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Propagation
log
period
10.
MoDERN
ANTENNAs
lg:LllEcoNFrGuRABle
aHreniie
t
A
reconJigurable
antenna
is
en
antenna
capabre
of
modifying
,tyrr*rroffi
/i"equency
and
radiation
properties
in
a
controlled
and
reversible
manner..
'
Reconfigt*able
antenncr.s,
with
the
ability
to
radiate
more
than
dffirent
frequencies
and
porarizations,
are
necessary
t
e
I
e
c
ommuni
c
ation
sy
s t e
m.s.
'
These
anlinnas
can
address
complex
system
requirements
by
modifying
their
eeometry
and
electrical
behavior.
thereby
adapting
to
changes
in
environmental
conditions
or
system
requirements
(i.e.,
enhanced
band-width.
changes
in
operating
frequency,
polarization,
and
radiation
pattem).
Neutralizing
line
Fig.
I
0J.
ReconJiguroble
antenno
one pqttern
al
in
modern
I
t_
I
I
RF
switches
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r0,
Antenna
and
Vl/ave
Propagation
In
order
to
provide
a
dynamical
response,
reconfigurable
antennas
integrate
an
inner
mechanism
(such
as
RF
switches.
varactors,
mechanical
actuators
or
tunable
materials)
that
enable
the
intentional
redistribution
of
the
RF
currents
over
the
antenna
surface
and
produce reversible
modifications
over its properties.
Reconfigurable
antennas
differ
from
smart
antennas
because
the reconfiguration
mechanism
lies
inside
the
antenna
rather
than
in
an
external
beamforming
network.
The
reconfiguration
capability
of
reconfigurable
antennas
is
used
to
maximize
the
antenna
performance
in
a
changing
scenario
or to
satisfy
the
changing
operating
requirements.
10.1.1.
TYPES
OF
ANTENNA
RECONFTGURATION
o
Reconfigurable
antennas
can
be
classified according
to
the
antenna
parameter
that
is
dynamically
adjusted,
typically
the
frequency
of
operation,
radiation
pattem
or
polarizatibn.
(i)
Freq
uency
Reconfig
uration
o
Frequency
reconfigurable
antennas
can
adjust
dynamically
their
frequency
oi
operation.
They
ate particularly
useful
in
situations
where
several
communications
systems
converge
because
the
multiple
antennas
required
can
be
replaced
by a
single
reconfigurable
antenna.
.
Frequency
reconfiguration
is
generally
achieved
by modifying
physically
electrically
the
antenna
dimensions
using
RF-switches,
impedance
loading
tunable
materials.
(ii)
Radiation
Pattern
Reconfig
uration
Radiation
pattern
reconfigurability
is
based
on the
intentional
modification
of
the
spherical
distribution
of
radiation
pattern.
Beam
steering
is
the most
extendec
application
and
consists
in
steering
the
direction
of
maximum
radiation
tc
maximize
the
antenna
gain
in
a
link with
mobile
devices.
Pattern
reconfigurable
antennas
are
usually
designed
using
movable/rotatabl;
structures
or
including
the switchable
and
reactively-loaded
parasitic
elements.
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Antenna
ancl
l|/crve
propagation
1g{.
AcilvEANTENNA
.
An
active
antenna
is
an
antenna
that
contains
active
electronic
components.
dipole
antenna.
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in^lo
^h+^.^-^
A
simple
definition
of
active
sntennas
are
ony
antennas
with
integrated
signar
amplifier'r
built
right
into
rhe
unit.
Passive
antennas
are
antennas
that
have
no
mplificution
stdges'
An
active
antenna
is
a
passive
ontenna
that
simpry
includes
an
onboard
amplifier.
Most
active
antennas
have
onry
one
erectricar
connection.
that
is,
RF
signar
and
he
power
for
the
amprifier
are
suppried
on
the
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o
f
I
I
t_
PdSsMEANTENNA
mrm
flril'i
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gnim
rilm
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&rc
U[t
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6t
ffIr;'
mmrir.ii
ACTIVEANTENNA
)lt
rutr1lilill
$*qwut*r/rmryd.t'ir.
Fig.l0.2.
Simple
active
antenna
An
active
design
allou's
the
construction
of
antennas
of
limited
size
and
/
or
wide
frequency
range'
and
are
primarily
used
in
situations
where
a
simpler
and
more
receptive
large
antenna
is
either
impractical
(inside
a
portable
radio)
or
impossible
(suburban
residentiar
area
trrat
disarows
use
of
rarge
outdoor
low_frequency
antennas).
Most
active
antenxas
use
antenna
parts
of
minimar
conducting
area,
e.g.,
a
smail
whip'
connected
to
the
active
component
(usually
a
FET).
The
signal
attenuation
caused
by
the
anten
circuit.
ura-srze-to-wa'erength
mismatch
is
compensated
by
an
active
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as
a
signal
hove
no
include,s
and
I
or
wide
more
a
small
active
\ odern
Antenn(ts
.
The
active
circuit
consists
of rmpedancc
translating
stage
and
an optional
amplification
stage.
This
arrangement
is
especially
useful
for
constructins
compact
low
frequency
antennas
due
to
budgetary,
spatial,
or
practical
requirements
(e.g.,
installation
in vehicles),
must be downsized.
Low frequencl,
signal
wavelengths
range
from
one
to ten kilometers.
.
Power
for
the
active
components
may
be
supplied by
batteries,
a filtered
power
supply,
or
through
the
signal
feeder
itself
(phantom
power).
Antennas
containing
active
impedance
translating
and
optionally
amplifuing stages
are usually
used
only
for receiving,
since
operation
of
such
stages
is
unidirectional.
.
Without
amplifiers,
the
loss
of
the coaxial
cable will have
a severe
impact
on
performance.
With
an
active
uirt.nnu,
it
is
possible
to
use
long
cables
and
still
maintain
pgrformance.
,
0.2.1.
INTEGRATED
ACTIVE
ANTENNAS
.
Integrated
active
antennas
are
a
single
column
antenna consists
of
multiple
elements.
If
all
the elements
are
fed
in
phase.
a narrow horizontal
beam
is
formed.
.
By
adjusting
the
phase
of
the
signal
to
each
element
causes the
beam
to tilt..This
is
known as
"electrical
downtilt"
and
it
is
in
common usage
throughout
the
digital
mobile
radio
industry.
:
JB
I
Fig. I 0.3.
Integrilecl
sctive ontenno
.
If
the
antenna
column
has
a
single
transceiver,
all carriers
will
be
approxirnate;ty
ifre same
amount
in
both
transmit
and receive directions.
rilted by
D
E
F
u
H
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To
achieve
this,
antenna
element
transceivers.
The
requirements
size,
weight,
cost,
receiver
electronic
it
a
is'required
to
have
a
.separate
transceiver
controller
would
be necessary
to
control
Antenna
and
Ll/ave
propagat
fbr
(ideally)
/synchronise
for
success
are
that
each
transceiver
is
and
performance.
Each
transceiver
has
devices.
acceptable
in
terms
the
transmitter
and
10.2.2.
ADVANTAGES
The
advantages
of
active
antennas
are
as follows:
(i)
Increased
coverage
and
capacity.
(ii)
Independent
optimisation
per
carrier
and
per
technology.
(iii)
Reduced
item
count
at
ground
level
and
on
the
mast
(iv)
Pcoducing
duplexing
filters
that
deliver
high performance.
ll-0f
:|p1F}EgISl_c.BFggNAroRANrENNA(DiA,
o
The
DRA
is
an
antenna
that
makes
use
of
a
radiating
mode
of
a
dielec
resonator
(DR)
and resonance
frequency
determined
by
the
its
dimensions
dielectric
constant
s
Fig.
1
0.4.
Dielectric
resonator
antenna
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Modern
Antennas
each
the
of
and
o
A
Dielectric
Resonator
Antenna
(DRA)
is
a radio
anteniia
mostl\,
used
at
the
microwave
frequencies
and
higher,
that
consists
of
a
block
of
ceramic
material
of
various
shapes,
the
dielectric
resonator,
mounted
on
a metal
surface
and
a
sround
plane.
'
Radio
waves
are
introduced
into
the
inside
of
the
resonator
material
from
the
transmitter
circuit
and
bounce
back
and
forth
between
the
resonator
rvalls.
forming
standing
waves.
The
walls
of
the
resonator
are
partially
transparent
to
radio
waves,
allowing
the
radio
power
to
radiate
into
space.
'
An
advantage
of
dielectric
resonhtor
antennas
is
they
lack
metal
parts,
which
become
lossy
at
high
frequencies,
dissipating
energy.
so
these
antennas
can
have
lower
losses
and
be
more
efficient
than
metal
antennas
at
high
microwave
and
millimeter
wave
frequencies.
'
The
dielectrii
waveguide
antennas
aro
used
in
some
compact
portable
wireless
devices,
and
military
millimeter_wave
radar
equipment.
'
The
dimension
of
a DRA
is
of
the
order
of
+
where
]..
is
the
free-space
It,
wavelength
and
e
,
is
the
dielectric
constant
of
the
resonator
material.
Thus,
by
choosing
a
high
value
of
e,
(=
r
0
-
r
00),
the
size
of
the
DRA
can
be'iignificantly
reduced.
10.3.1.
FEATURES
Dielectric
resonator
antennas
offer
the
following
attractive
features:
(i)
There
is
no
inherent
conductor
loss
in
dielectric
resonators.
This
leads
to
high
radiation
efficiencS'
of the
antenna.
This
feature
is
especially
attractive
for
millimeter
(mm)-wave
antennas,
where
the
loss
in
metal
fabricated
antennas
can
be
high.
(ii)
DRAs
offer
sirnple
coupling
schemes
to
nearly
all transmission
lines
used
at
microwave
and
mm-wave
frequencies.
This
makes
them
suitable
fbr
integration
into
different
planar
techno
I
o
gi
es.
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Antenna
and
Wave
Propagation
(iiD
The
operating
bandwidth
of
a DRA
can
be varied
over
a wide
range
by
suitably
choosing
resonator
parameters.
(iv)
Use
of
multiple
modes
radiating
identically
has
also
been
succegsfully
addressed.
(v)
Each
mode
of a DRA
has
a
unique
intemal
and
associated
exterpal
field
distribution.
Therefore,
different
radiation
characteristics
can
be obtained
by
exciting
different
modes
of
a DRA.
10.4.
.TIIUO:MARKS
QUESTIONS
AND
ANSWERS
't:
n,
,&
lt.
U
l:Tl
[ ilf,r:c
;
:"-
a
-'.
1.
What
is
reconJigurable
antenna?
A
reconfigurable
antenna
is
an
antenna capable
of
modifying
dynamically
its
frequency
and
radiation
properties
in
a controlled
and
reversible
manner.
Reconfigurabli'antennas,
with
the
ability
to
radiate
more
than
one
pattern
at
different
frequencies
and
polarizations,
are
necessary
in modem
telecommunication
systems.
List
the
types
of
reconfiguration.
Reconfigurable
antennas
can
be
classified
according
to the
antenna parameter
that
is
dynamically
adjusted,
typically
the
frequency
of
operation,
radiation
pattern
or
polarization.
(i)
Frequency
Reconfiguration
(ii)
RadiationPatternReconfiguration
(iii)
Polarization
Reconfiguration
(iv)
CompoundReconfiguration
Deft
ne
freq
uency
reconfigurab
I
e
ante
nnas.
Frequency
reconfigurable
antennas
can adjust
dynamically
their
frequency
of
operation.
They
are
particularly
useful
in
situations
where
several
communications
systems
converge
because
the
multiple
antennas
required
can
be replaced
by
a single
reconfigurable
antenna.
2.
3.