1 fs antennas for wireless systems
TRANSCRIPT
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Chap. 1: Antennas for Wireless Systems 1
FS: Funksysteme Terrestrial and Satellite-based Radio Systems 1-antennas-for-wireless-systems.doc
1 Antennas for Wireless Systems
In this chapter,antennas are taken into account as a
part within a radio system. For calculating thepower-
link budgetof radio systems in the next chapter, we
have to understand some fundamentals of antennas as
a key component of a wireless system.Fig. 1-1 exhibits a communication link by connecting
two Earth stations at different continents via a satellite
rely station in the geostationary orbit about 36.000 km
above the equator. These links require very high-gain
antennas at both Earth or Ground Stations as well as at
the satellite to overcome the extremely large path
losses, primarily because of the huge distances. The antennas on board of a satellite serve for
up-linksignal receiving and for radiation ofdown-linksignals.
The various types of antennas in wireless systems ranges from dipole antennas with omni-
directional characteristics as in mobile phones to antennas with a narrow beamwidth for high
gain used for long-distance links as shown above. Hence, various antenna types are used in
wireless systems, depending on
Performance, Properties Examples
the frequency range, e.g. 1) dipole antennas f< 2 GHz
2) aperture antennas f> 1 GHz
the requirements 3) omni-directional antenna diagram,
e.g. in a mobile terminal for terrestrial/satellite MobCom.
4) very directional antenna diagram,
e.g. receiving antenna for Satellite-TV5) polarization (linear or circular)
6) low-profile (flat, compact)
7) cost
8) steerable / scanning (mechanically or electronically)
Within the scope of this lecture, we will focus on three types of antennas most often used in
wireless communication systems (more types and the fundamental principles are given in the
antenna lecture):
1. dipole antennas (crossed-dipole) f< 2 GHz, G < 3 dB
2. aperture antennas (high-gain antennas) f> 3 GHz, G = 10 to 40 dB3. antenna arrays f> 1 GHz, G = 3 to 40 dB
to scan the antenna diagram (tracking)
to increase the directivity and resolution
for multi-beams to increase the capacity
for interference suppression by space filtering
Fig. 1-1: Long-distance satellite links (up- and down-link) with two Earth stations at different continents.
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Chap. 1: Antennas for Wireless Systems 2
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1.1 Antenna Fundamentals and Basic Parameters
The space surrounding an antenna is usually subdivided into three regions (see Fig. 1-2):
1. the reactive near-field region, 2. the radiating near-field (Fresnel) region and 3. thefar-field
(Fraunhofer) region. If the antenna has a maximum overall dimensionD, the far-field region is
commonly taken to exist at distances greater than
rPQ > 2D2/ (1-1)
from the antenna, being the wavelength. In this far-field region, a spherical TEM-wave
essentially exists, i.e. the field components are perpendicular to each other and transverse to the
radial propagation direction. In addition, the angular distribution is independent of the radial
distance where the measurements are made.
Electromagnetic waves are used to transport information through a wireless medium or a
guiding structure, from one point to the other. It is then natural to assume that power and energy
are associated with electromagnetic fields. The quantity used to describe the average radiated
power density associated with an electromagnetic wave is the time average Poynting vector:
avS (r, , ) = Re{ E(r, , ) H*(r, , ) } in W/m2 (1-2a)
The factor appears because the E and H fields represent peak values and it should be
omitted for RMS values.
As mentioned above, in the far-field region only aspherical TEM-wave propagates, i.e. the ,
-field components are perpendicular to each other and transverse to the radial propagation
direction, e.g.EundH. Both quantities are linked via the free-space impedance 0 =376.6 :H =E /0 . For this case the average radiated power density is:
0
** Re
2
1Re
2
1),,(
EuEuHuEurSav
r=
2
0
),,(2
1
rEur
. (1-2b)
The average power radiated by an antenna (radiated power) can simply be determined by
integrating the average radiated power density over a closed surface in the far field, usually asphere with radius r:
rff
rnf= 0.62 /3D
rff= 2D2/rnf
Fig. 1-2: Field regions of an antenna and calculated radiation patterns of a paraboloidantenna for different distances from the antenna [Bal].
Normalizedpowerpattern
[dB]
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Prad= dsnSsphere
av ),( =
2
0 0
2sin),( ddrSav . (1-2c)
The power pattern of the antenna is just a measure as a function of direction, of the average
power density radiated by the antenna. The observations are usually made on a large sphere of
constant radius extending into the far field. In practice, absolute power patterns are usually not
desired. However, the performance of the antenna is measured in terms of the gain (to bediscussed subsequently) and in terms of relative power patterns.
The spherical wave is an elementary form of
electromagnetic waves. Their wavefronts are made
of imagined surfaces of concentric spheres. The
center of the spherical wavesis the center of these
spheres. From there the energy flows radialinto the
space. An antenna, which radiates the same energy
in all directions is named isotropic radiator. The
radiated powerPradof the isotropic radiator will be
distributed in any distance rover the whole possible
surface of a sphere 4r2 according to Fig. 1-3a.Then, the power flux density respectively the
average radiated power densitySi(r) of the isotropic
radiator in a distance ris:
24)(
r
PrS radi
. (1-3)
A perfect isotropic radiator is impossible to realize
in praxis. It serves as an ideal reference for realistic
antennas. Only a very short dipole produces an
approximate isotropic field, except in itslongitudinal axis.
In most cases, the radiation pattern of a real
antenna is represented as a function of the
directional coordinates and and looks like that
in Fig. 1-3b. A graph of the spatial variation of the
electric (or magnetic) field along a constant radius
is called anfield-strength or amplitude field pattern
E(, ). That of the received power at a constant
radius is called the power patternP(, ) = S(,
)Ae, where S(, ) is the average radiated powerdensity in the given direction , and Ae the effective aperture of the antenna. For most
practical applications, it is usually given as
Normalized pattern 1),(
),(0
E
Ec
with ),( 000 EE = max. or (1-4a)
Power pattern0
),(),(
P
Pp
=
re
e
AS
AS
0
),( =
)2/(
)2/(),(
0
2
0
0
2
E
E=
2),( c (1-4b)
with ),( 000 SS = max.E0,P0 and S0 are the maximum field strength, average power and
average radiated power density, respectively, at the direction 0, 0. Thus, the maximum valueis c0 = c(0,0) = 1. These pattern are mostly recorded in decibels, showing the variations over
a sphere centered on the antenna.
24)(
r
PrS radi
r
Isotropic radiator
Surface element
44 344 21
d
ddrdA sin2
Solid angle
ddr
dAd sin
2Prad
Fig. 1-3a: Spherical waves from an isotropic radiatorwith radiated powerPrad. Definition of a solid anglewith its vertex at the center of a sphere with radius rand a spherical surface element dA.
Side Lobe Level(SLL) in dB
Half Power Beam
Width (HPBW)
Main or major lobe axis 0, 0(boresight axis)
Main or
major lobe
E(, )E0
Side lobes
Back lobes
Minor lobes
First Null Beam
Width (FNBW)
Fig. 1-3b: Typical antenna pattern (radiation lobesand beamwidths) of a narrow-beam antenna.
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One very important description of an antenna is
how much it concentrates energy in one direction
in preference to radiation in other directions. This
characteristic of an antenna is called the directivity,
which is defined in the far field at a distance ras
D(, ) =
rS
S
,
,=
directionsalloverintegrateddensitypowerradiatedaverage
,directiongivenaindensitypowerradiatedaverage .
The average radiated power density in the far field
above a sphere with radius r corresponds to the
average radiated power density of an isotropic radiator fed by the same radiated powerPrad:
2
2
2 2
0 0
Average radiated
Radiated power power density ofover a sphere in the far field an isotropic radiator
1, , sin ( )
4 4
radi
radP
PS S r d d S r
r r
1 2 31 4 4 4 4 4 4 2 4 4 4 4 4 43.
In most applications, only the maximum directivity in the direction 0, 0 is of interest, i.e.
000 ),(),( SSS :
000 ,DD =
rS
S
,
,=
44 344 21
434 21
d
c
rad ddS
SP
Sr
sin,
44
,
00
2
0
02
2
(1-5)
In this formula, S(, ) is the averageradiated power density in the preferred
direction , , S0 the maximum radiation
intensity at 0, 0, the solid angle and d
an infinitesimal element of the solid angle of
a sphere according to Fig. 1-3a. The table on
the left indicates the directivity of some
antennas.
Since it is usually much easier to measure the input
powerPin than the radiated powerPrad of an antenna in
practice, very often thegainG(, ), which is related tothe input terminals, i.e. the input powerPin, is often used
as a measure describing the performance of an antenna
instead of the directivityD(, ), which is related to the
output terminals, i.e. the radiated powerPrad. Thus,
taking into account the efficiency of the antenna, their
relation are derived in Fig. 1-5: G0 = G(0, 0) =
0Drad . The antenna radiation efficiency is
in
rad
radP
P with rad = 1 for a lossless antenna.
The average radiated power Prad of an antenna differsfrom the input power Pin by thepower lossesPL due to the Ohmic losses, because of the finite
conductivity of the metallic walls of the antenna and the absorption in the dielectrics.
Antenna Types Directivity
in dB
Dipole, Loop & Slot Antennas 1.7 - 3
Patch, Dielectric Rod 2.5 - 9
Yagi, Helix, Small Arrays 5 - 17
Horn, Medium Arrays 10 -22
Reflector, Lens, Large Arrays 22 - 70
ci
ci
dir
Horn antenna
0
0
ZZ
ZZr
in
in
Pin PL
Prad
21 rr
Input terminals
(gain reference)Output terminals
(directivity reference)
c= conduction efficiency
d= dielectric efficiency
r= reflection (mismatch)efficiency
total antenna efficiency
tot=321
rad
dcr
inP
SrG
,4,
2
radP
SrD
,4, 2
),(,
4,
4,
,
22
DP
Sr
P
SrG rad
D
rad
rad
radrad
44 344 21
Pin rad = Prad
Fig. 1-5: Directivity and gain of a horn antenna.
00 ,D
24
)(r
PrS radi
;
Major or main lobeSide lobes
Isotropic
radiator
Prad
HPBWr Main lobe axis
ri
S
DS
),(,
Fig. 1-4: Antenna pattern of an narrow-beam antennawith respect to a pattern of an isotropic radiator fedby the same average radiated power Prad and thesame polarization.
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TheHalf-Power BeamWidth (HPBW) is defined as: In a plane containing the direction of the
maximum of a beam, the angle between the two directions in which the radiation intensity is
one-half the maximum value of the beam [Bal]. The 3 dB beamwidth HPBWof the antenna
is a very important figure-of-merit, and it often used to as a tradeoff between it and the side
lobe level; that is, as the beamwidth decreases the sidelobe increases and vice versa. In addition,
the beamwidth of the antenna is also used to describe the resolution capabilities of the antenna
to distinguish between two adjacent radiating sources or radar targets. The most commonresolution criterion states that the resolution capability of an antenna to distinguish between two
sources is equal to half theFirst Null BeamWidth (FNBWorFNBW), which is usually used to
approximate theHalf-Power BeamWidth (HPBW). That is, two sources separated by angular
distances equal or greater thanFNBW/2 HPBW(orFNBW/2 HPBW) of an antenna with auniform distribution can be resolved. If the separation is smaller, then the antenna will tend to
smooth the angular separation distance (see Fig. 1-6). In summarizing, the beamwidth is a very
important figure-of-merit, characterizing e.g.
1) the spatial illumination area of satellite and terrestrial point-to-multipoint systems,2) the space filtering capabilities against multipath in mobile communications and3) the resolution capabilities to distinguish between two sources in radar, radiometry or mobile communications.
Fig. 1-6: Spatial illumination area of a satellite (left), space filtering capability against multipath
propagation effects in mobile communications (middle) and resolution capability to distinguish betweentwo sources in radar systems (right).
The polarization of a wave can be defined in terms
of a wave radiated (transmitted) or receivedby an
antenna in a given direction. The polarization of a
wave radiatedby an antenna in a specified direction
at a point in the far field is defined as the
polarization of the (locally) plane wave which is
used to represent the radiated wave at that point. At
any point in the far field of an antenna the radiated
wave can be represented by a plane wave whoseelectric field strength is the same direction as that of
the wave and whose direction of propagation is in
the radial direction from the antenna. As the radial
distance approaches infinity, the radius of curvature
of the radiated waves phase front also approaches
infinity and thus in any specified direction the wave
appears locally as a plane wave. The polarization
of a wave receivedby an antenna is defined as the
polarization of a plane wave, incident from a given
direction and having a given power flux density,
which results in maximum available power at the antenna terminals.
Fig. 1-7: Polarization factor for transmitting andreceiving aperture (top) and linear wire (bottom)antenna.
MS
MS
BS
Space filtering
capability
Spatialillumination area
Geostationary
Satellite
Low Earth Orbit
Resolution
capability
< res can not beresolved
> res can beresolved
Antennaposition 1
Antennaposition 2
Rotatingantenna
res
res
Source 1
Source 2
Source 3
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1.2 Dipole Antennas
1.2.1 Ideal dipole with uniform current distribution
Let us first consider an ideal dipole orinfinitesimal currentelementto be z-directed and placed
in the origin of a co-ordinate system (Fig. 1-8). It is ideal in the sense that it has
a very short length (incremental length) compared to the wavelength zQ
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The Fig. 1-9 shows the far field of an ideal dipole, e.g. the angular variation ofE andH over
a sphere with constant radius r. An electric field probe antenna moved over the sphere surface
and oriented parallel toE will have an output proportional to the normalized pattern c() =
sin. Any plane containing the z-axis has the same radiation pattern of sinsince there is no
-variation in the fields due to the symmetry of the source. A pattern taken in one of these
planes is called a E-plane pattern because it contains the electric field vectorE. A pattern
taken in a plane perpendicular to the E-plane (thex,y-plane in this case) is anH-plane patternbecause it contains the magnetic fieldH [Stutz]. The complete three-dimensional pattern for
the ideal dipole is shown in Fig. 1-9(d). It is
an omni-directional pattern in azimuth since
it is uniform in the x,y-plane. Omni-
directional antennas in azimuth are very
popular particularly in ground-based mobile
communications because of the time- and
space-dependent angular (primarily -)
variations of the incident wave of the mobile
station due to shadowing and multi-patheffects. Themaximum directivityof the ideal
dipole is given by:2
0, 0
0, 0 0 2
(( , ) 4
| ( , ) |u u
cD D
c d
=
0
2
2
0
sinsin
4
dd
=
0
3sin2
4
d
=
0
2
cos3
2
3
cossin
2
= 2
3
1.76 dB
This means, that in the direction of maximum radiation 0, the radiation intensity is 50% morethan that which would occur from an isotropic source radiating the same total power.Below, there are examples of the power density and directivity of dipole antennas. They all
exhibit similar far-field characteristics with broad major lobes and maximum directivity inboresight direction (perperdicular to the dipole axis) and "nulls" along the dipole axis. In
satellite mobile communications, omni-directional pattern are aimed for, which can be achievede.g. by crossed-dipole configurations given below.
Examples of the power density and directivity of dipole antennas.
0
1
1.5
1.64
x112
2(sin )4
1.5rad
iD r
PS u
r
r r
rad
/ 2 2
2cos( / 2 cos )
1.64092sin4
r
PS u
r
r r
rad
21
4i r
PS u
r
r r
( , ) 1i
D
2( , ) 1.5 ( sin )iDD
2
/2
cos( / 2 cos )( , ) 1.64092
sinD
0, 1iD
0, 1.5iDD
0, / 2 1.64092D
Isotropicradiator,
ideal
dipole
and
/2dipole:
Fig. 1-9: Normalized far-field pattern of the ideal dipole[Bal]: (a) Field components; (b) E-plane radiation pattern;(c) H-plane radiation pattern; (d) 3D-radiation pattern.
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1.2.2 Crossed-dipole antenna (Ohmori page 101)
In satellite mobile communications, omni-directional pattern are aimed for, which can beachieved e.g. by crossed-dipole configurations for a ship given below.
A dipole antenna with a half-wavelength (/2) is the most widely used, e.g. in mobile satellitecommunications. A half-wavelength dipole is a linear antenna, whose current amplitude varies
one-half of a sine wave with a maximum at the center.
Far-field of an2
-dipole:
00
2
( )
cos cos
2 sin
Pjkr
P
c
I eE j
r
1 4 4 2 4 4 3
; 0/H E
As a dipole antenna radiates linearly polarized waves, two crossed-dipole antennas have been
used in order to generate circular-polarized waves. The two dipoles are geometrically
orthogonal (x andy axes in the Fig. 3-10), and equal amplitude signals are fed to them with /2
in-phase difference.
Characteristic of a crossed-dipole antenna 1 2, , ,c c c
dipole#1: along thex-axis rotational symmetrical around = angle between
= angle between length axis and Pr only dependent from
1
2cos cos
( )sin
c
dipole#2: along they-axis rotational symmetrical around
= angle between length axis and Pr only dependent from = 90 -
2
2 2cos cos cos sin
( )sin cos
c
Overall pattern of the crossed-dipole with equal amplitudes but with2
in-phase difference:
2 2cos cos cos sin,sin cos
c j
The patterns 1c and 2c are indicated in by the thick and thin lines respectively, withina coordination system. The radiation pattern of a crossed-dipole antenna is also indicated by the
thick line in Fig. 1-10, which is nearly omni-directional in the horizontal plane. A dipole
antenna needs a balun to be excited by coaxial cables, which is an unbalanced feed line. Further,
a 3-dB hybrid (power divider) is generally used to feed a cross-dipole in order to be able to feed
the same power a phase difference of/2 for each dipole element.A crossed-dipole antenna has a maximum gain in the boresight direction (zaxis direction in
Fig. 1-10). In mobile satellite communications, especially in land-mobile communications,
y
x
I(zQ)2l
rPQ
rP
zQP
HPBW
78
ZQ Angle between length axis of the dipole
and far-field point P.
0
0
2 1 for 0
2( )
2 1 for 0
2
Q
z Q
Q
z Q
zu I z
I zz
u I z
l
l
l
l
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elevation angles to the satellite are not 90 except immediately under the satellite. In order to
optimize the radiation pattern, a set of dipole antennas are bent toward the ground as shown in
Fig. 1-11, which is called a drooping dipole antenna. The crossed drooping dipole is one of the
most interesting candidates for land-mobile satellite communications, where the required
angular coverage is narrow and almost constant in elevation. By adjusting the height between
the dipole elements and the ground plane and the bending angle of the dipoles, the gain and
elevation pattern can be optimized for the coverage region of interest. Fig. 1-11 shows theradiation patterns for the antenna designed by Jet Propulsion Laboratory (JPL) which is to be
used over the entire continental Unites States (CONUS). It has a 4-dBi gain [8].
Fig. 1-10: Radiation patterns of a dipole, a cross-dipole,and the coordination system.
Fig. 1-11: (a) Crossed drooping dipole antenna and
(b) its radiation pattern.
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1.3 Effective Aperture of Antennas
The following approach of effective
aperture of antennas, in particular of an
ideal dipole, and then the relation between
the effective aperture and gain of antennas
will be derived in detail in the antenna
lecture in Chap. 3.1. Here, only theresults will be taken into account.
Fig. 1-12 shows an example of an
receiving antenna, e.g. a short linear wire
dipole. For determining the effective
aperture or areaAe,p of this receiving
antenna, an incident homogeneous local
plane wave is assumed, having an average
radiated power density Si in W/m2 in the
far field of an transmitting antenna. The
effective aperture or area Ae,p of this
receiving antenna is defined by
{
2 2
, cos cos
e
e p p ei
A
pRA A
P
S , (1-6a)
where Si = )2/( 02
0 E is the average
radiated power density of the incident homogeneous locally plane wave,p the angle between
the polarization (orientation) of the receiving antenna and the locally plane wave andPRis the
maximum received powerfor an terminating impedanceZte=Rte+jXte and ideal orientation p
=0. The receiving antenna collects all the power which propagates through this effective
aperture, which is not necessarily correlated with the physical aperture or length of the antenna.The power at the terminating resistanceRte of the receiving antenna is given by
21
2AR te
P R I =
2 2
2 2 2
1 1
2 2
ind ind te
te
A te A te A te
U R UR
Z Z R R X X
with 0 teAAind ZZIU teA
ind
AZZ
UI
.
Hence, the effective aperture or area Ae,p of the receiving antenna is:
2
,cosR
e p pi
PA
S
=
2 212
2
20 0
/cos
/ 2
indte A te
p
R U Z Z
E
, (1-6b)
For optimal orientation 0p and power matching (Zte=
AZ Rte=RA = (Rrad+RL),Xte = -
XA) and in the next step for lossless antennas it will be:
2 21 22
0
2200 0
/ 2
4/ 2
indte Aind
e
A
R U R UA
R EE
emA =
rad
ind
RE
U
2
0
2
04
forRA =Rrad,RL=0 (1-6c)
From that, the effective aperture with respect to the maximum gain of the antenna is given by:
0 0
e rad em
rad
A A
G D
, independent of the losses of the antenna.
a)
p
Incident wave induces a voltage
indU along the antenna
Incident field
0 EuE x
i
r
Incident homogeneous plane wave
0
0
EuH y
i
ReceivedpowerPR
Lossless
antenna
Average power density
of the incident field2
0
02
1 EuS z
i
r
p
b)indU
IA
Terminating
impedance teZ
PR Rte
te
a
A
Rrad
AU
Antenna impedante AZ
Radiation resistance Rradscattered (re-radiated) powerPrad
Ohmic or loss resistanceRLpower dissipated as heat
LP
Antenna reactanceXA
RL
Fig. 1-12: Assumed configuration for the derivation of theeffective aperture of an antenna: a) Receiving antenna forthe incidence of a homogeneous locally plane wave; b)Impedance of an antenna with a terminating impedance.
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Effective aperture of an ideal lossless dipole (see antenna lecture section 3.1.2)
Since the ideal dipole is very short with respect to the wavelength (zQ
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Example: Parabolic reflector with ap = 0.6 for 12 GHz.
D in m 0.4 0.6 0.8 1 2 5 10 20 30
D/ 16 25 32 40 80 200 400 800 1200
G in dB 31.8 35.3 37.8 39.8 45.8 53.7 59.8 65.8 69.3
HPBW
in deg
4.38 2.9 2.2 1.75 0.88 0.36 0,18 0.091 0.063
The relationD/is the decisive parameter (value?) in above equations, because the gain factor
is proportional to (D/)2 and the beamwidth is inversely proportional toD/.
1.4.1 Area coverage (footprint)
The area coverage can be determined by the following approximation, where the satellite
antenna with diameterD1 produces a footprint (area coverage) with diameter of about a1 on
Earth for smallHPBW.2
111
DG ap and 1
11
57.3
ap
HPBWD
d
aHPBW 11tan 1
11
57.3
180
ap
a dD
for smallHPBW1
Antenna
gain[dB]
1.5 GHz
4 GHz
50 GHz
30 GHz
12 GHz
100 GHz
Aperture diameter [m]
1.5 2 2.5 3 3.50.5 100
70
20
10
30
60
50
40
0.6ap
2
10 lg apD
G
Fig. 1-13: Gain and half-power beamwidth of aperture antennas.
Half-Pow
erBeamWidth[deg]
0 0.5 1.0 1.5 2.0 2.5 3.0
Aperture diameter [m]
1.5 GHz
4 GHz
12 GHz
30 GHz
50
40
30
20
10
0
157.3
ap
HPBWD
6.0ap
Half-Pow
erBeamWidth[deg]
0 0.5 1.0 1.5 2.0 2.5 3.0
Aperture diameter [m]
1.5 GHz
4 GHz
12 GHz
30 GHz
50
40
30
20
10
0
157.3
ap
HPBWD
6.0ap
D1
D2
Elliptical
reflector
Feed horn
HPBW1 HPBW2
3 dB gain
contour
Service area
=Required flux density (dBW/m2)
at given frequency and polarization1 21 2
57.3 57.3;
ap ap
HPBW HPBWD D
22
1 1 2
1 2
57.3
apG D DHPBW HPBW
Earth
Satellite
Fig. 1-14: Elliptically shaped area coverage on Earth
by an elliptically shaped reflector antenna.
Sat 1Area
coverage
HPBW1a1
Earth
d
d
aHPBW 11)tan(
daHPBW 11 for smallHPBW1
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1.4.2 Multibeam, multimode antennas
1.4.3 Cross-polarisation of an reflector antenna
Cross-polarization is produced from the reflector curventure deviating from a plane. It causes
cross coupling or cross talk from one polarization channel (e.g. a channel with horizontal
polarization) to another polarization channel (e.g. a channel with vertical polarization) in the
same frequency band or in overlapping frequency bands.
Reduction of cross polarization by
highf/D (accomplishable through compact Cassegrain or Gregorian configurations)
Offset-feeding
Especially for ground stations (Earth stations) high-gain Cassegrain-, Gregorian- or
Parabolic-antennas are utilized with the following properties
extreme high gain (G)
extreme narrow Beam (HPBW)
reduction of interferences with adjacent satellite links in the same frequency range
Examples: see below
Emerging markets for telecommunication satellites dominatedby multiple spot beam scenarios with overlapping spots
Many spot beams with different information Higher satellite capacity Frequency reuse
Two basic principles for multi spot beam antennaswith overlapping spots
Single feed per beam (SFB) Multiple feeds per beam (MFB)
Overlapping feed aperture sub-arrayswith alternating polarization and
frequency sub-bands
Antenna farm with separateantennas per color and singlefeed er beam allocation
1
2
3
Service
areacontour
TX 1
TX 2
TX 3
Feeds Signals atdifferent
frequencies
Fig. 1-15: Operational principle of a multibeam
antenna with three contiguous beams operating at the
same or at different frequencies.
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Fig. 1-16a: Parabolic reflector antennas.
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Fig. 1-16b: Cassegrain antenna.
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Fig. 1-16c: Earth station antenna (Cassegrain).
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Fig. 1-16d: Antenna requirements.
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1.5 Phased-Array Antennas
Array theory is given within the antenna lecture. The focus here is of the possibilities and
technologies for phased arrays, since of their increasing demand and importance in wireless
systems.
Example:
Fig. 1-17: Linear phased array with four dipoles.
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Several antennas can be arrayed in space to make a directional pattern or one with a desired
radiation pattern. This type of antenna is called an array antenna, which consists of more than
two elements. Each element of an array antenna is excites by equal amplitude an phase, and its
radiation pattern is fixed. On the other hand, the radiation pattern can be scanned in space by
controlling the phase of the exciting current in each element of the array. This type of antenna
is called a phased-array antenna [13], which has many advantages in terms of mobile satellite
communications such as compactness, light weight, high-speed tracking performance, andpotentially low cost.
Arrays are found in many geometrical configurations. The most typical type in mobile satellite
communications is the planar array, in which elements are arrayed in a plane to scan the beam
at both azimuth and elevation angles to track the satellite. Fig. 1-17 shows the most simple
linear phase array that is composed of four elements, which have the same electrical
characteristics, and are arrayed at equal spaces ofdalong thex axis. In Fig. 1-17, if each element
is excited equally in amplitude, but with different phases, the far field of the array antenna is
given by
1 2 1 23 3
2 2 2 2sin sin sin sind d d d
jkrjk jk jk jke
E c e e e er
1 2 1 23 3
2 2 2 2sin sin sin sind d d d
jkrjk jk jk jke
E c e e e er
1 232 22 cos sin cos sinjkr
kd kd e cr
42jkre
c AFr
where the phase center is at the coordinate origin, and c() is the radiation pattern of theelement. The 1 and 2 including their signed are the values of phase shifters, as shown in Fig.
1-17. the coefficientAFis called the array factor. The radiation pattern for the array antenna is
found by multiplying the radiation pattern of the element antenna and the array factor.
The array factors AF2 andAF4 of linear arrays with two and four elements, excites by equal
phase (1 = 2 = 0), whose spacing between elements is half a wavelength (d= /2), are as
follows:
4 32 2cos sin cos sinAF
Figure 1-17 below shows patterns of array factors for the four-element linear array. The space
between element is half a wavelength. The maximum value was obtained in the boresight
direction (y axis). The array factor will reach maximum in direction 0 when cos( ) = 1are
satisfied. This can physically be explained by the fact that the phases of wave fronts become
equal, as shown in Fig. 1-17.
0 1 0 23
2 2sin sin ( 0, 1, 2, )kd kd n n
Therefore, in case ofn = 0
1 02sinkd and 2 0
32
sinkd
It is found that maximum gain can be obtained in the desired direction, and the beam can be
scanned into a desired angle off the boresight direction. The radiation pattern of phased array
antennas with four elements can be calculated by the following equation:
4 0 03
2cos sin cos sin sinsc sin
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Chap. 1: Antennas for Wireless Systems 20
where 0 denotes the angle of scanned direction. Each element is assumed to be non-directional,
and element spacing is half a wavelength (d= /2).
Fig. 1-17 shows radiation patterns of phased array antennas for four-element linear arrays. The
beam is scanned at an angle of 30 degrees.
AppendixArray theory is given within the antenna lecture.
The focus here is of the possibilities and
technologies for phased arrays, since the
Non-uniformly excited equally spaced linear array with linear phase progression and
different amplitude taper.
I
I
I I
I
z
Uniform Triangular
Binominal Dolph-Chebyshev,
SLL = -20 dB
Dolph-Chebyshev, SLL = -30 dB
z
z
z
z
z
z