fraunhofer zone
I became interested deeper into zones of antenna - I mean Rayleigh and Fresnel as "near zone" and Fraunhofer as "far field".
I need your comments...
Is the difference between these zones only in simplifying Green function exp^{-jkR}/R resulting in Fourier transform when considering far field?
Or there are some next effects associated with antenna function (transformating process of guided waves into free space)?
What do you think?
Thank in advance,
Eirp
Are you talking about reactive field, near field and far field? If yes, then I can add somthing in terms of radiating property.
Yes, g86, this is it...
I started to think about vector potential which can be expressed (in far field) as Fourier transform of the source. This is also connected with optics.
From physical point of view I think that near field area is the are where spheiracal waves are forming from guided waves (E-lines must connect together). This is shown in attachment - fig. e) and f) . In fig g) r means radiated energy (radiation resistance) and x reflected energy (imaginary part)..BTW there are only two E lines shown, with distance lambda/2...
Is this really a near field
One aspect of Fresnel zones is the far field where the rays take different paths from the transmitter to the receiver. The zones are the cross sectional areas of any perpindicular plane along the path where the path length differs by half a wavelength. These are concentric circles on the plane.
This becomes important when there is an obstical (like a building or a mountain) in a direct optical path. The received signal strength is reduced even thouth you can easily see the receive antenna when standing next to the transmitting antenna beause the bottom parts of the zones are blocked by the obstical.
A very small addition:
We can just think the problem as electric, magnetic and electromagnetic radiation dominant zones.
At reactive field the effect of accumulation of charge takes the major role. Say for a dipole we know the far field pattern is figure of eight [8]. But at near fiels the accumulated charges will provide a pattern like dumble [o-o] due to the presense of electrostatic field and here the effect of radiation will be proportional to 1/r^3. These fields are frequency dependent.
At near field the major role is due to the magnetic fields and also frequency independent. and the effect varies with 1/r^2. Normally this field is due to the magnetic induction from the movement of electrical charge in antenna.
At far field power varies with 1/r and here electromagnetic field is predominant and direction of power follows the ponting vector. electromagnetic fields are due to the acceleration of charges.
So in terms of electric charge motion the regions can be defined. At very close fields major role due to static charge, charge velocity takes major role at near fields and charge acceleration holds its major role at far field.
A very interesting thing about all these fields is that they are of equal value at \lamda/\pi distance from the antenna.
I've read that distance lambda/2pi is valid only for small antennas as dipole.
g86: You demonstrated dipole near field, that's OK.
And what about aperture antennas?
There's far-field limit r>D^2/lambda which is discussed in optics too and is derived from Green function aproximation (expansion in series, you know..) in Rayleigh-Sommerfeld diffraction formula (similar to our vector potential as I suppose). What about connection between optics and elmag. point of view?
Eirp
I have had some further thoughts after reading the discussion.
1. The various field types that fall off at 1/(r^n) are derived from a infinitely small dipole of length dl. Once you make the dipole larger the r relationships become more complex.
2. The various zones that started off this section come from optics and also apply to sound. When a plane wave of limited width is transmitted by a source such as a lens or a piston, the wave stays the same diameter as the emitter for a while and then diverges. These terms apply to the different ranges from the emitter.
Is it really? Maybe thats why my avatar is so colorful:)
Oho, you are right. I forget to tell that lambda/2pi is true for Harzian dipole.
Yes, the names used for the zones started out in optics because it came first. They all refer to difraction patterns caused by truncating an infinite plane wave to a finite cross section.
These are discussed for optics in Principles of Optics by Born and Wolf.
They are discussed for acoustics in Acoustic Waves by Kino.
The derivation of the 1/(r^n) radiation terms for a infinitely short antenna with uniform current distribution is described in Electromagnetics by Kraus and Antennas by Kraus. What is interesting is the 90 degree phase shift between some of the magnetic and electric fields. These produce no real power which can radiate which explains the transfer of energy back and forth between the near area and the radiating element.
The extension to finite sized dipoles with nonuniform current distribution is described in Antennas by Kraus.
For engineering approximation purposes, a finite length diipole with nonuniform current distribution is close enough to a short one as described by the classical equations.
Check ANTENNAs for ALL APPLICAIONS by KRAUS.It has the best explanation (in 2nd chapter).
Nearest zone (Rayleigh): field strength falls off at inverse distance cubed rate.
Near field: (Fresnel): field strength falls off as inverse distence squared.
Transition region: Fresnel field strength equals Fraunhofer (far-field) strength.
Far field: field strength falls off as inverse distance.
The critical transition region distance is different for each type of antenna.
Dipole: lambda divided by 2pi
Monopole (whip): 3 lambda
Surface types (horn, dish, billboard, aperture): twice distance squared divided by
lambda.
Check out "Introduction to Electromagnetic Compatibility" by Clayton A. Paul,
page 423 of second edition. Wiley, 2006 ISBN: 0-471-75500-1
I have this expl. from BALANIS