1 FS Antennas for Wireless Systems

7/27/2019 1 FS Antennas for Wireless Systems

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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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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.


11/20



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


12/20



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


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


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


13/20



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.


15/20



Fig. 1-16b: Cassegrain antenna.


16/20



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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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

1 FS Antennas for Wireless Systems

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