It's difficult and even the term noise figure gets a bit fuzzy as the antenna gain starts to play a role.

If you just measure the noise power, and tune for lowest noise power, that is one way but then you don't know if the tuning has mismatched the antenna so much that the signal gain drops more than the noise power.

I'm familiar with a measurement called G/T that measures the effective gain/noise of an active antenna. I don't have a method for SA measurement but I wrote a white paper on using a PNA-X to measure this, which I've attached.

If you just measure the noise power, and tune for lowest noise power, that is one way but then you don't know if the tuning has mismatched the antenna so much that the signal gain drops more than the noise power.

I'm familiar with a measurement called G/T that measures the effective gain/noise of an active antenna. I don't have a method for SA measurement but I wrote a white paper on using a PNA-X to measure this, which I've attached.