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Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
If you think of it in terms of complex potentials, the storage field is the imaginary part, the radiation field is the real part. It's as simple as that. Power can't be transferred by the storage field: any time it's being transferred, there is a real component to the field.
The radiation field is not necessarily the far field, because EM power travelling through free space must have an impedance of 377 ohms. You can have radiation fields whose impedance is far from this, and they can't travel in free space. This is what happens in the transformer, Tesla wireless power, and so on. It's not a storage field (that would be the field associated with the magnetizing current) but it's not a radiant field either.
Paul Nicholson explains this by saying that the apparatus forms a cavity or waveguide with an impedance different to free space. For instance, when two Tesla coils are coupled capacitively, the field transferring power between them has a characteristic impedance of hundreds of thousands of ohms, on account of the high voltage and low current.
And yes, if you surrounded a TV mast with an enormous sphere of TV sets, they would absorb most of the power. You should read Nicholson's article on non-Hertzian waves.
Registered Member #29
Joined: Fri Feb 03 2006, 09:00AM
Location: Hasselt, Belgium
Posts: 500
The difference between near field and far field is a bit of a fuzzy one. In a nutshell, whenever you have something with time-varying currents flowing on its surface, electric and magnetic fields are produced in the surrounding space. These fields exhibit two forms of behaviour. On one hand, the local "near fields" do not go anywhere...they look a lot like static fields and contribute to reactive power only; circulating around the structure. They typically die off as 1/r^2, 1/r^3, etc. as you move away from the current source. On the other hand, the radiation fields decay ar 1/r from the structure and contribute to power loss by power flow away from the structure. At a distance where the intensity of the radiation fields exceed the intensity of the stationary reactive fields defines the "far field" region.
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Difference between the near field and far field
You should read Nicholson's article on non-Hertzian waves.
