I think one of the most important properties of near-field is that the
receiving (secondary) antenna "interacts" with the sending antenna
(primary).
That is, the loading caused by the receiving antenna can be sensed by
the sending antenna as a change in some electrical property (inductance,
flux, etc.), while with far field the sending antenna doesn't notice any
changes in its properties irrespectively if zero, one or n receiving
antennas are receiving its signal.

Isaac


Em 01/04/2013 01:30, RussellMc escreveu:
> Here is some "sort of correct" comment for people to tear holes in and
> improve. Or not:
>
> It is important to note that the terms: EM, electromagnetic wave  and
> electromagnetic, all TEND to refer to true RF where energy is launched fr=
om
> a transmitting structure, as opposed to M or E coupling where the
> transmitted "signal" is still directly coupled to the sending structure a=
t
> all times. At significant distances from the transmitting structure
> (usually held to be "a few wavelengths") the E & M fields have "settled
> down", you have a true EM or RF signal - this region is referred to as th=
e
> "far field". As you approach the transmitting structure "all hell breaks
> loose" [tm] and E and M ratios var widely. The majority of the energy in
> this region is still directly coupled to the transmitting structure and h=
as
> not been "launched" as energy which can travel across the void unaided.
> This is the "near field" region.
> For interest, the near field of the Jodrell Bank radio telescope extends
> beyond the atmosphere. The RT is used as one means of communicating with
> inter-planetary spacecraft. Having your far field start somewhere in low
> earth orbit makes pre-testing of the overall local & remote comms systems=
 "
> difficult".
>
> True or "far field" "RF" consists of a combined E & M field "signal" in a
> fixed ratio. The ratio defines the impedance of free space (Or the
> impedance of free space defines the ratio).
>
> A far field RF signal is "launched" from the transmitting structure and i=
s
> independent of it. You can pack up the sending aerial and take it home an=
d
> any already launched RF signals "will not notice". You can move the
> transmitter source or rotate it at high speed or turn off its power and a=
nd
> RF signal already created is not affected. This sort of non-related effec=
t
> is most often relevant to aliens or distant space-craft due to the speed =
of
> propagation. Most RF signals are launched and received (or lost as heat)
> while the sending aerial is still in its initial state, so the Independen=
ce
> of the two is not noticed.
>
> As you move away from the transmitting aerial it takes a bit of distance
> for the signal to 'get its act together' properly and for E & M signals t=
o
> settle down to their fixed ratios. As you come nearer than a few
> wavelengths away from the transmitting structure, the variations in signa=
ls
> still "tied" to aerial interfere with bits of E & M that are trying to
> properly couple into their free space E:M ratio and what you measure at a=
ny
> given point will be somewhat chaotic.  How great this effect is depends i=
n
> part on antenna structure, which may be altered to enhance or decrease
> various effects. Use of eg top-hat capacitive loads or loading coil in li=
ne
> inductive loads will affect how the aerial affects the fields out to  a f=
ew
> wavelengths distant.
>
> This chaotic field area is known as the near field. In this area you can
> design (or attempt to) equipment which emphasises either E or M field
> coupling. E field (capacitive) coupling is used by some systems for short
> range coupling but M (magnetic) field usually proves more useful for mode=
st
> power transfer.
>
> Very importantly, while RF deals with hunks of energy which have 'been
> shaken free' of the sending antenna and are travelling through free space
> on their own recognisance, E fields alone and M fields alone are still
> driven by and associated with the generating antenna. Their magnitudes an=
d
> waveforms are affected by what happens at the antenna. If the antenna pac=
ks
> up its bags and goes home the E & M near-field signals immediately also
> cease.
>
> The relevance of the dependence of E & M field to transmitting structure
> and of RF to free space is that for E & M fields energy is not "used" by
> their mere existence. There may (and will) re losses in the antenna
> structure, the generating electronics and power supplies will have losses
> BUT the fields  are "out there" losslessly, as long as they are not
> intercepted.
>
> Conversely, the RF signal has a certain amount of energy in it and this
> will be spread out increasingly as the wavefront increases with distance
> and will be dissipated by anything it encounters that can absorb it (eg a
> cup of water in a microwave oven).
>
> If we now take an essentially lossless M field and couple a tuned circuit
> to it that is resonant at the frequency of the M field we can "draw energ=
y
> from the field". This energy is supplied by direct magnetic coupling to t=
he
> transmit antenna and NOT like RF by picking up energy travelling in the
> aether [tm].  The transmit aerial will be loaded, power will be drawn fro=
m
> the TX circuit and the circuit will respond in some designed (or undesign=
ed)
> manner. In a magnetic power transfer system, the transmitter MAY respond =
by
> increasing the available voltage drive to the TX circuit to increase
> received signal and so increase available power level.
>
>
>                 Russell McMahon

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