in the case of measuring the impedance/VSWR/S11 with VNA? As I said, these rings or this kind of choke prevents or attenuates the RF current flowing on the outer-side of the coax cable shield. This current can affect your measurement. That is, if you hold your antenna with the hand or a mechanical structure, you must ensure that the RF current does not flow on the coax shield. But where unavoidable, one must use these rings stuck on the cable to reduce this RF current...