<s>
The	O
near	O
field	O
and	O
far	B-Algorithm
field	I-Algorithm
are	O
regions	O
of	O
the	O
electromagnetic	O
(	O
EM	O
)	O
field	O
around	O
an	O
object	O
,	O
such	O
as	O
a	O
transmitting	O
antenna	O
,	O
or	O
the	O
result	O
of	O
radiation	O
scattering	O
off	O
an	O
object	O
.	O
</s>
<s>
Non-radiative	O
near-field	O
behaviors	O
dominate	O
close	O
to	O
the	O
antenna	O
or	O
scattering	O
object	O
,	O
while	O
electromagnetic	O
radiation	O
far-field	B-Algorithm
behaviors	O
dominate	O
at	O
greater	O
distances	O
.	O
</s>
<s>
Far-field	B-Algorithm
E	O
(	O
electric	O
)	O
and	O
B	O
(	O
magnetic	O
)	O
field	O
strength	O
decreases	O
as	O
the	O
distance	O
from	O
the	O
source	O
increases	O
,	O
resulting	O
in	O
an	O
inverse-square	O
law	O
for	O
the	O
radiated	O
power	O
intensity	O
of	O
electromagnetic	O
radiation	O
.	O
</s>
<s>
This	O
generates	O
an	O
oscillating	O
(	O
or	O
reversing	O
)	O
electrical	O
dipole	O
,	O
which	O
affects	O
both	O
the	O
near	O
field	O
and	O
the	O
far	B-Algorithm
field	I-Algorithm
.	O
</s>
<s>
The	O
boundary	O
between	O
the	O
near	O
field	O
and	O
far	B-Algorithm
field	I-Algorithm
regions	O
is	O
only	O
vaguely	O
defined	O
,	O
and	O
it	O
depends	O
on	O
the	O
dominant	O
wavelength	O
(	O
)	O
emitted	O
by	O
the	O
source	O
and	O
the	O
size	O
of	O
the	O
radiating	O
element	O
.	O
</s>
<s>
The	O
general	O
purpose	O
of	O
conventional	O
antennas	O
is	O
to	O
communicate	O
wirelessly	O
over	O
long	O
distances	O
,	O
well	O
into	O
their	O
far	B-Algorithm
fields	I-Algorithm
,	O
and	O
for	O
calculations	O
of	O
radiation	O
and	O
reception	O
for	O
many	O
simple	O
antennas	O
,	O
most	O
of	O
the	O
complicated	O
effects	O
in	O
the	O
near	O
field	O
can	O
be	O
conveniently	O
ignored	O
.	O
</s>
<s>
The	O
near	O
field	O
has	O
been	O
of	O
increasing	O
interest	O
,	O
particularly	O
in	O
the	O
development	O
of	O
capacitive	B-Device
sensing	I-Device
technologies	O
such	O
as	O
those	O
used	O
in	O
the	O
touchscreens	O
of	O
smart	O
phones	O
and	O
tablet	O
computers	O
.	O
</s>
<s>
Although	O
the	O
far	B-Algorithm
field	I-Algorithm
is	O
the	O
usual	O
region	O
of	O
antenna	O
function	O
,	O
certain	O
devices	O
that	O
are	O
called	O
antennas	O
but	O
are	O
specialized	O
for	O
near-field	O
communication	O
do	O
exist	O
.	O
</s>
<s>
Magnetic	O
induction	O
as	O
seen	O
in	O
a	O
transformer	B-Algorithm
can	O
be	O
seen	O
as	O
a	O
very	O
simple	O
example	O
of	O
this	O
type	O
of	O
near-field	O
electromagnetic	O
interaction	O
.	O
</s>
<s>
For	O
example	O
send	O
/	O
receive	O
coils	O
for	O
RFID	B-Application
,	O
and	O
emission	O
coils	O
for	O
wireless	B-Application
charging	I-Application
and	O
inductive	O
heating	O
;	O
however	O
their	O
technical	O
classification	O
as	O
"	O
antennas	O
"	O
is	O
contentious	O
.	O
</s>
<s>
The	O
interaction	O
with	O
the	O
medium	O
can	O
fail	O
to	O
return	O
energy	O
back	O
to	O
the	O
source	O
,	O
but	O
cause	O
a	O
distortion	O
in	O
the	O
electromagnetic	O
wave	O
that	O
deviates	O
significantly	O
from	O
that	O
found	O
in	O
free	O
space	O
,	O
and	O
this	O
indicates	O
the	O
radiative	O
near-field	B-Algorithm
region	I-Algorithm
,	O
which	O
is	O
somewhat	O
further	O
away	O
.	O
</s>
<s>
It	O
is	O
approximately	O
one	O
wavelength	O
from	O
the	O
antenna	O
,	O
and	O
is	O
where	O
the	O
electric	O
and	O
magnetic	O
parts	O
of	O
the	O
radiated	O
waves	O
first	O
balance	O
out	O
:	O
The	O
electric	O
field	O
of	O
a	O
linear	O
antenna	O
gains	O
its	O
corresponding	O
magnetic	O
field	O
,	O
and	O
the	O
magnetic	O
field	O
of	O
a	O
loop	B-Architecture
antenna	I-Architecture
gains	O
its	O
electric	O
field	O
.	O
</s>
<s>
It	O
can	O
either	O
be	O
considered	O
the	O
furthest	O
part	O
of	O
the	O
near	O
field	O
,	O
or	O
the	O
nearest	O
part	O
of	O
the	O
far	B-Algorithm
field	I-Algorithm
.	O
</s>
<s>
In	O
contrast	O
,	O
the	O
far	B-Algorithm
field	I-Algorithm
is	O
the	O
region	O
in	O
which	O
the	O
field	O
has	O
settled	O
into	O
"	O
normal	O
"	O
electromagnetic	O
radiation	O
.	O
</s>
<s>
In	O
the	O
far-field	B-Algorithm
region	I-Algorithm
of	O
an	O
antenna	O
,	O
radiated	O
power	O
decreases	O
as	O
the	O
square	O
of	O
distance	O
,	O
and	O
absorption	O
of	O
the	O
radiation	O
does	O
not	O
feed	O
back	O
to	O
the	O
transmitter	O
.	O
</s>
<s>
In	O
the	O
far-field	B-Algorithm
region	I-Algorithm
,	O
each	O
of	O
the	O
electric	O
and	O
magnetic	O
parts	O
of	O
the	O
EM	O
field	O
is	O
"	O
produced	O
by	O
"	O
(	O
or	O
associated	O
with	O
)	O
a	O
change	O
in	O
the	O
other	O
part	O
,	O
and	O
the	O
ratio	O
of	O
electric	O
and	O
magnetic	O
field	O
intensities	O
is	O
simply	O
the	O
wave	O
impedance	O
in	O
the	O
medium	O
.	O
</s>
<s>
Also	O
known	O
as	O
the	O
radiation-zone	O
,	O
the	O
far	B-Algorithm
field	I-Algorithm
carries	O
a	O
relatively	O
uniform	O
wave	O
pattern	O
.	O
</s>
<s>
The	O
radiation	O
zone	O
is	O
important	O
because	O
far	B-Algorithm
fields	I-Algorithm
in	O
general	O
fall	O
off	O
in	O
amplitude	O
by	O
This	O
means	O
that	O
the	O
total	O
energy	O
per	O
unit	O
area	O
at	O
a	O
distance	O
is	O
proportional	O
to	O
The	O
area	O
of	O
the	O
sphere	O
is	O
proportional	O
to	O
,	O
so	O
the	O
total	O
energy	O
passing	O
through	O
the	O
sphere	O
is	O
constant	O
.	O
</s>
<s>
This	O
means	O
that	O
the	O
far-field	B-Algorithm
energy	O
actually	O
escapes	O
to	O
infinite	O
distance	O
(	O
it	O
radiates	O
)	O
.	O
</s>
<s>
The	O
amplitudes	O
of	O
the	O
far-field	B-Algorithm
components	O
fall	O
off	O
as	O
,	O
the	O
radiative	O
near-field	O
amplitudes	O
fall	O
off	O
as	O
,	O
and	O
the	O
reactive	O
near-field	O
amplitudes	O
fall	O
off	O
as	O
.	O
</s>
<s>
All	O
of	O
the	O
field	O
components	O
overlap	O
everywhere	O
,	O
so	O
for	O
example	O
,	O
there	O
are	O
always	O
substantial	O
far-field	B-Algorithm
and	O
radiative	O
near-field	O
components	O
in	O
the	O
closest-in	O
near-field	O
reactive	O
region	O
.	O
</s>
<s>
Because	O
of	O
these	O
nuances	O
,	O
special	O
care	O
must	O
be	O
taken	O
when	O
interpreting	O
technical	O
literature	O
that	O
discusses	O
far-field	B-Algorithm
and	O
near-field	B-Algorithm
regions	I-Algorithm
.	O
</s>
<s>
The	O
term	O
near-field	B-Algorithm
region	I-Algorithm
(	O
also	O
known	O
as	O
the	O
near	O
field	O
or	O
near	O
zone	O
)	O
has	O
the	O
following	O
meanings	O
with	O
respect	O
to	O
different	O
telecommunications	O
technologies	O
:	O
</s>
<s>
In	O
optical	B-Architecture
fiber	I-Architecture
communications	O
,	O
the	O
region	O
near	O
a	O
source	O
or	O
aperture	B-General_Concept
that	O
is	O
closer	O
than	O
the	O
Rayleigh	O
length	O
.	O
</s>
<s>
(	O
Presuming	O
a	O
Gaussian	O
beam	O
,	O
which	O
is	O
appropriate	O
for	O
fiber	B-Architecture
optics	I-Architecture
.	O
)	O
</s>
<s>
For	O
such	O
an	O
antenna	O
,	O
the	O
near	O
field	O
is	O
the	O
region	O
within	O
a	O
radius	O
,	O
while	O
the	O
far-field	B-Algorithm
is	O
the	O
region	O
for	O
which	O
.	O
</s>
<s>
For	O
antennas	O
physically	O
larger	O
than	O
a	O
half-wavelength	O
of	O
the	O
radiation	O
they	O
emit	O
,	O
the	O
near	B-Algorithm
and	I-Algorithm
far	I-Algorithm
fields	I-Algorithm
are	O
defined	O
in	O
terms	O
of	O
the	O
Fraunhofer	O
distance	O
.	O
</s>
<s>
This	O
distance	O
provides	O
the	O
limit	O
between	O
the	O
near	B-Algorithm
and	I-Algorithm
far	I-Algorithm
field	I-Algorithm
.	O
</s>
<s>
Conversely	O
,	O
when	O
a	O
given	O
antenna	O
emits	O
high	O
frequency	O
radiation	O
,	O
it	O
will	O
have	O
a	O
near-field	B-Algorithm
region	I-Algorithm
larger	O
than	O
what	O
would	O
be	O
implied	O
by	O
a	O
lower	O
frequency	O
(	O
i.e.	O
</s>
<s>
Additionally	O
,	O
a	O
far-field	B-Algorithm
region	I-Algorithm
distance	O
must	O
satisfy	O
these	O
two	O
conditions	O
.	O
</s>
<s>
where	O
is	O
the	O
largest	O
physical	O
linear	O
dimension	O
of	O
the	O
antenna	O
and	O
is	O
the	O
far-field	B-Algorithm
distance	O
.	O
</s>
<s>
The	O
far-field	B-Algorithm
distance	O
is	O
the	O
distance	O
from	O
the	O
transmitting	O
antenna	O
to	O
the	O
beginning	O
of	O
the	O
Fraunhofer	B-Algorithm
region	I-Algorithm
,	O
or	O
far	B-Algorithm
field	I-Algorithm
.	O
</s>
<s>
The	O
transition	O
zone	O
between	O
these	O
near	B-Algorithm
and	I-Algorithm
far	I-Algorithm
field	I-Algorithm
regions	O
,	O
extending	O
over	O
the	O
distance	O
from	O
one	O
to	O
two	O
wavelengths	O
from	O
the	O
antenna	O
,	O
is	O
the	O
intermediate	O
region	O
in	O
which	O
both	O
near-field	O
and	O
far-field	B-Algorithm
effects	O
are	O
important	O
.	O
</s>
<s>
In	O
this	O
region	O
,	O
near-field	O
behavior	O
dies	O
out	O
and	O
ceases	O
to	O
be	O
important	O
,	O
leaving	O
far-field	B-Algorithm
effects	O
as	O
dominant	O
interactions	O
.	O
</s>
<s>
(	O
See	O
the	O
"	O
Far	B-Algorithm
Field	I-Algorithm
"	O
image	O
above	O
.	O
)	O
</s>
<s>
As	O
far	O
as	O
acoustic	O
wave	O
sources	O
are	O
concerned	O
,	O
if	O
the	O
source	O
has	O
a	O
maximum	O
overall	O
dimension	O
or	O
aperture	B-General_Concept
width	O
(	O
)	O
that	O
is	O
large	O
compared	O
to	O
the	O
wavelength	O
,	O
the	O
far-field	B-Algorithm
region	I-Algorithm
is	O
commonly	O
taken	O
to	O
exist	O
at	O
distances	O
,	O
when	O
the	O
Fresnel	O
parameter	O
is	O
larger	O
than	O
1	O
:	O
</s>
<s>
For	O
a	O
beam	O
focused	O
at	O
infinity	O
,	O
the	O
far-field	B-Algorithm
region	I-Algorithm
is	O
sometimes	O
referred	O
to	O
as	O
the	O
Fraunhofer	B-Algorithm
region	I-Algorithm
.	O
</s>
<s>
Other	O
synonyms	O
are	O
far	B-Algorithm
field	I-Algorithm
,	O
far	O
zone	O
,	O
and	O
radiation	O
field	O
.	O
</s>
<s>
In	O
the	O
far	B-Algorithm
field	I-Algorithm
,	O
the	O
relationship	O
between	O
the	O
electric	O
field	O
component	O
and	O
the	O
magnetic	O
component	O
is	O
that	O
characteristic	O
of	O
any	O
freely	O
propagating	O
wave	O
,	O
where	O
and	O
have	O
equal	O
magnitudes	O
at	O
any	O
point	O
in	O
space	O
(	O
where	O
measured	O
in	O
units	O
where	O
[ [ 	O
speed	O
of	O
light|	O
)	O
.	O
</s>
<s>
In	O
contrast	O
to	O
the	O
far	B-Algorithm
field	I-Algorithm
,	O
the	O
diffraction	O
pattern	O
in	O
the	O
near	O
field	O
typically	O
differs	O
significantly	O
from	O
that	O
observed	O
at	O
infinity	O
and	O
varies	O
with	O
distance	O
from	O
the	O
source	O
.	O
</s>
<s>
Also	O
,	O
unlike	O
the	O
far	B-Algorithm
field	I-Algorithm
where	O
electromagnetic	O
waves	O
are	O
usually	O
characterized	O
by	O
a	O
single	O
polarization	O
type	O
(	O
horizontal	O
,	O
vertical	O
,	O
circular	O
,	O
or	O
elliptical	O
)	O
,	O
all	O
four	O
polarization	O
types	O
can	O
be	O
present	O
in	O
the	O
near	O
field	O
.	O
</s>
<s>
The	O
near	O
field	O
is	O
a	O
region	O
in	O
which	O
there	O
are	O
strong	O
inductive	O
and	O
capacitive	O
effects	O
from	O
the	O
currents	O
and	O
charges	O
in	O
the	O
antenna	O
that	O
cause	O
electromagnetic	O
components	O
that	O
do	O
not	O
behave	O
like	O
far-field	B-Algorithm
radiation	O
.	O
</s>
<s>
These	O
effects	O
decrease	O
in	O
power	O
far	O
more	O
quickly	O
with	O
distance	O
than	O
do	O
the	O
far-field	B-Algorithm
radiation	O
effects	O
.	O
</s>
<s>
Non-propagating	O
(	O
or	O
evanescent	O
)	O
fields	O
extinguish	O
very	O
rapidly	O
with	O
distance	O
,	O
which	O
makes	O
their	O
effects	O
almost	O
exclusively	O
felt	O
in	O
the	O
near-field	B-Algorithm
region	I-Algorithm
.	O
</s>
<s>
The	O
outer	O
boundary	O
of	O
the	O
reactive	O
near-field	B-Algorithm
region	I-Algorithm
is	O
commonly	O
considered	O
to	O
be	O
a	O
distance	O
of	O
times	O
the	O
wavelength	O
(	O
i.e.	O
,	O
or	O
approximately	O
)	O
from	O
the	O
antenna	O
surface	O
.	O
</s>
<s>
The	O
radiative	O
near	O
field	O
(	O
also	O
called	O
the	O
Fresnel	O
region	O
)	O
covers	O
the	O
remainder	O
of	O
the	O
near-field	B-Algorithm
region	I-Algorithm
,	O
from	O
out	O
to	O
the	O
Fraunhofer	O
distance	O
.	O
</s>
<s>
The	O
energy	O
in	O
the	O
radiative	O
near	O
field	O
is	O
thus	O
all	O
radiant	O
energy	O
,	O
although	O
its	O
mixture	O
of	O
magnetic	O
and	O
electric	O
components	O
are	O
still	O
different	O
from	O
the	O
far	B-Algorithm
field	I-Algorithm
.	O
</s>
<s>
Depending	O
on	O
antenna	O
characteristics	O
and	O
frequencies	O
,	O
such	O
coupling	O
may	O
be	O
far	O
more	O
efficient	O
than	O
simple	O
antenna	O
reception	O
in	O
the	O
yet-more-distant	O
far	B-Algorithm
field	I-Algorithm
,	O
so	O
far	O
more	O
power	O
may	O
be	O
transferred	O
to	O
the	O
secondary	O
"	O
antenna	O
"	O
in	O
this	O
region	O
than	O
would	O
be	O
the	O
case	O
with	O
a	O
more	O
distant	O
antenna	O
.	O
</s>
<s>
When	O
a	O
secondary	O
radiating	O
antenna	O
surface	O
is	O
thus	O
activated	O
,	O
it	O
then	O
creates	O
its	O
own	O
near-field	B-Algorithm
regions	I-Algorithm
,	O
but	O
the	O
same	O
conditions	O
apply	O
to	O
them	O
.	O
</s>
<s>
This	O
fall-off	O
is	O
far	O
more	O
rapid	O
than	O
the	O
classical	O
radiated	O
far-field	B-Algorithm
(	O
and	O
fields	O
,	O
which	O
are	O
proportional	O
to	O
the	O
simple	O
inverse-distance	O
(	O
)	O
.	O
</s>
<s>
Again	O
,	O
this	O
is	O
the	O
same	O
principle	O
that	O
applies	O
in	O
induction	O
coupled	O
devices	O
,	O
such	O
as	O
a	O
transformer	B-Algorithm
,	O
which	O
draws	O
more	O
power	O
at	O
the	O
primary	O
circuit	O
,	O
if	O
power	O
is	O
drawn	O
from	O
the	O
secondary	O
circuit	O
.	O
</s>
<s>
This	O
is	O
different	O
with	O
the	O
far	B-Algorithm
field	I-Algorithm
,	O
which	O
constantly	O
draws	O
the	O
same	O
energy	O
from	O
the	O
transmitter	O
,	O
whether	O
it	O
is	O
immediately	O
received	O
,	O
or	O
not	O
.	O
</s>
<s>
As	O
an	O
example	O
of	O
such	O
an	O
effect	O
,	O
power	O
is	O
transferred	O
across	O
space	O
in	O
a	O
common	O
transformer	B-Algorithm
or	O
metal	O
detector	O
by	O
means	O
of	O
near-field	O
phenomena	O
(	O
in	O
this	O
case	O
inductive	O
coupling	O
)	O
,	O
in	O
a	O
strictly	O
short-range	O
effect	O
(	O
i.e.	O
,	O
the	O
range	O
within	O
one	O
wavelength	O
of	O
the	O
signal	O
)	O
.	O
</s>
<s>
Solving	O
Maxwell	O
's	O
equations	O
for	O
the	O
electric	O
and	O
magnetic	O
fields	O
for	O
a	O
localized	O
oscillating	O
source	O
,	O
such	O
as	O
an	O
antenna	O
,	O
surrounded	O
by	O
a	O
homogeneous	O
material	O
(	O
typically	O
vacuum	B-General_Concept
or	O
air	O
)	O
,	O
yields	O
fields	O
that	O
,	O
far	O
away	O
,	O
decay	O
in	O
proportion	O
to	O
where	O
is	O
the	O
distance	O
from	O
the	O
source	O
.	O
</s>
<s>
These	O
are	O
the	O
radiating	O
fields	O
,	O
and	O
the	O
region	O
where	O
is	O
large	O
enough	O
for	O
these	O
fields	O
to	O
dominate	O
is	O
the	O
far	B-Algorithm
field	I-Algorithm
.	O
</s>
<s>
Such	O
an	O
antenna	O
can	O
be	O
an	O
assemblage	O
of	O
conductors	O
in	O
space	O
typical	O
of	O
radio	O
devices	O
or	O
it	O
can	O
be	O
an	O
aperture	B-General_Concept
with	O
a	O
given	O
current	O
distribution	O
radiating	O
into	O
space	O
as	O
is	O
typical	O
of	O
microwave	O
or	O
optical	O
devices	O
.	O
</s>
<s>
These	O
simplified	O
distributions	O
have	O
been	O
termed	O
the	O
"	O
far	B-Algorithm
field	I-Algorithm
"	O
and	O
usually	O
have	O
the	O
property	O
that	O
the	O
angular	O
distribution	O
of	O
energy	O
does	O
not	O
change	O
with	O
distance	O
,	O
although	O
the	O
energy	O
levels	O
still	O
vary	O
with	O
distance	O
and	O
time	O
.	O
</s>
<s>
Such	O
an	O
angular	O
energy	O
distribution	O
is	O
usually	O
termed	O
an	O
antenna	B-Device
pattern	I-Device
.	O
</s>
<s>
Typically	O
one	O
finds	O
simple	O
relations	O
describing	O
the	O
antenna	O
far-field	B-Device
patterns	I-Device
,	O
often	O
involving	O
trigonometric	O
functions	O
or	O
at	O
worst	O
Fourier	B-Algorithm
or	O
Hankel	B-Algorithm
transform	I-Algorithm
relationships	O
between	O
the	O
antenna	O
current	O
distributions	O
and	O
the	O
observed	O
far-field	B-Device
patterns	I-Device
.	O
</s>
<s>
While	O
far-field	B-Algorithm
simplifications	O
are	O
very	O
useful	O
in	O
engineering	O
calculations	O
,	O
this	O
does	O
not	O
mean	O
the	O
near-field	O
functions	O
cannot	O
be	O
calculated	O
,	O
especially	O
using	O
modern	O
computer	O
techniques	O
.	O
</s>
<s>
The	O
electromagnetic	O
field	O
in	O
the	O
far-field	B-Algorithm
region	I-Algorithm
of	O
an	O
antenna	O
is	O
independent	O
of	O
the	O
details	O
of	O
the	O
near	O
field	O
and	O
the	O
nature	O
of	O
the	O
antenna	O
.	O
</s>
<s>
The	O
wave	O
impedance	O
is	O
the	O
ratio	O
of	O
the	O
strength	O
of	O
the	O
electric	O
and	O
magnetic	O
fields	O
,	O
which	O
in	O
the	O
far	B-Algorithm
field	I-Algorithm
are	O
in	O
phase	O
with	O
each	O
other	O
.	O
</s>
<s>
Thus	O
,	O
the	O
far	B-Algorithm
field	I-Algorithm
"	O
impedance	O
of	O
free	O
space	O
"	O
is	O
resistive	O
and	O
is	O
given	O
by	O
:	O
</s>
<s>
The	O
electromagnetic	O
field	O
in	O
the	O
near-field	B-Algorithm
region	I-Algorithm
of	O
an	O
electrically	O
small	O
coil	O
antenna	O
is	O
predominantly	O
magnetic	O
.	O
</s>
<s>
The	O
electromagnetic	O
field	O
in	O
the	O
near-field	B-Algorithm
region	I-Algorithm
of	O
an	O
electrically	O
short	O
rod	O
antenna	O
is	O
predominantly	O
electric	O
.	O
</s>
<s>
In	O
both	O
cases	O
,	O
the	O
wave	O
impedance	O
converges	O
on	O
that	O
of	O
free	O
space	O
as	O
the	O
range	O
approaches	O
the	O
far	B-Algorithm
field	I-Algorithm
.	O
</s>
