<s>
A	O
loop	B-Architecture
antenna	I-Architecture
is	O
a	O
radio	O
antenna	O
consisting	O
of	O
a	O
loop	O
or	O
coil	O
of	O
wire	O
,	O
tubing	O
,	O
or	O
other	O
electrical	O
conductor	O
,	O
that	O
is	O
usually	O
fed	O
by	O
a	O
balanced	O
source	O
or	O
feeding	O
a	O
balanced	O
load	O
.	O
</s>
<s>
Large	O
loop	B-Architecture
antennas	I-Architecture
(	O
or	O
self-resonant	O
loop	B-Architecture
antennas	I-Architecture
or	O
full-wave	O
loops	O
)	O
have	O
a	O
perimeter	O
close	O
to	O
one	O
or	O
more	O
whole	O
wavelengths	O
at	O
the	O
operating	O
frequency	O
,	O
which	O
makes	O
them	O
self-resonant	O
at	O
that	O
frequency	O
.	O
</s>
<s>
Large	O
loop	B-Architecture
antennas	I-Architecture
have	O
a	O
two-lobe	O
radiation	B-Device
pattern	I-Device
at	O
their	O
first	O
,	O
full-wave	O
resonance	O
,	O
peaking	O
in	O
both	O
directions	O
perpendicular	O
to	O
the	O
plane	O
of	O
the	O
loop	O
.	O
</s>
<s>
Some	O
writers	O
prefer	O
to	O
exclude	O
them	O
from	O
loop	B-Architecture
antennas	I-Architecture
,	O
since	O
they	O
can	O
be	O
well-understood	O
as	O
bent	O
dipoles	O
,	O
others	O
make	O
halos	O
an	O
intermediate	O
category	O
between	O
large	O
and	O
small	O
loops	O
,	O
or	O
the	O
extreme	O
upper	O
limit	O
for	O
small	O
loops	O
:	O
In	O
shape	O
and	O
performance	O
halo	O
antennas	O
are	O
very	O
similar	O
to	O
small	O
loops	O
,	O
only	O
distinguished	O
by	O
being	O
self	O
resonant	O
and	O
having	O
much	O
higher	O
radiation	O
resistance	O
.	O
</s>
<s>
Small	O
loop	B-Architecture
antennas	I-Architecture
(	O
or	O
magnetic	O
loops	O
or	O
tuned	O
loops	O
)	O
have	O
a	O
perimeter	O
smaller	O
than	O
half	O
the	O
operating	O
wavelength	O
(	O
typically	O
no	O
more	O
than	O
~	O
wave	O
)	O
.	O
</s>
<s>
The	O
radiation	B-Device
pattern	I-Device
of	O
small	O
loop	B-Architecture
antennas	I-Architecture
is	O
maximum	O
at	O
directions	O
within	O
the	O
plane	O
of	O
the	O
loop	O
,	O
so	O
perpendicular	O
to	O
the	O
maxima	O
of	O
large	O
loops	O
.	O
</s>
<s>
For	O
all	O
of	O
the	O
large	O
loops	O
described	O
in	O
this	O
section	O
,	O
the	O
radio	O
's	O
operating	O
frequency	O
is	O
assumed	O
to	O
be	O
tuned	O
to	O
the	O
loop	B-Architecture
antenna	I-Architecture
's	O
first	O
resonance	O
.	O
</s>
<s>
Self-resonant	O
loop	B-Architecture
antennas	I-Architecture
for	O
so-called	O
“	O
short	O
”	O
wave	O
frequencies	O
are	O
relatively	O
large	O
,	O
with	O
a	O
perimeter	O
just	O
greater	O
than	O
the	O
intended	O
wavelength	O
of	O
operation	O
,	O
hence	O
for	O
circular	O
loops	O
diameters	O
between	O
roughly	O
175ft	O
(	O
53m	O
)	O
at	O
the	O
largest	O
,	O
around	O
1.8MHz	O
.	O
</s>
<s>
Large	O
loop	B-Architecture
antennas	I-Architecture
can	O
be	O
thought	O
of	O
as	O
a	O
folded	O
dipole	O
whose	O
parallel	O
wires	O
have	O
been	O
split	O
apart	O
and	O
opened	O
out	O
into	O
some	O
oval	O
or	O
polygonal	O
shape	O
.	O
</s>
<s>
The	O
loop	O
's	O
shape	O
can	O
be	O
a	O
circle	O
,	O
triangle	O
,	O
square	O
,	O
rectangle	O
,	O
or	O
in	O
fact	O
any	O
closed	O
polygon	B-General_Concept
,	O
but	O
for	O
resonance	O
the	O
loop	O
perimeter	O
must	O
be	O
slightly	O
larger	O
than	O
a	O
wavelength	O
.	O
</s>
<s>
Loop	B-Architecture
antennas	I-Architecture
may	O
be	O
in	O
the	O
shape	O
of	O
a	O
circle	O
,	O
a	O
square	O
or	O
any	O
other	O
closed	O
geometric	O
shape	O
that	O
allows	O
the	O
total	O
perimeter	O
to	O
be	O
slightly	O
more	O
than	O
one	O
wavelength	O
.	O
</s>
<s>
There	O
may	O
be	O
one	O
or	O
more	O
additional	O
loops	O
stacked	O
parallel	O
to	O
the	O
first	O
as	O
'	O
parasitic	O
 '	O
director	O
and	O
/	O
or	O
reflector	O
element(s )	O
,	O
creating	O
an	O
antenna	B-Algorithm
array	I-Algorithm
which	O
is	O
unidirectional	O
with	O
gain	O
that	O
increases	O
with	O
each	O
additional	O
parasitic	O
element	O
.	O
</s>
<s>
Unlike	O
a	O
dipole	O
antenna	O
,	O
the	O
polarization	O
of	O
a	O
resonant	O
loop	B-Architecture
antenna	I-Architecture
is	O
not	O
obvious	O
from	O
the	O
orientation	O
of	O
the	O
loop	O
itself	O
,	O
but	O
depends	O
on	O
the	O
placement	O
of	O
its	O
feedpoint	O
.	O
</s>
<s>
The	O
radiation	B-Device
pattern	I-Device
of	O
a	O
first-resonance	O
loop	B-Architecture
antenna	I-Architecture
peaks	O
at	O
right	O
angles	O
to	O
the	O
plane	O
of	O
the	O
loop	O
.	O
</s>
<s>
Additional	O
gain	O
(	O
and	O
a	O
uni-directional	O
radiation	B-Device
pattern	I-Device
)	O
is	O
usually	O
obtained	O
with	O
an	O
array	O
of	O
such	O
elements	O
either	O
as	O
a	O
driven	O
endfire	O
array	O
or	O
in	O
a	O
Yagi	O
configuration	O
(	O
with	O
all	O
but	O
one	O
loop	O
being	O
parasitic	O
elements	O
)	O
.	O
</s>
<s>
Low	B-Algorithm
frequency	I-Algorithm
one	O
wavelength	O
loops	O
"	O
lying	O
down	O
"	O
are	O
sometimes	O
used	O
for	O
NVIS	O
operation	O
.	O
</s>
<s>
Its	O
radiation	B-Device
pattern	I-Device
consists	O
of	O
a	O
single	O
lobe	O
straight	O
up	O
(	O
radiation	O
toward	O
the	O
ground	O
which	O
is	O
n't	O
absorbed	O
is	O
reflected	O
upward	O
)	O
.	O
</s>
<s>
The	O
radiation	B-Device
pattern	I-Device
and	O
especially	O
the	O
input	O
impedance	O
is	O
affected	O
by	O
its	O
proximity	O
to	O
the	O
ground	O
.	O
</s>
<s>
As	O
the	O
frequency	O
increases	O
,	O
the	O
radiation	B-Device
pattern	I-Device
breaks	O
up	O
into	O
multiple	O
lobes	O
which	O
peak	O
at	O
lower	O
angles	O
relative	O
to	O
the	O
horizon	O
which	O
is	O
advantageous	O
especially	O
at	O
higher	O
frequencies	O
.	O
</s>
<s>
Although	O
it	O
could	O
be	O
categorized	O
as	O
a	O
bent	O
dipole	O
,	O
it	O
has	O
the	O
omnidirectional	O
radiation	B-Device
pattern	I-Device
very	O
nearly	O
the	O
same	O
as	O
a	O
small	O
loop	O
.	O
</s>
<s>
On	O
the	O
VHF	B-Algorithm
bands	I-Algorithm
and	O
above	O
,	O
the	O
physical	O
diameter	O
of	O
a	O
halo	O
is	O
small	O
enough	O
to	O
be	O
effectively	O
used	O
as	O
a	O
mobile	O
antenna	O
.	O
</s>
<s>
The	O
horizontal	O
radiation	B-Device
pattern	I-Device
of	O
a	O
horizontal	O
halo	O
is	O
nearly	O
omnidirectional	O
–	O
to	O
within	O
3dB	O
or	O
less	O
–	O
and	O
that	O
can	O
be	O
evened	O
out	O
by	O
making	O
the	O
loop	O
slightly	O
smaller	O
and	O
adding	O
more	O
capacitance	O
between	O
the	O
element	O
tips	O
.	O
</s>
<s>
Not	O
only	O
will	O
that	O
even	O
out	O
the	O
gain	O
,	O
it	O
will	O
reduce	O
upward	O
radiation	O
,	O
which	O
for	O
VHF	B-Algorithm
is	O
typically	O
wasted	O
–	O
radiated	O
into	O
space	O
.	O
</s>
<s>
For	O
brevity	O
,	O
introductory	O
articles	O
on	O
small	O
loop	B-Architecture
antennas	I-Architecture
sometimes	O
confine	O
discussion	O
to	O
loops	O
smaller	O
in	O
circumference	O
than	O
,	O
since	O
for	O
loops	O
with	O
circumferences	O
larger	O
than	O
the	O
simplifying	O
assumption	O
of	O
uniform	O
current	O
around	O
the	O
entire	O
loop	O
becomes	O
untenably	O
inaccurate	O
(	O
small	O
transmitting	O
loops	O
are	O
discussed	O
below	O
)	O
.	O
</s>
<s>
Some	O
writers	O
mistakenly	O
consider	O
the	O
gap	O
in	O
the	O
halo	O
antenna	O
's	O
loop	O
to	O
distinguish	O
it	O
from	O
a	O
small	O
loop	B-Architecture
antenna	I-Architecture
–	O
since	O
there	O
is	O
no	O
DC	O
connection	O
between	O
the	O
two	O
ends	O
.	O
</s>
<s>
Contrary	O
to	O
the	O
pattern	O
of	O
large	O
loop	B-Architecture
antennas	I-Architecture
,	O
the	O
reception	O
and	O
radiation	O
strength	O
of	O
small	O
loops	O
peaks	O
inside	O
the	O
plane	O
of	O
the	O
loop	O
,	O
rather	O
than	O
broadside	O
(	O
perpendicular	O
)	O
to	O
it	O
.	O
</s>
<s>
As	O
with	O
all	O
antennas	O
that	O
are	O
physically	O
much	O
smaller	O
than	O
the	O
operating	O
wavelength	O
,	O
small	O
loop	B-Architecture
antennas	I-Architecture
have	O
small	O
radiation	O
resistance	O
which	O
is	O
dwarfed	O
by	O
ohmic	O
losses	O
,	O
resulting	O
in	O
a	O
poor	O
antenna	O
efficiency	O
.	O
</s>
<s>
The	O
small	O
loop	B-Architecture
antenna	I-Architecture
is	O
also	O
known	O
as	O
a	O
magnetic	O
loop	O
since	O
the	O
response	O
of	O
an	O
electrically	O
small	O
receiving	O
loop	O
is	O
proportional	O
to	O
the	O
rate	O
of	O
change	O
of	O
magnetic	O
flux	O
through	O
the	O
loop	O
.	O
</s>
<s>
If	O
the	O
perimeter	O
of	O
a	O
loop	B-Architecture
antenna	I-Architecture
is	O
much	O
smaller	O
than	O
the	O
intended	O
operating	O
wavelengths	O
–	O
say	O
to	O
of	O
a	O
wavelength	O
–	O
then	O
the	O
antenna	O
is	O
called	O
a	O
small	O
loop	B-Architecture
antenna	I-Architecture
.	O
</s>
<s>
To	O
increase	O
the	O
magnetic	O
field	O
in	O
the	O
loop	O
and	O
thus	O
its	O
efficiency	O
,	O
while	O
greatly	O
reducing	O
size	O
,	O
the	O
coil	O
of	O
wire	O
is	O
often	O
wound	O
around	O
a	O
ferrite	O
rod	O
magnetic	O
core	O
;	O
this	O
is	O
called	O
a	O
ferrite	O
loop	B-Architecture
antenna	I-Architecture
.	O
</s>
<s>
Such	O
ferrite	O
loop	B-Architecture
antennas	I-Architecture
are	O
used	O
in	O
almost	O
all	O
AM	O
broadcast	O
receivers	O
with	O
the	O
notable	O
exception	O
of	O
car	O
radios	O
,	O
since	O
the	O
antenna	O
for	O
the	O
AM	B-Algorithm
band	I-Algorithm
needs	O
to	O
be	O
outside	O
the	O
obstructing	O
metal	O
car	O
chassis	O
.	O
</s>
<s>
Small	O
loop	B-Architecture
antennas	I-Architecture
are	O
also	O
popular	O
for	O
radio	B-Device
direction	I-Device
finding	I-Device
,	O
in	O
part	O
due	O
to	O
their	O
exceedingly	O
sharp	O
,	O
clear	O
"	O
null	O
"	O
along	O
the	O
loop	O
axis	O
:	O
When	O
the	O
loop	O
axis	O
is	O
aimed	O
directly	O
at	O
the	O
transmitter	O
,	O
the	O
target	O
signal	O
abruptly	O
vanishes	O
.	O
</s>
<s>
Consequently	O
,	O
most	O
of	O
the	O
power	O
delivered	O
to	O
a	O
small	O
loop	B-Architecture
antenna	I-Architecture
will	O
be	O
converted	O
to	O
heat	O
by	O
the	O
loss	O
resistance	O
,	O
rather	O
than	O
doing	O
useful	O
work	O
.	O
</s>
<s>
At	O
these	O
lower	O
frequencies	O
,	O
atmospheric	O
noise	O
(	O
static	O
)	O
and	O
man-made	O
noise	O
(	O
radio	O
frequency	O
interference	O
)	O
even	O
a	O
weak	O
signal	O
from	O
an	O
inefficient	O
antenna	O
is	O
far	O
stronger	O
than	O
the	O
internal	O
thermal	O
or	O
Johnson	O
noise	O
generated	O
in	O
the	O
radio	O
receiver	O
's	O
own	O
circuitry	O
,	O
so	O
the	O
weak	O
signal	O
from	O
a	O
loop	B-Architecture
antenna	I-Architecture
can	O
be	O
amplified	O
without	O
degrading	O
the	O
signal-to-noise	O
ratio	O
.	O
</s>
<s>
If	O
a	O
small	O
loop	B-Architecture
antenna	I-Architecture
's	O
loss	O
is	O
50dB	O
(	O
as	O
if	O
the	O
antenna	O
included	O
a	O
50dB	O
attenuator	O
)	O
the	O
electrical	O
inefficiency	O
of	O
that	O
antenna	O
will	O
have	O
little	O
influence	O
on	O
the	O
receiving	O
system	O
's	O
signal-to-noise	O
ratio	O
.	O
</s>
<s>
Instead	O
,	O
the	O
radiation	B-Device
pattern	I-Device
peaks	O
in	O
directions	O
lying	O
in	O
the	O
plane	O
of	O
the	O
loop	O
,	O
because	O
signals	O
received	O
from	O
sources	O
in	O
that	O
plane	O
do	O
not	O
quite	O
cancel	O
owing	O
to	O
the	O
phase	O
difference	O
between	O
the	O
arrival	O
of	O
the	O
wave	O
at	O
the	O
near	O
side	O
and	O
far	O
side	O
of	O
the	O
loop	O
.	O
</s>
<s>
Since	O
the	O
magnetic	O
(	O
and	O
electric	O
)	O
fields	O
of	O
an	O
electromagnetic	O
wave	O
in	O
free	O
space	O
are	O
transverse	O
(	O
no	O
component	O
in	O
the	O
direction	O
of	O
propagation	O
)	O
,	O
it	O
can	O
be	O
seen	O
that	O
this	O
magnetic	O
field	O
and	O
that	O
of	O
a	O
small	O
loop	B-Architecture
antenna	I-Architecture
will	O
be	O
at	O
right	O
angles	O
,	O
and	O
thus	O
not	O
coupled	O
.	O
</s>
<s>
Since	O
a	O
small	O
loop	B-Architecture
antenna	I-Architecture
is	O
essentially	O
a	O
coil	O
,	O
its	O
electrical	O
impedance	O
is	O
inductive	O
,	O
with	O
an	O
inductive	O
reactance	O
much	O
greater	O
than	O
its	O
radiation	O
resistance	O
.	O
</s>
<s>
Since	O
a	O
good	O
loop	B-Architecture
antenna	I-Architecture
will	O
have	O
a	O
high	O
factor	O
(	O
narrow	O
bandwidth	O
)	O
,	O
the	O
capacitor	O
must	O
be	O
variable	O
and	O
is	O
adjusted	O
to	O
match	O
the	O
receiver	O
's	O
tuning	O
.	O
</s>
<s>
As	O
long	O
as	O
the	O
loop	O
perimeter	O
is	O
kept	O
below	O
about	O
wave	O
,	O
the	O
directional	O
response	O
of	O
small	O
loop	B-Architecture
antennas	I-Architecture
includes	O
a	O
sharp	O
null	O
in	O
the	O
direction	O
normal	O
to	O
the	O
plane	O
of	O
the	O
loop	O
,	O
so	O
small	O
loops	O
are	O
favored	O
as	O
compact	O
radio	B-Device
direction	I-Device
finding	I-Device
antennas	O
for	O
long	O
wavelengths	O
.	O
</s>
<s>
The	O
procedure	O
is	O
to	O
rotate	O
the	O
loop	B-Architecture
antenna	I-Architecture
to	O
find	O
the	O
direction	O
where	O
the	O
signal	O
vanishes	O
–	O
the	O
“	O
null	O
”	O
direction	O
.	O
</s>
<s>
One	O
method	O
is	O
to	O
rely	O
on	O
a	O
second	O
loop	B-Architecture
antenna	I-Architecture
located	O
at	O
a	O
second	O
location	O
,	O
or	O
to	O
move	O
the	O
receiver	O
to	O
that	O
other	O
location	O
,	O
thus	O
relying	O
on	O
triangulation	B-Algorithm
.	O
</s>
<s>
Instead	O
of	O
triangulation	B-Algorithm
,	O
a	O
second	O
dipole	O
or	O
vertical	O
antenna	O
can	O
be	O
electrically	O
combined	O
with	O
a	O
loop	O
or	O
a	O
loopstick	O
antenna	O
.	O
</s>
<s>
Called	O
a	O
sense	O
antenna	O
,	O
connecting	O
and	O
matching	O
the	O
second	O
antenna	O
changes	O
the	O
combined	O
radiation	B-Device
pattern	I-Device
to	O
a	O
cardioid	O
,	O
with	O
a	O
null	O
in	O
only	O
one	O
(	O
less	O
precise	O
)	O
direction	O
.	O
</s>
<s>
The	O
general	O
direction	O
of	O
the	O
transmitter	O
can	O
be	O
determined	O
using	O
the	O
sense	O
antenna	O
,	O
and	O
then	O
disconnecting	O
the	O
sense	O
antenna	O
returns	O
the	O
sharp	O
nulls	O
in	O
the	O
loop	B-Architecture
antenna	I-Architecture
pattern	O
,	O
allowing	O
a	O
precise	O
bearing	O
to	O
be	O
determined	O
.	O
</s>
<s>
Small	O
loop	B-Architecture
antennas	I-Architecture
are	O
lossy	O
and	O
inefficient	O
for	O
transmitting	O
,	O
but	O
they	O
can	O
be	O
practical	O
receiving	O
antennas	O
for	O
frequencies	O
below	O
10MHz	O
.	O
</s>
<s>
Especially	O
in	O
the	O
mediumwave	B-Algorithm
(	O
520	O
–	O
1710kHz	O
)	O
band	O
and	O
below	O
,	O
where	O
wavelength-sized	O
antennas	O
are	O
infeasibly	O
large	O
,	O
and	O
the	O
antenna	O
inefficiency	O
is	O
irrelevant	O
,	O
due	O
to	O
large	O
amounts	O
of	O
atmospheric	O
noise	O
.	O
</s>
<s>
AM	O
broadcast	O
receivers	O
(	O
and	O
other	O
low	B-Algorithm
frequency	I-Algorithm
radios	O
for	O
the	O
consumer	O
market	O
)	O
typically	O
use	O
small	O
loop	B-Architecture
antennas	I-Architecture
,	O
even	O
when	O
a	O
telescoping	O
antenna	O
may	O
be	O
attached	O
for	O
FM	O
reception	O
.	O
</s>
<s>
A	O
multiband	O
receiver	O
may	O
contain	O
tap	O
points	O
along	O
the	O
loop	O
winding	O
in	O
order	O
to	O
tune	O
the	O
loop	B-Architecture
antenna	I-Architecture
at	O
widely	O
different	O
frequencies	O
.	O
</s>
<s>
Ferrite	O
loop	B-Architecture
antennas	I-Architecture
are	O
made	O
by	O
winding	O
fine	O
wire	O
around	O
a	O
ferrite	O
rod	O
.	O
</s>
<s>
Often	O
,	O
at	O
mediumwave	B-Algorithm
and	O
lower	O
shortwave	O
frequencies	O
,	O
Litz	O
wire	O
is	O
used	O
for	O
the	O
winding	O
to	O
reduce	O
skin	O
effect	O
losses	O
.	O
</s>
<s>
A	O
typical	O
AM	O
broadcast	O
radio	O
loop	B-Architecture
antenna	I-Architecture
wound	O
on	O
ferrite	O
may	O
have	O
a	O
cross	O
sectional	O
area	O
of	O
only	O
at	O
a	O
frequency	O
at	O
which	O
an	O
ideal	O
(	O
lossless	O
)	O
antenna	O
would	O
have	O
an	O
effective	O
area	O
some	O
hundred	O
million	O
times	O
larger	O
.	O
</s>
<s>
A	O
small	O
transmitting	O
loop	B-Architecture
antenna	I-Architecture
with	O
a	O
perimeter	O
of	O
10%	O
or	O
less	O
of	O
the	O
wavelength	O
will	O
have	O
a	O
relatively	O
constant	O
current	O
distribution	O
along	O
the	O
conductor	O
,	O
and	O
the	O
main	O
lobe	O
will	O
be	O
in	O
the	O
plane	O
of	O
the	O
loop	O
.	O
</s>
<s>
Loops	O
in	O
this	O
size	O
range	O
may	O
have	O
neither	O
the	O
uniform	O
current	O
of	O
the	O
small	O
loop	O
,	O
nor	O
the	O
double	O
peaked	O
current	O
of	O
the	O
full	O
sized	O
loop	O
and	O
thus	O
cannot	O
be	O
analyzed	O
using	O
the	O
concepts	O
developed	O
for	O
the	O
small	O
receiving	O
loops	O
nor	O
full-wave	O
loop	B-Architecture
antennas	I-Architecture
.	O
</s>
<s>
Performance	O
is	O
best	O
determined	O
with	O
NEC	B-Protocol
analysis	I-Protocol
.	O
</s>
<s>
The	O
combination	O
is	O
in	O
effect	O
a	O
transformer	B-Algorithm
,	O
with	O
power	O
in	O
the	O
near-field	O
inductively	O
coupled	O
from	O
the	O
feed	O
loop	O
to	O
the	O
main	O
loop	O
,	O
which	O
itself	O
is	O
connected	O
to	O
the	O
resonating	O
capacitor	O
and	O
is	O
responsible	O
for	O
radiating	O
most	O
of	O
the	O
power	O
.	O
</s>
<s>
Some	O
antennas	O
look	O
very	O
much	O
like	O
loops	O
,	O
but	O
are	O
designed	O
to	O
couple	O
with	O
the	O
inductive	O
near-field	O
over	O
distances	O
of	O
or	O
rather	O
than	O
to	O
transmit	O
or	O
receive	O
long-distance	O
electromagnetic	O
waves	O
in	O
the	O
radiative	O
far-field	B-Algorithm
.	O
</s>
<s>
The	O
use	O
of	O
coupling	O
coils	O
for	O
inductive	O
systems	O
,	O
including	O
their	O
use	O
at	O
LF	B-Algorithm
and	O
HF	B-Algorithm
,	O
is	O
outside	O
the	O
scope	O
of	O
this	O
article	O
.	O
</s>
<s>
Inductive	O
heating	O
systems	O
,	O
induction	O
cooking	O
stovetops	O
,	O
and	O
RFID	B-Application
tags	I-Application
and	O
readers	O
all	O
interact	O
by	O
near	B-Algorithm
field	I-Algorithm
magnetic	O
induction	O
rather	O
than	O
far	B-Algorithm
field	I-Algorithm
transmitted	O
waves	O
.	O
</s>
<s>
However	O
,	O
they	O
are	O
more	O
usefully	O
thought	O
of	O
as	O
analogs	O
to	O
the	O
windings	O
in	O
loosely	O
coupled	O
transformers	B-Algorithm
.	O
</s>
<s>
Although	O
the	O
magnetic	O
coils	O
in	O
these	O
inductive	O
systems	O
sometimes	O
seem	O
indistinguishable	O
from	O
the	O
small	O
loop	B-Architecture
antennas	I-Architecture
discussed	O
above	O
,	O
such	O
devices	O
can	O
only	O
operate	O
over	O
short	O
distances	O
,	O
and	O
are	O
specifically	O
designed	O
to	O
avoid	O
transmitting	O
or	O
receiving	O
radio	O
waves	O
.	O
</s>
<s>
Because	O
Inductive	O
heating	O
systems	O
and	O
RFID	B-Application
readers	O
only	O
use	O
near	B-Algorithm
field	I-Algorithm
alternating	O
magnetic	O
fields	O
,	O
their	O
performance	O
criteria	O
are	O
dissimilar	O
to	O
far	B-Algorithm
field	I-Algorithm
radio	O
antennas	O
discussed	O
in	O
this	O
article	O
.	O
</s>
