madengr replied to the topic: Impedance of free space

Well if a 50 Ohm coaxial airline is measured to be 50 Ohms, and the dimensions are exact, then E0/u0 have the same ratio in the airline as in free space, so free space must be 377 Ohms/meter. I suppose the historical direct measurement is the speed of light c=1/sqrt(E0*u0), but that still leaves an unknown term. Some smart people obviously figured it out.

I suppose if you had a hypothetical sheet of material of 377 Ohms/sq. then it would have no reflection, but any material has Er>1 so you'd still get a reflection. Once, on paper, I came up with an tapered lossy line of sheet resistance, that if you made it infinitely long, would have zero reflection down to DC. Making it shorter should provide a good termination without needing a ground connection at the other end. Of course any infinitely long line would do that, but the taper let it use an arbitrary sheet resistance and start at a width of arbitrary Z0. I suppose something along those lines, but in 3D.

Desert Sage replied to the topic: Impedance of free space

The ratio of mu zero to epsilon zero has units of ohms (per meters cancel).

In a carefully controlled experiment with parallel plate capacitor with plates spacing settable with micrometer precise epsilon zero can be determined empirically. Know or measuring the speed of light you can solve for Z zero or mu zero.

Hertz measured wavelength in earliest of experiments and if he knew frequency he would know the speed of light. (He basically used reflections off a wall and a probe to measure VSWR without a slotted line.)

I am sure there are many ways but not too easy or we would have done experiments in EE or Physics labs!

steve barak replied to the topic: Impedance of free space

Z=Sqrt(L/C), but I think it's a distributed parameter, not a lumped one. Think Transmission Lines.

tc replied to the topic: Impedance of free space

Z=Sqrt(L/C) it works both ways. The L and the C are usually per unit length but lumped values are frequently used.

Microwaves101 replied to the topic: Impedance of free space

I like the idea of measuring permittivity of free space E0 by a testing a capacitor. Then solve for eta=1/(c x E0)

Steve

andysek replied to the topic: Impedance of free space

Direct Z measurement is not possible, as is the "normal" R resistance.
We know that Z is a coefficient, convenient in practice and defined as E / H.
(more on theory, for example in the book V.Volman Engineering Electrodynamics).

By definition - Z = E / H

It is therefore necessary to measure E and H at a given point. this can be done using a field level meter, eg SMP2 WaveControl. The polarities of both vectors should be perpendicular to each other and to the direction of the source.

The second method involves the use of power gauge measurements. We measure E and P (power - using a power meter). The power meter antenna should be calibrated to an isotropic antenna.
Then S = P / A, S - dense power (Poyting vector), A - effective area of the antenna aperture.

hence S = E H or E ^ 2 / Z

with E ^ 2 / Z = P / A

Z = E ^ 2 A / P
The measurements are best done in the anechoic chamber.