Hello everyone as you know when light scattered from any object then both kind of waves produced propagating and non propagating or evanescent waves. From the scattering spectra of the propagating waves how can we identify that this is the boundary or interface between the near and far field? How can find that interface or point or bridge between near and far field?
Thanks Sir for yours reply but my question is that from the visualization of the scattered field image how can you decide and tell that this is the interface between the near and far field???
You can find different fields from the antenna by measuring distance.
please see the image which is attached.
Ullah
Far fields fall with 1/r
Near fields fall with 1/r^n where n>1.
Due to the rate of decay, after r large only 1/r componentes are left. How big this r should be depends on the definition you use.
As far as I know there isnt a definite boundary where you visualize the fields and decide - antenna guys use one definition for Farfield limit, EMC people use another.
Thanks all for your's reply.@Anvesh Kumar Nella no this is just an explanation which tell us about near and far field.@Marcelo B. Perotoni is there any method from which we can say that in 20 micrometer scattering spectra from an object there is boundary between near and far field??? so sir @Elman Shahverdiev is there two definition one related to optics and other related to quantum mechnanics???? attached file contains two images of the scattered field from 20 micrometer from a nanoparticle surface can you be able to describe its three regio??? i mean Near field,Near and far field boundary and far field???
The near field is the ordinary large scale magnetic field that is associated with the electric current in the wire. A magnetic field, being solenoidal has finite extent. Everybody ought to be familiar with the pattern, which can be demonstrated using iron filings. The near field is reversible, in that the energy collapses back into the wire when the power is switched off. The boundary of the near field is where the compass needle will stop deflecting.
The far field on the other hand is EM radiation that arises when the electric current is increasing in time, and it radiates to infinity. It is not reversible, in that the energy doesn't return when we switch the power off.
In electromagnetics, the boundary between the near and far zones is determined by the field penetration depth (skin depth) in the frequency domain of field diffusion depth in the time domain. This in case of a conductive medium. Otherwise, the wave length is the parameter.
Ullah
What is the frequency of the EM wave youre talking about? According to antenna book by Balanis, farfield lies @ distances further than 2D^2/lambda, where lambda refers to the EM wave and D you can take as the nanoparticle diameter (Balanis, eq. 4.47).
I only see a problem applying this definition, your scattering its not due to one single particle, its an array. But I guess its a first starting point.
@Marcelo B. Perotoni i am not asking this i am saying from spectra can we find the boundarycan we do an analysis to find out that region? if we do then how???
Dear Ullah I dont know what you meant. The picture you've shown before is related to a certain wavelength range, right? That would be the wavelength you use for the criterium.
In the far field, there is no phase shift between the electric and magnetic component of the field and both are orthogonal. That a true transversal wave. It is sufficient to measure the E-component, because E/H=Zo=377 Ohms.
In the near field, there is no pure transversal wave and there are additional radial components. Other formulas apply: The phase shift is a function of location, and the ratio E/H is a complex number. There is no fixed border, where suddenly other formulas apply.
Dear Mr. Ullah,
the boundary is depent from the wavelenght and the impedance of the source field.
All the best
F. Gräbner
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