<s>
An	O
antenna	B-Algorithm
array	I-Algorithm
(	O
or	O
array	B-Algorithm
antenna	I-Algorithm
)	O
is	O
a	O
set	O
of	O
multiple	O
connected	O
antennas	O
which	O
work	O
together	O
as	O
a	O
single	O
antenna	O
,	O
to	O
transmit	O
or	O
receive	O
radio	O
waves	O
.	O
</s>
<s>
More	O
sophisticated	O
array	B-Algorithm
antennas	I-Algorithm
may	O
have	O
multiple	O
transmitter	O
or	O
receiver	O
modules	O
,	O
each	O
connected	O
to	O
a	O
separate	O
antenna	O
element	O
or	O
group	O
of	O
elements	O
.	O
</s>
<s>
An	O
antenna	B-Algorithm
array	I-Algorithm
can	O
achieve	O
higher	O
gain	O
(	O
directivity	O
)	O
,	O
that	O
is	O
a	O
narrower	O
beam	O
of	O
radio	O
waves	O
,	O
than	O
could	O
be	O
achieved	O
by	O
a	O
single	O
element	O
.	O
</s>
<s>
Some	O
antenna	B-Algorithm
arrays	I-Algorithm
(	O
such	O
as	O
military	O
phased	O
array	O
radars	B-Application
)	O
are	O
composed	O
of	O
thousands	O
of	O
individual	O
antennas	O
.	O
</s>
<s>
Arrays	O
can	O
be	O
used	O
to	O
achieve	O
higher	O
gain	O
,	O
to	O
give	O
path	O
diversity	B-Protocol
(	O
also	O
called	O
MIMO	O
)	O
which	O
increases	O
communication	O
reliability	O
,	O
to	O
cancel	O
interference	O
from	O
specific	O
directions	O
,	O
to	O
steer	O
the	O
radio	O
beam	O
electronically	O
to	O
point	O
in	O
different	O
directions	O
,	O
and	O
for	O
radio	B-Device
direction	I-Device
finding	I-Device
(	O
RDF	O
)	O
.	O
</s>
<s>
The	O
term	O
antenna	B-Algorithm
array	I-Algorithm
most	O
commonly	O
means	O
a	O
driven	O
array	O
consisting	O
of	O
multiple	O
identical	O
driven	O
elements	O
all	O
connected	O
to	O
the	O
receiver	O
or	O
transmitter	O
.	O
</s>
<s>
A	O
phased	O
array	O
usually	O
means	O
an	O
electronically	O
scanned	O
array	O
;	O
a	O
driven	O
array	B-Algorithm
antenna	I-Algorithm
in	O
which	O
each	O
individual	O
element	O
is	O
connected	O
to	O
the	O
transmitter	O
or	O
receiver	O
through	O
a	O
phase	B-Algorithm
shifter	I-Algorithm
controlled	O
by	O
a	O
computer	O
.	O
</s>
<s>
However	O
the	O
term	O
"	O
phased	O
array	O
"	O
is	O
sometimes	O
used	O
to	O
mean	O
an	O
ordinary	O
array	B-Algorithm
antenna	I-Algorithm
.	O
</s>
<s>
A	O
second	O
technique	O
is	O
to	O
use	O
multiple	O
antennas	O
which	O
are	O
fed	O
from	O
the	O
same	O
transmitter	O
or	O
receiver	O
;	O
this	O
is	O
called	O
an	O
array	B-Algorithm
antenna	I-Algorithm
,	O
or	O
antenna	B-Algorithm
array	I-Algorithm
.	O
</s>
<s>
The	O
radiation	B-Device
pattern	I-Device
of	O
such	O
an	O
antenna	O
consists	O
of	O
a	O
strong	O
beam	O
in	O
one	O
direction	O
,	O
the	O
main	O
lobe	O
,	O
plus	O
a	O
series	O
of	O
weaker	O
beams	O
at	O
different	O
angles	O
called	O
sidelobes	O
,	O
usually	O
representing	O
residual	O
radiation	O
in	O
unwanted	O
directions	O
.	O
</s>
<s>
The	O
largest	O
array	B-Algorithm
antennas	I-Algorithm
are	O
radio	O
interferometers	O
used	O
in	O
the	O
field	O
of	O
radio	O
astronomy	O
,	O
in	O
which	O
multiple	O
radio	O
telescopes	O
consisting	O
of	O
large	O
parabolic	O
antennas	O
are	O
linked	O
together	O
into	O
an	O
antenna	B-Algorithm
array	I-Algorithm
,	O
to	O
achieve	O
higher	O
resolution	O
.	O
</s>
<s>
In	O
the	O
technique	O
called	O
Very	O
Long	O
Baseline	O
Interferometry	O
(	O
VLBI	O
)	O
dishes	O
on	O
separate	O
continents	O
have	O
been	O
linked	O
,	O
creating	O
"	O
array	B-Algorithm
antennas	I-Algorithm
"	O
thousands	O
of	O
miles	O
in	O
size	O
.	O
</s>
<s>
Most	O
array	B-Algorithm
antennas	I-Algorithm
can	O
be	O
divided	O
into	O
two	O
classes	O
based	O
on	O
how	O
the	O
component	O
antennas	O
 '	O
axis	O
relates	O
to	O
the	O
radiation	O
direction	O
.	O
</s>
<s>
This	O
is	O
a	O
high	O
gain	O
omnidirectional	O
antenna	O
,	O
often	O
used	O
in	O
the	O
VHF	B-Algorithm
band	I-Algorithm
as	O
broadcasting	O
antennas	O
for	O
television	O
stations	O
and	O
base	O
station	O
antennas	O
for	O
land	O
mobile	O
two-way	O
radios	O
.	O
</s>
<s>
High	O
gain	O
omnidirectional	O
radiation	B-Device
pattern	I-Device
with	O
wide	O
bandwidth	O
.	O
</s>
<s>
Planar	B-Algorithm
array	I-Algorithm
-	O
a	O
flat	O
two-dimensional	O
array	O
of	O
antennas	O
.	O
</s>
<s>
Reflective	O
array	O
-	O
a	O
planar	B-Algorithm
array	I-Algorithm
of	O
antennas	O
,	O
often	O
half-wave	O
dipoles	O
fed	O
in	O
phase	O
,	O
in	O
front	O
of	O
a	O
flat	O
reflector	O
such	O
as	O
a	O
metal	O
plate	O
or	O
wire	O
screen	O
.	O
</s>
<s>
Used	O
as	O
UHF	O
television	O
antennas	O
and	O
radar	B-Application
antennas	O
.	O
</s>
<s>
Curtain	O
array	O
-	O
an	O
outdoor	O
wire	O
shortwave	O
transmitting	O
antenna	O
consisting	O
of	O
a	O
planar	B-Algorithm
array	I-Algorithm
of	O
wire	O
dipoles	O
suspended	O
in	O
front	O
of	O
a	O
vertical	O
reflector	O
made	O
of	O
a	O
"	O
curtain	O
"	O
of	O
parallel	O
wires	O
.	O
</s>
<s>
Used	O
on	O
HF	B-Algorithm
band	O
as	O
long	O
distance	O
transmitting	O
antenna	O
for	O
shortwave	O
broadcasting	O
stations	O
.	O
</s>
<s>
Microstrip	B-Algorithm
antenna	I-Algorithm
-	O
an	O
array	O
of	O
patch	O
antennas	O
fabricated	O
on	O
a	O
printed	O
circuit	O
board	O
with	O
copper	O
foil	O
on	O
the	O
reverse	O
side	O
functioning	O
as	O
a	O
reflector	O
.	O
</s>
<s>
Phased	O
array	O
or	O
electronically	O
scanned	O
array	O
-	O
A	O
planar	B-Algorithm
array	I-Algorithm
in	O
which	O
the	O
beam	O
can	O
be	O
steered	O
electronically	O
to	O
point	O
in	O
any	O
direction	O
over	O
a	O
wide	O
angle	O
in	O
front	O
of	O
the	O
array	O
,	O
without	O
physically	O
moving	O
the	O
antenna	O
.	O
</s>
<s>
The	O
current	O
from	O
the	O
transmitter	O
is	O
fed	O
to	O
each	O
component	O
antenna	O
through	O
a	O
phase	B-Algorithm
shifter	I-Algorithm
,	O
controlled	O
by	O
a	O
computer	O
.	O
</s>
<s>
Widely	O
used	O
in	O
military	O
radars	B-Application
,	O
this	O
technique	O
is	O
rapidly	O
spreading	O
to	O
civilian	O
applications	O
.	O
</s>
<s>
Passive	B-Algorithm
Electronically	I-Algorithm
Scanned	I-Algorithm
Array	I-Algorithm
(	O
PESA	O
)	O
-	O
A	O
phased	O
array	O
as	O
described	O
above	O
,	O
in	O
which	O
the	O
antenna	O
elements	O
are	O
fed	O
from	O
a	O
single	O
transmitter	O
or	O
receiver	O
through	O
phase	B-Algorithm
shifters	I-Algorithm
.	O
</s>
<s>
Active	B-Architecture
Electronically	I-Architecture
Scanned	I-Architecture
Array	I-Architecture
(	O
AESA	O
)	O
-	O
A	O
phased	O
array	O
in	O
which	O
each	O
antenna	O
element	O
has	O
its	O
own	O
transmitter	O
and/or	O
receiver	O
module	O
,	O
controlled	O
by	O
a	O
central	O
computer	O
.	O
</s>
<s>
This	O
second	O
generation	O
phased	O
array	O
technology	O
can	O
radiate	O
multiple	O
beams	O
at	O
multiple	O
frequencies	O
simultaneously	O
,	O
and	O
is	O
mostly	O
used	O
in	O
sophisticated	O
military	O
radars	B-Application
.	O
</s>
<s>
The	O
individual	O
elements	O
are	O
driven	O
by	O
phase	B-Algorithm
shifters	I-Algorithm
which	O
compensate	O
for	O
the	O
varying	O
path	O
lengths	O
,	O
allowing	O
the	O
antenna	O
to	O
radiate	O
a	O
plane	O
wave	O
beam	O
.	O
</s>
<s>
Smart	O
antenna	O
,	O
reconfigurable	O
antenna	O
or	O
adaptive	O
array	O
-	O
a	O
receiving	O
array	O
that	O
estimates	O
the	O
direction	O
of	O
arrival	O
of	O
the	O
radio	O
waves	O
and	O
electronically	O
optimizes	O
the	O
radiation	B-Device
pattern	I-Device
adaptively	O
to	O
receive	O
it	O
,	O
synthesizing	O
a	O
main	O
lobe	O
in	O
that	O
direction	O
.	O
</s>
<s>
Like	O
a	O
phased	O
array	O
it	O
consists	O
of	O
multiple	O
identical	O
elements	O
with	O
phase	B-Algorithm
shifters	I-Algorithm
in	O
the	O
feed	O
lines	O
,	O
controlled	O
by	O
a	O
computer	O
.	O
</s>
<s>
The	O
parasitic	O
elements	O
function	O
as	O
resonators	O
,	O
absorbing	O
radio	O
waves	O
from	O
the	O
driven	O
element	O
and	O
reradiating	O
them	O
with	O
a	O
different	O
phase	O
,	O
to	O
modify	O
the	O
radiation	B-Device
pattern	I-Device
of	O
the	O
antenna	O
,	O
increasing	O
the	O
power	O
radiated	O
in	O
the	O
desired	O
direction	O
.	O
</s>
<s>
They	O
are	O
widely	O
used	O
on	O
the	O
HF	B-Algorithm
,	O
VHF	B-Algorithm
,	O
and	O
UHF	O
bands	O
as	O
television	O
antennas	O
,	O
shortwave	O
communication	O
antennas	O
,	O
and	O
in	O
radar	B-Application
arrays	O
.	O
</s>
<s>
Quad	O
antenna	O
-	O
This	O
consists	O
of	O
multiple	O
loop	B-Architecture
antennas	I-Architecture
in	O
a	O
line	O
,	O
with	O
one	O
driven	O
loop	O
and	O
the	O
others	O
parasitic	O
.	O
</s>
<s>
In	O
an	O
antenna	B-Algorithm
array	I-Algorithm
providing	O
a	O
fixed	O
radiation	B-Device
pattern	I-Device
,	O
we	O
may	O
consider	O
that	O
the	O
feed	O
network	O
is	O
a	O
part	O
of	O
the	O
antenna	B-Algorithm
array	I-Algorithm
.	O
</s>
<s>
Thus	O
,	O
the	O
antenna	B-Algorithm
array	I-Algorithm
has	O
a	O
single	O
port	O
.	O
</s>
<s>
If	O
,	O
in	O
addition	O
,	O
the	O
amplitude	O
of	O
the	O
excitation	O
received	O
by	O
each	O
element	O
(	O
during	O
emission	O
)	O
is	O
also	O
well	O
chosen	O
,	O
it	O
is	O
possible	O
to	O
synthesize	O
a	O
single-port	O
array	O
having	O
a	O
radiation	B-Device
pattern	I-Device
that	O
closely	O
approximates	O
a	O
specified	O
pattern	O
.	O
</s>
<s>
Changes	O
in	O
current	O
distribution	O
are	O
known	O
to	O
affect	O
the	O
radiation	B-Device
pattern	I-Device
when	O
applied	O
to	O
arrays	O
.	O
</s>
<s>
The	O
design	O
of	O
an	O
electronically	O
steerable	O
antenna	B-Algorithm
array	I-Algorithm
is	O
different	O
,	O
because	O
the	O
phasing	O
of	O
each	O
element	O
can	O
be	O
varied	O
,	O
and	O
possibly	O
also	O
the	O
relative	O
amplitude	O
for	O
each	O
element	O
.	O
</s>
<s>
Here	O
,	O
the	O
antenna	B-Algorithm
array	I-Algorithm
has	O
multiple	O
ports	O
,	O
so	O
that	O
the	O
subject	O
matters	O
of	O
matching	O
and	O
efficiency	O
are	O
more	O
involved	O
than	O
in	O
the	O
single-port	O
case	O
.	O
</s>
<s>
An	O
antenna	B-Algorithm
array	I-Algorithm
used	O
for	O
spatial	O
diversity	B-Protocol
and/or	O
spatial	O
multiplexing	O
(	O
which	O
are	O
different	O
types	O
of	O
MIMO	O
radio	O
communication	O
)	O
always	O
has	O
multiple	O
ports	O
.	O
</s>
<s>
Here	O
also	O
,	O
the	O
subject	O
matters	O
of	O
matching	O
and	O
efficiency	O
are	O
involved	O
,	O
especially	O
in	O
the	O
case	O
of	O
an	O
antenna	B-Algorithm
array	I-Algorithm
of	O
a	O
mobile	O
device	O
(	O
see	O
chapter	O
10	O
of	O
)	O
,	O
since	O
,	O
in	O
this	O
case	O
,	O
the	O
surroundings	O
of	O
the	O
antenna	B-Algorithm
array	I-Algorithm
influence	O
its	O
behavior	O
,	O
and	O
vary	O
over	O
time	O
.	O
</s>
