# electric field source and magnetic field source differences

#### massi

Joined Mar 16, 2017
10
Hi why close to electric field source the free space wave inpedance Z0 is high (and the electric field is also high) and then only at long distances (Frouhnofer region) the ratio of electric field over the magnetic field ( that is Z0) becomes the same as for a magnetic field source? I would expect for all distances the same ratio Z0. It seems that for electric field source, the electric field close to the source decreases very fast compared with magnetic field; the opposite for a magnetic field source where the magnetic field decrease rapidly. Maybe it can look as a trivial question but what is your opinion/suggestions or experiences about that? thanks in advance !
Regards
Massimo

#### BR-549

Joined Sep 22, 2013
4,938
Can you give a little more context to your post? Several of your statements doesn't make any sense to me.

#### massi

Joined Mar 16, 2017
10
both electric field source (basically a voltage) and magnetic field source (basically a charge current) can be electromagnetic waves sources;
look in proximity of these sources (Fresnel region):
the free space wave impedance Z0=E/H (ratio of electric and magnetic fields) increases by the distance in the case of magnetic field source;
Z0 decreases by the distance in the case of electric field source.
At long distances the ratio Z0 is constant and it is equal to 377 Ohm in both cases (electric source and magnetic source).
In a few words I would like to understand why Z0 changes by the distance in two different ways for the electric field source and for magnetic field source.

#### BR-549

Joined Sep 22, 2013
4,938
Space "Z0" has a different character than normal circuit Z0.

This "Fresnel region" is the area where the angular character of a field changes into a linear character of a wave.

The actual physical structure, and therefore the relationship......of the fields are changing to the structure of a wave at this time.

#### massi

Joined Mar 16, 2017
10
these changing of the relationships of the fields , can be related with the existence of the source itself that perturbs the fields?
I mean: going further from the source the perturbation of the source itself decreases and there will be only the radiated electromagnetic wave free of any perturbation (idealizing)..
Close to the source can be a coupling of wave impedance Z0 and the electrical impedance of the source Z ?

#### BR-549

Joined Sep 22, 2013
4,938
"these changing of the relationships of the fields , can be related with the existence of the source itself that perturbs the fields?"

Yes. At least in ever case that I am aware of.

"Close to the source can be a coupling of wave impedance Z0 and the electrical impedance of the source Z ?"

What does close mean? The whole purpose/process of emission.........is DE-coupling.

#### massi

Joined Mar 16, 2017
10
close I mean in the Fresnel region

in what consists the DE-coupling?

#### BR-549

Joined Sep 22, 2013
4,938
What is your background and where are you at in your studies?

#### kyka

Joined Jun 7, 2015
24
I'm not sure I understand 100% what you're asking. I suppose you want to know why the wave impedance is different in the Fresnel region than in the Far-field region. That's because the electric field in that region is not composed purely by the Far-field electric field (the one that decays as the inverse of the distance) but contains also components from the near-field electric field (which decays as the square inverse of the distance).

#### massi

Joined Mar 16, 2017
10
close I mean in the Fresnel region

in what consists the DE-coupling?
What is your background and where are you at in your studies?
I am a phd student in plasma physics and now I have to deal with some kind of antenna radiations
I'm not sure I understand 100% what you're asking. I suppose you want to know why the wave impedance is different in the Fresnel region than in the Far-field region. That's because the electric field in that region is not composed purely by the Far-field electric field (the one that decays as the inverse of the distance) but contains also components from the near-field electric field (which decays as the square inverse of the distance).
yes thank you

#### BR-549

Joined Sep 22, 2013
4,938
massi.......If you are working for a degree........you do not want to listen to me.

I do not believe in the modern theory.

There are many people here that will re-enforce you.

#### nsaspook

Joined Aug 27, 2009
7,890
close I mean in the Fresnel region

in what consists the DE-coupling?
It's not a trivial question
It's complex subject that's almost impossible to answer in this format so I will post some background information.
Perhaps a few pages from one of my old texts can help.

35.1 We have the antenna being feed RF energy at the resonant frequency.

Near the antenna (Fresnel region) you can see the contours of the electric field being affected by the two electric field components.

As you move from the antenna the electric field changes to that of a regular plane wave.

#### massi

Joined Mar 16, 2017
10
massi.......If you are working for a degree........you do not want to listen to me.

I do not believe in the modern theory.

There are many people here that will re-enforce you.

It's not a trivial question
It's complex subject that's almost impossible to answer in this format so I will post some background information.
thank you a lot!
Perhaps a few pages from one of my old texts can help.

35.1 We have the antenna being feed RF energy at the resonant frequency.

Near the antenna (Fresnel region) you can see the contours of the electric field being affected by the two electric field components.

As you move from the antenna the electric field changes to that of a regular plane wave.

#### nsaspook

Joined Aug 27, 2009
7,890
One thing that's important to remember about an antenna is how currents and fields reinforce in the out-going direction instead of cancel like in a transmission line to create a standing wave nodes on the antenna that combine with the propagating radiation to affect the wave impedance at that point in space near the antenna.