electromagneticelectromagnetismimpedancepropagationwave
I am looking for the exact formulas for the impedance of an electric and magnetic wave in the near field region.
We have some approximates:
Zwe≈ 1 / (ω * ε0 *r)
Zwm≈ (ω * µ0 *r)
where r is the distance, ω is the angular frequency, ε0 vacuum permittivity, µ0 vacuum permeability
However these formulas are only approximations, and I believe the accurate formula involves complex numbers.
I could not find the full formula for this, I was hoping somebody here would know it.
Magnetic field falls with distance cubed so, if the distance is increased by 10:1 then the field has fallen by 1000:1.
On the other hand if the radius increases by \$\sqrt{1000}\$ then you receive the same magnetic flux density. For confirmation of your formula see this: -
Picture taken from hyperphysics website
That PDF provides various 1/distance^N contributors to field strength.
The Efield has [1/D^2 - 1/D^4 + 1/D^6]
The Hfield has [1/D^2 + 1/d^4]
I recall similar equations in Corson and Lorrain. I ain't a trained E&M jock, just trying to understand the mechanisms of shielding for reliable 16 and 24 bit systems.
Best Answer
I haven't got formulae but I have this picture that may help a little bit: -
Picture taken from here http://incompliancemag.com/article/guide-to-testing-conducted-emissions-based-on-the-methods-in-en-55022-and-en-55011/