antennas1.v1

1

ANTENNAS FOR COMMUNICATIONS

Course Outline

Antenna in a System: parameters, modeling and antennas in a system.

Input characteristics: Impedance and matching, fundamental limitations.

Printed resonant antennas. Microstrip patches, slots.

Broadbanding techniques Broadbanding techniques.

Miniaturization techniques. Fractal antennas.

UWB antennas design and specification.

Multielement antennas. Arrays. MIMO.

Smart and reconfigurable antennas.

CAD methods for antennas.

2ANTENNAS FOR COMMUNICATIONS

Antenna measurement.

2

Introduction

Antenna Definition

antenna. That part of a transmitting or receiving system that is designed to radiate or to receive electromagnetic waves.electromagnetic waves.

IEEE Standard Definitions of Terms for Antennas IEEE Std 145-1993http://ieeexplore.ieee.org/iel1/2785/7109/00286098.pdf?tp=&isnumber=7109&arnumber=286098

In this course we will follow as much as possible the IEEE standard for antenna term definitions.

The concept of antenna is inherently associated with the system and electromagnetic waves.

It was the IRE (IEEE) that embraced the new field of wireless and radio, which became


the fertile field for electronics and later the computer age. But antennas and propagation

will always retain their identity, being immune to miniaturization or digitization.

Harold A. Wheeler, Antenna topics in my experience, IEEE Transactions on Antennas and Propagation, vol. AP-33, no.2, February 1985.

Antenna in a System

Antenna Parameters


3

Antenna in a System

3. Fundamental Antenna Parameters

Balanis, Antenna Theory Ed. Wiley Cardama, Antenas, Edicions UPC (In Spanish) Orfanidis, Electromagnetic Waves and Antennas, available on-line


: radiation intensity [W/sr] : power density [W/m2]

GLOSSARY

U

Antenna in a System

W

p y [ ] D : directivity D0 : maximum directivity G : gain G0 : maximum gain et : efficiency (total) erad : radiation efficiency eref : reflection/mismatch efficiency ZA : antenna input impedance []


ZG : generator input impedance [] ZL : load impedance [] ap : aperture efficiency

4

RADIATION PATTERN The radiation pattern (or antenna pattern) is the representation of the

radiation properties of the antenna as a function of space coordinates (, ).

3D Radiation PatternAntenna

Antenna in a System

3D Radiation PatternAntenna


The radiation pattern is measured in the far-field region, where the spatial (angular) distribution of the radiated power does not depend on the distance.

RADIATION PATTERN

Antenna in a System

One can measure and plot the field intensity or the radiated power at a constant radius from the antenna:

The plot of the spatial variation of the power density at a constant radius from the antenna is called

The plot of the spatial variation of the electric (magnetic) field at aconstant radius from the antenna is

ll d h lit d fi ld tt

0, ,E r r

2

01 , ,W E r r


the power pattern.called the amplitude field pattern.

2 2 2 2 22 2 2 2 2x y z r zE E E E E E E E E E

NOTE: RMS values for E, H, V and I will be always considered.

5

RADIATION PATTERN

Usually, the pattern describes the normalized field (power) values withrespect to the maximum value.

Antenna in a System

p

0max

, ,E r r

E

2

02

max

, ,E r r

E

The power pattern and the amplitude field pattern are the same when computed and plotted in dB.


2

0 02

max max

, , , ,20log 10log

E r r E r r

E E

RADIATION PATTERN The pattern can be a 3-D plot or a 2-D plot: a 2-D plot is obtained as an

intersection of the 3-D one with a given plane.

Antenna in a System


6

RADIATION PATTERN

2-D plots could be done in polar or cartesian axes: Cartesian plots are useful to show the details of directive antennas.

Antenna in a System

Polar plots clearly show the spatial distribution of power.


[Image from: A. Cardama et al., Antenas, Ed. UPC, 2da. Edicin, 2002 (Barcelona)]

If antenna #1 is a transmitting antenna and antenna #2 is the receiving one, the ratio of transmitted to the received power will not change if antenna #1 becomes the receiving antenna and antenna #2 becomes the transmitting one.

RECIPROCITY OF THE RADIATION PATTERN

Antenna in a System

The ratio of received to transmitted power does not change if the antennas interchange receiving with transmitting mode.

The radiation pattern is the same in receiving and in transmitting modes

Ridge Horn /2 Dipole

#1 #22 1

1 2

R R

T T

P PP P


The radiation pattern is the same in receiving and in transmitting modes.

The reciprocity theorem holds only if the whole system (antennas + propagation environment) is homogeneous, isotropic and linear. In antenna systems including circulators or isolators these properties do not hold.

7

RADIATION PATTERN CONCEPTS: DIRECTIONALITY

Isotropic pattern is an hypothetical pattern of an antenna having equal radiation in all directions. It is used as a reference to compare antennas.

Antenna in a System

A directional antenna radiates (receives) much more power (efficiently) in some directions than in others (e.g. parabolic antennas).

An omnidirectional antenna has a non-directional pattern in a given plane, and a directional pattern in any orthogonal plane (e.g. single-wire antennas).


Isotropic Omnidirectional Directional

[Images from: A. Cardama et al., Antenas, Ed. UPC, 2da. Edicin, 2002 (Barcelona)]

Principal patterns are the 2-D patterns measured in the E-plane and in the H-plane.

For linearly polarized antennas the E-plane is the plane that includes the direction of maximum radiation of the antenna and the orientation of the E-field in that di ti

RADIATION PATTERN CONCEPTS: MAIN Planes

Antenna in a System

direction. The H-plane is the plane that includes the direction of maximum radiation of the

antenna and the orientation of the H-field in that direction. Both planes are orthogonal.


[Image from: A. Cardama et al., Antenas,Ed. UPC, 2da. Edicin, 2002 (Barcelona)]

8

RADIATION PATTERN CONCEPTS: LOBES

Pattern lobe is a portion of the radiation pattern whose local radiation intensity maximum is relatively strong.

Antenna in a System

Major lobe (main beam): lobe containing the direction of maximum radiation.

Minor lobe: any lobe except the main beam.

Side lobe: lobes adjacent to the main beam or adjacent to a intended one.

Secondary lobe: lobe adjacent to


Secondary lobe: lobe adjacent to the maximum.

Back lobe: lobe making at angle of approximately 180 from the main beam.

[Images from: C.A. Balanis, Antenna Theory, Wiley, 2nd Edition, 1997 (New York)]

RADIATION PATTERN CONCEPTS: LEVELS Side Lobe Level (SLL): ratio between the maximum value of the side lobes and the maximum level of the main lobe.

Front to back ratio (F/BR): ratio between the maximum value of the back lobe and the maximum level of the main lobe.

Antenna in a System

Radiation intensity

Main Lobe

Secondary LobeSLL F/BR


[deg]00 1800900

Back Lobe

9

Half-power beamwidth (HPBW): is the angle in the major lobe where the radiation intensity is half its maximum (measured in the plane that contains the maximum of the main beam).First null beamwidth (FNBW): angular separation between the first nulls

RADIATION PATTERN CONCEPTS: BEAMWIDTHS

Antenna in a System

First-null beamwidth (FNBW): angular separation between the first nulls of the pattern.


[Images from: C.A. Balanis, Antenna Theory, Wiley, 3rdEdition, 2005 (New Jersey)] Field pattern Power pattern Power pattern (dB)

RADIATION INTENSITY Radiation intensity in a given direction is the power per unit solid angle

(steradian) radiated in this direction by the antenna.

[W/sr]raddPUd

Antenna in a System

4

[W]radP U d

[ ]d

22 2

0 0 4

sin rad rad rad radS

P W dS W r d d W r d

2U r W


The power pattern is a plot of the function U (usually normalized to its maximum value).

, ,radU r W

10

RADIATION INTENSITY Radiation intensity for an antenna at far-field:

2* 1 ReradW E H E r

Antenna in a System

2 2rU E

Infinitesimal dipole:

in 4 0 jkresrlkIjrE

jk

Isotropic antenna: radiation independent of spatial direction.

0 04 4

radP U d U d

Does this radiation intensity depend on distance?


2 20

1 sinU E

0 in jkreE r E s

r

04 radP U

0 4radPU

Steradians are a measure of the angular 'area' subtended by a two dimensional surface in a three dimensional space (just as a radian is a measure of the angle in a one dimension space).

RADIATION INTENSITY: SOLID ANGLE

Antenna in a System

radian steradian

[steradians]A[radians]S


2 [steradians]R [radians]

R

A sphere subtends 4steradians about the origin.

A circle subtends 2radians about the origin.

11

DIRECTIVITY The directivity of an antenna is the ratio of the radiation intensity of the

antenna in a given direction and the radiation intensity of an isotropic radiator fed by the same amount of radiated power.

U

Antenna in a System

,, 4rad

UD

P

Directivity is a dimensionless quantity.

Directivity can be zero but the maximum directivity D0 is always 1.

If not specified, directivity will refer only to the direction of maximum radiation (D )


(D0).

Directivity is also expressed in dB (10 log D).

The spatial integration of the directivity is a constant:

4 ,4

dD

DIRECTIVITY

The radiation intensity of an isotropic radiator is equal to the total power radiated by the antenna divided by 4.

For directive antennas: 44

Antenna in a System

-3 dB beamwidths in two orthogonal planes (in radians)

440D


12

GAIN The gain of an antenna is the ratio of the radiation intensity in a given direction

and the radiation intensity that would be obtained if the power accepted by the antenna were radiated isotropically.

,4 UG

Antenna in a System

, 4in

GP

The gain of the antenna takes into account the losses in the antenna. It is calculated via the accepted power Pin, which is a measurable quantity.

This definition does not account for the mismatch between the antenna and the transmission line.

lossesPincP P heat


radP

refPinP heat

ANTENNA EFFICIENCY AND RADIATION EFFICIENCY

incPP inP

lossesR

radR

lossesP

radP2

1 0 refe10 de

Antenna in a System

The power accepted by the antenna is related to the input power through a reflection/mismatch efficiency eref .This efficiency accounts for the dielectric/conduction losses of the antenna.

The radiated power is related to the accepted power h h ffi i ll d h di i ffi i

refP in 1 0 rade

2-1 referad

radRe

R R


through a coefficient called the radiation efficiency erad .

The antenna (total) efficiency et is used to estimate the total loss of energy at the input terminals of the antenna and within the antenna structure.

radreft eee

radrad lossesR R

13

INPUT IMPEDANCE AND RADIATION RESISTANCEradiation

Antenna in a System

losses (heat)

Antenna input impedance:

The radiation resistance relates the radiated power to the voltage (or current)

A A A losses rad AZ R jX R R jX


The radiation resistance relates the radiated power to the voltage (or current) at the antenna terminals.

2A

radrad

IPR if IA is rms:

EQUIVALENT CIRCUIT OF TRANSMITTING ANTENNAGenerator directly connected to

the antenna: impedance of the generator

Antenna in a System

Maximum power delivered to the antenna when conjugate matching of i d

Generator connected to the antenna through a transmission line: impedance of the generator transferred to the terminals of the antenna


impedances:

A losses rad G

A G

R R R RX X

14

EQUIVALENT CIRCUIT OF RECEIVING ANTENNA

Incident wave induces voltage VA at the antenna terminals.

Conjugate impedance matching is required between the antenna and the load:

Antenna in a System

load:


L A losses rad

L A

R R R RX X

ANTENNA EFFECTIVE AREA When an antenna is receiving, it is convenient to define an effective area (or

effective aperture) Ae.

Antenna in a System

ZL

antenna + load

incW

L inc eP W A

power delivered to the load


incident wavefront

power delivered to the load

15

EFECTIVE ANTENNA AREA

The effective antenna area (aperture) is the area which multiplied by the

Antenna in a System

The effective antenna area (aperture) is the area which multiplied by the incident power density of a plane wave incident upon the antenna produces the power delivered to the load.

2[ ]Leinc

PA mW


APERTURE EFFICIENCY

The aperture efficiency of an antenna is the ratio of the effective antenna aperture and its physical area.

Antenna in a System

p p y

For wire antennas, the effective aperture is much larger than the surface of wire itself.

eap

AA


For aperture-type antennas, the effective area is smaller than the physical aperture area.

16

DIRECTIVITY AND APERTURE AREA RELATIONSHIP The ratio directivity vs. effective area is the same for any antenna (easy to

show for an Hertzian dipole):

02

4DA

Antenna in a System

The subscript m denotes that it is the maximum effective area because there are no heat losses in the antenna and no impedance mismatch. If losses are included then the directivity is substituted for gain:

2me

A

2

4GA


The formula holds for any type of antenna that has a well-defined aperture, or surface area through which all of the radiated power flows.

eA

This formula does not hold for monopoles.

FREQUENCY BANDWIDTH

FBW is the range of frequencies within which the antenna characteristics conform to a specified standard (input impedance, radiation pattern, beamwidth, polarization, side-lobe level, gain, beam direction and width,

Antenna in a System

beamwidth, polarization, side lobe level, gain, beam direction and width, radiation efficiency,)

FBW of broadband antennas:

Fractional Frequency Bandwidth FFBW of narrowband antennas (f0 is the center frequency):

max

min

fFBWf

FBW:1Notation

from fmin to fmax


max min

0

100%f fFBWf

Fractional bandwidth

17

FREQUENCY BANDWIDTH

Example:

Antenna in a System

[Images and text from: Choi, W.; Kwon, S.; Lee, B.: Ceramic chip antenna using meander conductor lines, Electronics Letters, 19th July 2001, 37, (15), pp. 933 934 ]


ANTENNA IN A SYSTEM


18

The communication between a transmitting and a receiving antenna requires a power budget that can be analyzed with the help of the concept of gain and effective area.

TRANSMITTING AND RECEIVING ANTENNAS

Antenna in a System

transmitting antenna

ZG

VG

ZLr

tt , rr ,

DGP DGP

receiving antenna


Each antenna should be at each others far-field :

22Dr

ttttradtt DeGP ,,,,, , rrrrradrr DeGP ,,,,, ,

RX antenna

The power budget relates de power fed to the transmitting antenna to the power received by the receiving antenna when both are at each others far field.

POWER BUDGET EQUATION

Antenna in a System

TX antenna

ZG

VG

ZLr

tt , rr ,

ttttradtt DeGP ,,,,, , rerrrrradrr ADeGP ,, ,,,,,,

2 ,1 t t tDP P A


2 , ,2, 1 ,4 t t t

r t t rad t e r r rP P e Ar

power radiated by the TX antenna

power density received by the RX antenna

effective area of the RX antenna

19

The effective area accounts for the mismatch, radiation losses and polarization loss factor of receiving antenna and its load.

FRIIS TRANSMISSION EQUATION

2

2 2 1A D

Antenna in a System

Consequently the power budget or Friis Transmission Equation is:

2

2 2 2, , 1 1 t t t r r rrD DP e e

2 2, , , 1 , 4e r r r t r r rad r r r rA e D

mismatch radiation

efficiencypolarization loss factor


, , 21 1 4 4rad t rad r t r t rt e eP r

FREE-SPACE LOSS AND GAIN, ADDITIONAL LOSSES The effect of the propagation path, which causes received power to attenuate

with the square of the distance r, can be quantified by defining the free-space loss factor and free-space gain by:

21G

Antenna in a System

For impedance and polarization matched antennas:

An additional loss factor may be introduced if necessary representing other

14f f

GL r

rrrf

ttttr GLGPP , 1 ,


An additional loss factor may be introduced, if necessary, representing other losses (such as atmospheric absorption and scattering).

rrrotherf

ttttr GLLGPP , 1 1 ,

20

It is common practice to express Friis equation additively in dB. For impedance and polarization matched, lossless antennas:

FRIIS TRANSMISSION EQUATION IN dB

dB dB dB dB dBdB r t t f other rP P G L L G

Antenna in a System

d d d d ddB Remark 1:

-60

-50

-40

-30

-20

e-sp

ace

gain

[dB

]

-20 dB/decad = - 6 dB/octave

Remark 3:

Radiated power times the directivity (or antenna gain times

Remark 2:

[dBmW]rP

[dBW]rP [W][dBW] 10log 1 Wr

rPP

[mW][dBmW] 10log

1 mWr

rPP


100 101 102 103-90

-80

-70

Distance in terms of [r/]

Free

rL f

4log20

directivity (or antenna gain times accepted power by the antenna) is called Equivalent Isotropically Radiated Power (EIRP):

dBtdBtdB DPEIRP

NOISE IN SYSTEMS Ways that noise follow to enter to communication systems:

From background radiation of the environment (not scattering of the systems signal by the background -clutter): the receiving antenna picks up noisy signals from the sky, the ground, the weather, and other natural

d i

Antenna in a System

or man-made noise sources. Noise is also generated by the components in the receive channel.

Under most conditions it is the internally generated thermal noise that dominates and limits the system performance.


The total sum of all the noises introduced by such a cascade of receiver components must be maintained at acceptably low levels (relative to the amplified desired signal).

21

NOISE IN SYSTEMS

Antenna in a System


A high noise level will hide a weak signal and possibly cause a loss in communications.

EQUIVALENT NOISE TEMPERATURE The average power N [W] of a noise source within a certain bandwidth B [Hz]

can be quantified by means of an equivalent noise temperature T [K] defined through:

N kTB G i Whit N i

Antenna in a System

23 W1.38 10 Hz K

k Boltzmanns constant:

The temperature T is not necessarily the physical temperature of the source, it only provides a convenient way to express the noise power (for a thermal source, T is indeed the physical

N kTB Gaussian White Noise


temperature). The equation is commonly expressed in dB:

dB dB dB dBN k T B (10log)

22

ANTENNA SIGNAL-TO-NOISE RATIO (SNR) The performance of a telecommunications system depends on the signal-to-

noise ratio (SNR) at the receivers input. It defines the minimum received power required by the system (sensitivity).

Antenna in a System

The SNR relates the system signal power and the system noise power.

For impedance and polarization-matched antennas:

kTBLL

GGP

NPSNR otherf

rrrttttr

11, ,


Gaussian White Noise

ANTENNA NOISE TEMPERATURE The antenna itself is an important source of noise:

Noise due to the loss resistance of the antenna Noise which the antenna picks up from the surrounding environment

(any object with a physical temperature radiates EM power): extraterrestrial

Antenna in a System

(sun, galactic noise), ground (dep. on physical temperature, rugosity, and composition) and atmosphere (rain, water steam, composition, sparks), and man-made noise (power lines, engines).

The average noise power Nant at the antenna terminals is characterized by an equivalent antenna noise temperature Tant, such that Nant = kTant B.

man-made

atmosphere

extraterrestrial


At frequencies ranging from 1GHz to 10GHz: The noise temperature of night sky is 3

K to 5 K (very low). The noise temperature of ground is 100

K to 300 K (quite high).

23

ANTENNA NOISE TEMPERATURE The temperature Tant represents the weighted contributions of all the

radiating noise sources picked up by the antenna through its mainlobe and sidelobes.

The value of Tant depends primarily on the orientation and elevation angle of the t d h t th t i l ki t

Antenna in a System

antenna, and what the antenna is looking at.

2


2

0 02

0 0

( , ) ( , ) sin

( , ) sin

B

ANT

T G d dT

G d d

TB: brightness temperature of the background;

G: antenna gain

SYSTEM NOISE TEMPERATURE In a receiving antenna system, the signal-to-noise ratio at the receiver must

take into account not only the noise picked up by the antenna, and quantified by Tant, but also all the internal noises introduced by the various components of the receiver.

Antenna in a System

System noise temperature Tsys will be the noise temperature at the input port of a noisless device that generates the same noise power at the output port than the real (noisy) device.

noisy device(real device)

GinS out inS G S


Gin inN kT B out outN kT B

out sys in eN GkT B Gk T T B

24

SYSTEM NOISE POWER The amount of added noise power (at the input of a device) is expressed in

terms of the effective noise temperature Te of the device.noisless device(ideal device)noise

Antenna in a System

out in e in eN G N N Gk T T B

in inN kT BG

e eN kT Bout outN kT B

inS out inS G S


System noise power Nsys is the sum of the noise power of the input signal Ninand the effective noise of the device Ne.

out sys sysN GN GkT B

SYSTEM SNR If a desired signal with noise power Sin is also input to the device, then the

signal power at the output will be Sout = GSin. The system signal-to-noise ratio is defined to be the ratio of the input signal power to total system noise power:

i l d ii

Antenna in a System

S GS S

noisless device(ideal device)

noise contribution

G

eNinNinS

outNoutSsysN


out in inout sys

out sys sys

S GS SSNR SNRN GN N

The SNR is the same whether it is measured at the input or the output of the (noisless) device.

25

THE CASE OF AN ATTENUATOR Last equation can be expressed in terms of the input and output powers

Nin=kTinB and Nout= kToutB:

1 11 hN N kT B

Antenna in a System

Thus, the noise input power is attenuated (as expected), but the attenuator also adds its own thermal noise power.

More generally, if the input power arises from signal plus noise Pin = Sin + Nin,

1out in physN N kT BL L

added noiseattenuated input noise power

1 1


1 11out out out in out in physP S N GS N P kT BL L

attenuated input power added noiseWhat happens to SNR?

THE ANTENNA RECEIVER The signal to system-noise ratio of the receiving system (referred to point a)

will be the ratio of the received power Pr to the system noise:

GGPPSNR rttr11

Antenna in a System

This ratio is also called the carrier-to-system-noise ratio and is denoted by C/N. For a given transmitter, PtGt, the receiver performance depends critically on the ratio Gr/Tsys, referred to as the G/T ratio of the receiving antenna (it is a figure of merit). In dB:

BkTLN sysfrtt

sys

dBdBdBsys

rdBfdBttdB

BkTGLGPSNR


dBsys The receiver SNR can be also be referred to LNA input (point b):

dBdBdBb

feedrdBfdBttdB

BkTGG

LGPSNR

26

APPLICATION TO DATA RATE LIMITS

An example of the application of the concepts of gain and effective area and the use of Friis formulas is the estimation of data rate limits.

Antenna in a System

According to Shannons theorem the maximum data rate C (Shannons channel capacity, in bits/sec) that can be achieved is:

2log (1 )C B SNR For data rates smaller than C, Shannons theorem states that there is an ideal coding scheme that would

C: capacity [bits/s]B: noise bandwidth [Hz]SNR: signal to noise ratio [adim]

C.E. Shannon (1916-2001)


guarantee error-free transmission.

Equation valid only for Gaussian noise.

The system SNR limits the data rates between the two antennas.

For practical digital communication systems there is a certain Bit Error Rate (BER) that is expressed as function of the ratio

b

o

EN

Antenna in a System

o

Eb is the energy per bit.No is the noise spectral density. o sysN kT

For a given transmission rate R (bits/s) and bandwidth B (Hz)


g ( ) ( )

PL bo

E RSNRN N B

27

Antenna in a System

For BPSK or QPSK modulation 12

b

o

EBER erfcN

erfc is the complementary error fuction 22 t

xerfc e dt

Maximum BER


Minimum Eb/No required

Antenna in a System

For a desired maximum allowable BER we can obtain the minimummin

b

o

EN

The maximum transmission rate is given byThe maximum transmission rate is given by

2

max

min min

1 /4

rad T R rad

sysb b

o o

B SNR P D D eR bits skT rE E

N N


28

Antenna in a System

Antenna parameters have fundamental role in the power balance of a wireless link.

Free-space losses imply a great penalty for short distances.

The antenna is a passive device. Increasing the radiated power in a certain directionoccurs at the expense of decreasing the radiated power in other directions.

Noise is always present.


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