Terminating Port 2 with a 50 Ohm impedance when measuring S11

Hello,

Can someone tell me if it is necessary to termintate Port 2 of the VNA with a 50ohm impedance when measuring S11 of an Antennae?

If this is the case then what is the reason behind this?

I am trying to measure the reflected wave when a signal from Port 1 is reflected back to the source. Surely simply isolating port 2 by distance from the circuit being measured is sufficient as opposed to going to the trouble of terminating port 2 in a 50ohm impedance.

If it is a normal, not too old type of VNA, without any strange bridges, is it no needed to terminate P2 during S11 measurement. At S11 mode are both RX and TX performed at P1 while P2 is inactive. To be safe, you can easily check if a termination affects measurement result.
At S21 mode are both ports active. Normally is P1 TX and P2 is RX.