course antenna engineering wir aopert micro strip 01

1Course Antenna Engineering

Dirk Heberling

3. Antenna Concepts and Analysis

• Wire Antennas• Aperture Antennas• Microstrip Antennas


Dirk Heberling

3.1 Wire Antennas

• Dipole Antennas and Derivates• Antenna Matching and Balancing• Loop Antennas• Yagi-Uda Antennas• Helix Antennas and Broadband

Antennas• Mobile Phone Antennas


Dirk Heberling

Wire antennas 1

• Oldest antenna form• Most prevalent antenna form• Nearly any imaginable antenna shape

and configuration• Simple concept• Easy construction• Inexpensive


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Wire antennas 2• Many analytical solutions have been

presented• Modern numerical solutions

- Simple concepts, e.g. Method of Moments (MoM)

- Easy application to computers- Usable for many wire configurations

• High accuracy of simple theory


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Example of a wire antennas

Base station antenna for GSM


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Straight Wire Dipole 1

current distribution

( ) sin , z2 2mL LI z I k z⎛ ⎞⎛ ⎞= − ≤⎜ ⎟⎜ ⎟⎝ ⎠⎝ ⎠

for L < λ /2

Maximum current at the terminals:

( )0 sin2mLI z I k⎛ ⎞= = ⎜ ⎟

⎝ ⎠Source: W.L. Stutzman, G.A. Thiele: Antenna Theory and Design, Wiley, New York, 1981


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Straight Wire Dipole 2

current distribution for various centre-fed dipoles

Source: W.L. Stutzman, G.A. Thiele: Antenna Theory and Design, Wiley, New York, 1981


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Straight Wire Dipolefarfield pattern 1

The radiation integral: ( ) ( ) ' cos2

2

' 'L

jkzLf I z e dzθθ −

−= ∫

leads to the far-zone electric field:

cos cos cos2 2

2 sin

jkr

m

kL kLeE j I

rθ

θη

π θ

−⎛ ⎞ ⎛ ⎞−⎜ ⎟ ⎜ ⎟⎝ ⎠ ⎝ ⎠

for L = λ/2:

( )cos cos

2sin

F

π θθ

θ

⎛ ⎞⎜ ⎟⎝ ⎠=



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for L = λ:

( ) ( )cos cos 12sin

Fπ θ

θθ

+=

for L = 3λ/2:

( )

3cos cos20.7148

sinF

π θθ

θ

⎛ ⎞⎜ ⎟⎝ ⎠=




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Radiation pattern for L = 1.25λSource: C. A. Balanis, Antenna Theory, 2nd Ed. Wiley, New York, 1997


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Straight Wire DipoleInput impedance 1

with the radiated power Pr:

( )

2

2 22 2

20 0

cos cos cos1 2 2 sin

2 sin2m

r

kL kLIP r d d

r

π π θη θ θ φ

η θπ

⎧ ⎫⎛ ⎞ ⎛ ⎞−⎜ ⎟ ⎜ ⎟⎪ ⎪⎪ ⎪⎝ ⎠ ⎝ ⎠= ⎨ ⎬⎪ ⎪⎪ ⎪⎩ ⎭

∫ ∫

the radiation resistance Rr gives:

2

2 rr

m

PRI

= for L = λ/2: 73rR = Ω


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for L = λ/2: 73 42.5inZ j= + Ω



Dirk Heberling Source: C. A. Balanis, Antenna Theory, 2nd Ed. Wiley, New York, 1997

Input resistance Rin (Ω)Length L

20π2(L/λ)20<L<λ/4


24.7(π L/λ)2.4λ/4<L< λ/220π2(L/λ)20<L<λ/4


11.14(π L/λ)4.17λ/2<L< 0.637λ24.7(π L/λ)2.4λ/4<L< λ/2

20π2(L/λ)20<L<λ/4


Approximations for the input impedance:


Dirk HeberlingSource: W.L. Stutzman, G.A. Thiele: Antenna Theory and Design, Wiley, New York, 1981

0.49λ

Resonant length L ShorteningL/2a

2%50000.475λ0.49λ


5%502%5000

Straight Wire Dipole, shortening by thick wires

0.455λ0.475λ0.49λ


9%105%502%5000


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Folded Dipole Antenna 1

Transmission line mode

Antenna mode



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4in DZ Z=

for L = λ/2

Folded Dipole Antenna 2

212F in FP Z I=

Dipole

212D in DP Z I=

in the antenna mode12F DI I=

280Ω

Folded dipole



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Antenna Matching and Feeding

Two primary feeding considerations:

• Matching between transmission line and antenna

• Excitation of the current distribution on the antenna



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Antenna Matching 1

Important point:• Good matching not

always necessary• High voltages can arise

on the feeding line with high power applications

Ways of matching:• Discrete matching

network• λ/4-line transformer• Tuning devices like

stubs etc.

Reflected and transmitted power in relation to VSWR


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Antenna Matching 2

sin2in m inLI I zβ⎡ ⎤⎛ ⎞= −⎜ ⎟⎢ ⎥⎝ ⎠⎣ ⎦

Change of input impedance:

2

2m

in rmin

IR RI

=

Off-centre feeding of a full wave dipole



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Antenna Matching 3

( )21in aZ Zα+

' / 4l λfor

α current division factorbetween the wires

4in aZ Z

for equal radii conductors

The T-Match

Source: C. A. Balanis, Antenna Theory, 2nd Ed. Wiley, New York, 1997


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Antenna Balancing 1

unbalanced currents I1 > I2

Example:Cross section of a coaxial transmission line feeding a

dipole at its centre

balanced currents I1 = I2



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BALanced to UNbalancedThe Balun

Coax-fed dipole

Sleeve balun-fed

dipole

Equivalent circuit

Cross section of a sleeve balun

Split coax balun



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Wire antennas above imperfect ground

Elevation pattern of a vertical short dipole at the surface of the ground plane

( )cos cossin4

jkrjkh jkh

VIL eE j e e

rθ θ

θ ωμ θπ

−−= + Γ

with 2

2

cos sincos sin

rV

r

ε θ ε θ

ε θ ε θ

′ ′− −Γ =

′ ′+ −

and ro

j σε εωε

′ = −

typical: 15rε =

213 210 3 10m

σ − −Ω

= − ⋅Source: W.L. Stutzman, G.A. Thiele: Antenna Theory and Design, Wiley, New York, 1981


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Loop AntennasThe radiation resistance

of a small loop is

2 2

2

2 31,1713r

kS SR πηλ λ

⎛ ⎞⎛ ⎞ ⎛ ⎞= ⎜ ⎟⎜ ⎟ ⎜ ⎟⎝ ⎠⎝ ⎠ ⎝ ⎠

Increase of the loop resistance by:

Several turns of number n2

231,171rSR n

λ⎛ ⎞⎜ ⎟⎝ ⎠

Introduction of a ferrite core of effective permeability μeff

2

231,171r effSR nμ

λ⎛ ⎞⎜ ⎟⎝ ⎠

Typical μeff: 100 - 10,000


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Square Loop Antennas 1

For the one-wavelength square loop antenna:

( )0ˆ cos x8

I kx λ′ ′= = − ≤1 2I I x

( )0ˆ sin y8

I ky λ′ ′= − = ≤4 3I I y


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yz-plane

xz-plane

Principle plane patterns for one-wavelength square loop antenna

xy-plane

Source: W.L. Stutzman, G.A. Thiele: Antenna Theory and

Design, Wiley, New York, 1981

Square Loop Antennas 2


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Square Loop Antenna, Input Impedance



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Circular Loop, Equivalent Circuit

( ) ( )in in in r L A iZ R jX R R j X X= + = + + +

Rr = radiation resistanceRL = loss resistance of loop conductor

XA = external inductive reactance = ω LA

Xi = internal high-frequency reactance = ω Li



Dirk Heberling

Yagi-Uda Antenna 1

A parasitic linear array of parallel dipoles is called aYagi-Uda antenna

or

Yagi-Uda arrayor

Yagi

First published by Shintaro Uda 1926

Simplification of an antenna array if only a few elements are fed directly.

Up to now,all arrays examined have had all elements active, requiring a direct

connection to each element.

Such an array is referred to as a parasitic array.


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Example of a Yagi-Antenna

Yagi-Antenna for TV and Radio reception


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Yagi-Uda Antenna 2

Field incident to a parasitic element is:

incident driverE E=

0 incident parasiteE E= +withtangential to the parasite

Consider a driver element that is a half-wave dipole and a parasitic element very close to it

parasite incident driverE E E= − = −then



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Yagi-Uda Antenna 3

Driver of length 0.4781λ

Parasite of length 0.49λ

Driver of length 0.4781λ

Parasite of length 0.45λ



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Yagi-Uda Antenna 4


Three-element Yagi-Uda antenna

- Driver of length 0.4781λ- Reflector of length 0.49λ- Director of length 0.45λ

H-plane

E-plane


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Yagi-Uda Antenna 5

Configuration of a general Yagi-Uda antenna



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Yagi-Uda Antenna 6Radiation pattern of a six-element Yagi-Uda antenna for TV Channel 15


H-plane E-plane


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Broadband Antennas 1An antenna with wide bandwidth is referred to as a

Broadband antennaThe term „broadband“ is a relative measure of the

bandwidth and varies with the circumstances

With fU and fL the upper and lower frequency of operation and fC the centre frequency

Bandwidth as a percent of the centre frequency 100U L

C

f ff−

× Bandwidth defined as a ratio

U

L

ff

If the impedance and the pattern of an antenna do not change significantly over about an octave (fU/fL=2) or more, we classify it as a

broadband antenna


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Broadband Antennas 2

• Broadband antennas– Helical antennas– Biconical antennas– Discone monopole

• Frequency independent antennas– Spiral antennas– Log-periodic antennas


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Helical Antennas

D = diameter of the helixC = circumference of the helix Dπ=S = spacing between turns

α = pitch angle 1tanS

C−

=

L = total length NS=

L0 = length of one turn 2 2S C= +



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Helical Antennas, Normal Mode

Radiation patternEquivalent model

for 0NL λFarfield consists of dipole field ED and loop field EL

EAR

Eθ

φ

=2 2

2SDλ

π=

Circular polarization for 2C Sλ=

Helical antenna



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Helical Antennas, Axial Mode

with: 3 44 3

Cλ

< <- Circumference in

the range of

4S λ- Spacing about

12 14α° ≤ ≤ °- Pitch angle usually

Typical farfield pattern

Left-hand sensed helix

Right-hand sensed helix

Axial (endfire) mode of helix



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Log-Periodic Dipole Array (LPDA)

Construction details of the LPDA


A log-periodic antenna is an antenna having a structural geometry such that its impedance and radiation characteristics repeat periodically as the

logarithm of frequency


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Log-Periodic Dipole Array 2A wedge of enclosed angle α bounds the dipole lengths!

11

1 1

n n N

n n N

L L LLR R R R

+

+

= = = =with

1 1 1n n

n n

R LR L

τ + += = <the scale factor τ is given by:

2n

n

dL

σ =and the spacing factor σ is defined as:


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Example of a

LPDA



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Example of a LPDAFarfield Pattern


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Helical Antenna

Loop Antenna

Inverted-F Antenna

Sleeve-Dipole

Basic antenna types

Antennas for Mobiles 1


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Influence of the human body on the electromagnetic field

Antennas for Mobiles 2


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Example of a printed loop antenna

Realisation forms of loop antennas

Antennas for Mobilesthe loop-antenna

Equivalent circuit and matching circuit


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Principle of a sleeve dipole Current distributionon a cellular phone

Antennas for Mobilesthe sleeve-dipole


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Model of a helical antenna Operational modes

Antennas for Mobilesthe helical antenna


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Examples of Inverted-F Antennas

λ/4-Monopol

InvertedL-Antenne

InvertedF-Antenne (IFA)

PlanareInvertedF-Anenne(PIFA)

λ/4-Monopole

InvertedL-Antenna

InvertedF-Antenna (IFA)

Planar InvertedF-Antenna (PIFA)

Antennas for Mobilesthe inverted-F antenna


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Shortening and loading of antennas

Antennas for MobilesMiniaturization

a) Introduction of an inductance b) Surrounding by dielectric or magnetic materialsc) Introduction of a capacitance


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X

YZ

X

YZ

CONCEPT-Model of a cellular phone with an

helical antenna

CONCEPT-Modelof a cellular phone at

the user

YZ

Antennas for Mobiles, an Example 1

X

YZ

Calculated radiation patterns at 450 MHz


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CONCEPT-Modelof the mobile phone

Calculated magnetic nearfield at450 MHz (cut plane through the device)

Simulated nearfield behaviour



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η = P rad (with user)

P rad (without user)

Overall efficiency

EID-Antenna: • Concentration of the nearfield in the feeding point• Electrical decoupled from the casing

lelektr =λ0/2

Principle of the EID-Antenna

L

Optimised nearfield distribution

Optimised overall efficiency

Optimised Antenna: EID-Antenna



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Mobile phone with EID-antenna

Cellular phone withhelical antenna

Calculated magnetic nearfield atf = 450 MHz (cut plane through the device)

CONCEPT: Nearfield Characteristics



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X

YZ

η = 38 % η = 84 %

Mobile phone with EID-antenna

with user

X

Y

Z

CONCEPT: Farfield @ 450 MHz

X

YZ

X

YZ

Mobile phone with helical antenna

with user



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x

y

φ=270°

φ=180°

φ=90°

φ=0°

Measurement Situation

Measurement: Farfield @ 450 MHz

0

45

90

135

180

225

270

315

-14

-10

-10

-6

-6

-2

-2

2

2

Handy, freistehendHandy mit EID am BenutzerHandy mit Helix am Benutzer

φ in °

x

y

Measured horizontal farfield characteristic

dB

Mobile Phone, without userMobile Phone, with EID and userMobile Phone, with helix and user



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Integrated Antennas

Dualband- and Multiband-Antennas

Antenna Interaction

Antennas for MobilesDevelopment Trends


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1. Metallic patch 2. 3D-MID-Antennas 3. Ceramic Antennas

• very popular• good electrical properties• easy fabrication• mech. fixation necessary

• difficult fabrication • flexible antenna design• electric properties depend

on the material

• antenna design difficult• small size• difficult fabrication

Antennas for MobilesIntegrated Antenna Technology


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0,8 1 1,2 1,4 1,6 1,8 2-20

-15

-10

-5

0

free spacetalking position

GHz

dB

S11

f

GSM

DC

S18

00

Return Loss (S11)

Antennas for MobilesDualband Helical Antenna

Double Helical Antenna


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Antennas for MobilesInteraction with the Head

course antenna engineering wir aopert micro strip 01

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