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I am not able to understand what is the origin of the evanescent field, how do they interact and what is the physics behind it.

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I can't give you the full works on this but for a transmitting antenna, the evanescent fields are the E fields and H fields directly produced by the antenna and localized about the antenna. Eventually they can give rise to an electromagnetic wave but this isn't an evanescent field.

Typical usage that I'm aware of is magnetic field coupling between two slightly distance devices such a wireless charger for a phone. Magnetic field flux density generally reduces with distance as a cube law unlike regular EM waves whose E or H content reduce with distance rather than distance squared (EM power).

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Near field and far field are names given to mathematical expressions. Consider a radiation source and a imaginary sphere around it. The power crossing the sphere is given by P= (4πr^2)P'= 4π(C1 + C2/r + C3/r^2+ C4/r^3+ ….) , where P' is the power density.

The first term is a constant, so no matter the size of the sphere this power will always be there. In other words, this power will always be radiated from the source. Thus, it is only due to radiated field.

Now, at small r (near source), all terms involving r will be significant and the constant term is insignificant. The power expression at small r thus becomes (approximately) P= (4πr^2)P'= 4π( C2/r + C3/r^2+ C4/r^3+ …). This collective expression is then termed to be the near field.

Hence the equation is then lumped into two terms, the first being the radiated field term, and the exponentially decaying fields lumped into another term called the ‘evanescent fields’(since their intensity decay with distance).

This grouping of the exponentially decaying components of the near field non-radiative EM field types are then mathematically examined as a single term called the evanescent fields, which is not in itself a physical entity, but a summation of the effects and characteristics of the various EM waves present near the source.

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