Wave impedance

what impedance you want to mesure! media? T.L.? waveguid? what!

of free space

I have never heard of a case of measuring it. All of the cases I have seen have been calculating it.

Since impedance is the ratio of electric and magntic fields, I suspect that you would have to measure these at a point. The problem being that your measurement sensor would disturb the field. (This reminds me of the atomic physics uncertainty principle.)

I think this amounts to measuring the permittivity and permeability of free space. The experiments for obtaining accurately the value of epsilon_0 and mu_0 are quite complicated. I think this is the job for people in the bureau of standards. What is the purpose of looking into this?

For a plane wave the impedance is what you say, the fundamental electric and magnetic constants of free space. In cases where the wave is not plane, like in a waveguide or the near field of an antenna, other factors affect the impedance.

:D

Whenever you are measuring any energy parameter you must have to disturbe the system. It may be voltage, current or light or temperature..so you should not be worry about too much. Just think how to perform it without disturb it much.