Impedance is easy to measure. But this is not the only parameter. A 50 Ohm resistor has a perfect impedance, but makes a terrible antenna. I wonder how they optimized the radiation characteristic this way.
I mistyped, he uses a vector network analyzer for tuning RF stuff. Measures the amplitude and phase of a circuit and generates a smith cart in real time. They've come down in prices a lot but they used to be $50k or more.
There are also simulation tools that allow you to simulate antenna's.
