In lot of the FM receivers I see around, come with a simple black wire that you connect to the antenna input of the FM receiver and dangle it. If I use the same wire with an FM receiver I am trying to build, and knowing that the antenna impedance changes depending on a lot of things (length, position near by objects, distance from ground,... etc), so I assume that if I measure the impedance of such wire in the FM band with a VNA, it will never be a single figure that I can use to calculate my matching network.
So, how do engineers deal with such case, and what assumptions do they put in place to proceed with their front end designs?