How the wave impedance of free space works?

[Massi]

Hi All,
what is the reason why very close to an electromagnetic source (Fresnel Region), the wave impedance Z0 in free space for electric field and magnetic field is not the same? Z0 for magnetic field increases and for electric field decreases by increasing the distance from the source. Actually they are different also in the Frounhofer region but they have practically the same values with a very insignificant error.
So why the wave impedance can be different for the electric and magnetic fields?
Regards
Massimo

 

[sophiecentaur]

So why

It's not a "why" it's just that the ratio of E and H fields (the definition of the Impedance) is different in the vicinity of, say, a piece of metal (a wire antenna).
If you want a fairly close parallel, imagine taking a sheet of resistive paper and you measure the resistance between two small pads, separated by 1m and you use that value to describe the Resistance of the paper. If you stick a conducting plate along the line between the pads and measure the resistance, it will be lower. Two different measured values for the same piece of paper. Introducing the plate has affected your measurement because it is 'in parallel' with the paper resistive path. The smaller the plate is, compared with the pad separation, the less difference it will make this is equivalent to looking 'near and far field'. When there is a conducting antenna present, it appears (perhaps) in parallel with the E Field and near your Impedance Measuring device and will alter the ratio of E/H.

So why the wave impedance can be different for the electric and magnetic fields?

I'm not sure you meant that because the wave impedance contains both quantities.

 

[Massi]

thanks for reply, so the source itself perturbates the wave impedance;
I did this question because I found on the web this picture that I show you in attachment (the reference is given in the file). I did a bit confusion in the previous message I sent, Actually it refers to electric field source and magnetic field sources for which the wave impedance has a different behavior close to the source. It is interesting to understand why there is different behaviour of the wave impedance close to magnetic source and electric source.
Regards
Massi

 

[sophiecentaur]

You have answered your own question there. As far as I can see, an "Electric Field Source" would be a source that maintains a given value of Electric Field under any conditions - like a (ideal) Voltage Source maintains volts whatever load is connected. Such a Field source would need to have a very low (zero) impedance to impress that field, come what may. So you already have an impedance that isn't 377 Ohms, when you are right next to the source. As you get further away from an E source, the direction of the vectors will become perpendicular to the propagating wave and an appropriate H field will appear.

 

[Massi]

thank you
massi