That’s not really how it works.
The weakest signal you see typically correlates with the noise level.
That noise can picked up by the antenna which is unavoidable or it’s introduced on the signal path after the antenna.
Just the dBFS won’t tell you which of the two scenarios is your limit.
Once you optimize the gain a bit, the lowest signal number becomes a little bit more meaningful: Thoughts on optimizing gain
Because then it’s the weakest signal compared to the strongest signal.