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 from
a transmitting structure, as opposed to M or E coupling where the
transmitted "signal" is still directly coupled to the sending structure at
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 the
"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 has
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 is
independent of it. You can pack up the sending aerial and take it home and
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 and
RF signal already created is not affected. This sort of non-related effect
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 Independence
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 to
settle down to their fixed ratios. As you come nearer than a few
wavelengths away from the transmitting structure, the variations in signals
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 any
given point will be somewhat chaotic.  How great this effect is depends in
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 line
inductive loads will affect how the aerial affects the fields out to  a few
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 modest
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 and
waveforms are affected by what happens at the antenna. If the antenna packs
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 energy
from the field". This energy is supplied by direct magnetic coupling to the
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 from
the TX circuit and the circuit will respond in some designed (or undesigned=
)
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
--=20
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