14. Micro Strip Antennas

Microwave and Antenna Lab.

14. Microstrip Antennas

Second Semester, 2004Dept. of Electrical and Electronic Engineering,

Yonsei University Prof. Young Joong Yoon


Aperture Antennas

Huygens’ Principle

Uniqueness Theorem

Field Equivalence Principle

Love’s equivalent

Electric conductor equivalent

Magnetic conductor equivalent

Horn Antenna, Reflector Antenna, Sl

ot Antenna, Microstrip Antenna

EEM , HHnJ 11

ss


Introduction

Feeding

E-planeH-plane

Patch

Ground

Substrate

• 1953: G.A.Deschamps1953: G.A.Deschamps

• 1955: French Patent1955: French Patent

Advantages

low profile, low weight

conformable to surfaces

easy and inexpensive to manufacture

flexible and versatile (pattern, polarization, …)

compatible with solid state devices: loaded and active antennas

arrays easily made

Disadvantages

High Q ☞ small bandwidth

spurious radiation (feeds, edges, ground, surface waves,…) low efficiency☞Need quality substrates (tanδ<0.002)

difficult to achieve polarization purity


Introduction

Basic Characteristics very thin substrate : 0.003λ g ≤ h ≤ 0.05λ g

rectangular patch : λ g/3 ≤ L ≤ λ g/2 , 0.5L ≤ W ≤ 2L

dielectric constant : 2.2 ≤ εr ≤ 12

bandwidth : typically 1 ~ 3 %

input resistance : Rin = ½ Rr = 60 λ g / W

W

L

유전체

er

h

Excitation


Introduction

Feeding Methods

Microstrip line feed

Easy to fabricate

Simple to match

Spurious feed radiation

Narrow bandwidth (2-5%)

Probe feed

Easy to fabricate

Simple to match

Low spurious radiation

Narrow bandwidth


Introduction

Aperture-coupled feed

Reduction of cross pol.

Easy to model

Difficult to fabricate

Narrow bandwidth

Proximity-coupled feed

Large bandwidth (13%)

Easy to model

Low spurious radiation

Difficult to fabricate


Transmission line model

Fringing Effect

The fields at the edges of the patch undergo fringing

The amount of fringing is a function of the dimensions of

the patch and the height of the substrate

Effective dielectric constant : 1/ 1212

1

2

12/1

hWW

hrrreff

Electric field

lines

Effective dielectric

constant



Effective Length, Resonant Frequency

Because of the fringing effects, electrically the patch of the microstrip antenna looks

greater than its physical dimensions

Effective length ( )

The resonant frequency

8.0258.0

264.03.0412.0

hW

hW

h

L

reff

reff

LLLeff 2

rr

rL

v

Lf

22

1 0

00010

Physical and effective

lengths



The resonant frequency (including edge effect)

Design procedure ( specify : ε r, fr, h and determine : W, L )

Determine the width :

Determine the effective dielectric constant

Determine the ΔL

Determine the effective length :

1

1

2

1

00

rrfW

0000

010 22

1

2

1

reffreffeff

rcLLL

f

Lf

Lreffr

22

1

00



Conductance

Y1=G1+jB1

Slot #2 is identical to slot #1

Resonant Input Resistance

Y2=Y1, G2=G1, B2=B1

The resonant input resistance (considering mutual effects)

11111111 2~~~

2 GGBGBGBGBYYLY effin

10

1 ln636.01

120

10

1

24

11

120

0

20

01

0

20

01

h

hkW

B

hhk

WG

Transmission model

equivalent

1212

1

GGRin


Cavity model

Cavity model

Electric conductors

the top and bottom walls

Magnetic walls

the perimeter of the patch

Modes

The field configurations within

the cavity and the boundary

conditions determine the

resonant frequency.

Dominant mode : TM010 mode

Resonant frequency :

Charge distribution and current density

TM010 modes

r

rL

v

Lf

22

1 0010


Cavity model

Equivalent Current densities ( , )

Ground plane image theory ( )

The electric and magnetic fields within the cavity

0sJ as EnM ˆ

as EnM ˆ2

No ground

plane

0

'sin 'cos 00

yxzy

zx

HHEE

yL

HHyL

EE


Cavity model

Radiating slot and nonradiating slot

Typical E- and H- plane patterns of each microstrip patch slot

Radiating slot Nonradiating slot

E-plane H-plane


Cavity model

Field radiated (TM010 mode)

The far-zone electric fields radiated by each slot

The array factor :

Total electric field for the two slots

For small values of h

θWk

Zφθhk

XWhere

Z

Z

X

X

r

ehWEkjEE

rjk

r

cos2

,cossin2

sinsin

sin2

E , 0

00

000

sinsin

2cos2 0 e

y

LkAF

sinsin

2cos

sinsinsinE 000

0e

rjk Lk

Z

Z

X

X

r

ehWEkj

000

0

00 , sinsin2

coscos

cos2

sinsin

2E

0

hEVLk

Wk

r

eEVj e

rjk


Cavity model

E-plane (θ=90°) H-plane (φ=0°)


Cavity model

Directivity

Single slot :

Approximate quantity

10

2max

0

124

I

W

P

UD

rad

WkX

X

XXXSXd

Wk

I i

0

3

2

0

0

1

sincos2sin

cos

cos2

sin

0

0

0

0 W 4

W5.2dBessdimensionl3.3

WD


Cavity model

Two slots :

Approximate quantity

12002 1

2

gDDDD AF

11212 conductor mutual normalized

AFFactor array ofy Directivit

GGg

DAF

0

0

0

2 W 8

W8.2dBessdimensionl6.6

WD


Circular Patch

Cavity model

The cavity is composed of two perfect

electric conductors at the top and bottom

to represent the patch and the ground

plane, and by a cylindrical perfect

magnetic conductor around the circular

periphery of the cavity

Electric and magnetic fields-TMzmnp

The homogeneous wave equation for TMz mode

0,,,, 22 zAkzA zz


Circular Patch

Electric and magnetic fields

Boundary condition

Magnetic vector potential

0 1

11

1

11

1 22

222

zzz

zzzz

HA

HA

H

Akz

jEz

AjE

z

AjE

0)'0,2'0,'(

0)',2'0,'0(

0)0',2'0,'0(

hzaH

hzaE

zaE

μεωkkkzkmBmAkJBA rrzρzmmnpz2222

22 , 'coscos'sin'cos'


Circular Patch

Resonant frequency

Effective radius

Electric field in far-field region

0200 ' cos

2

0

Jr

eVakjE

rjke

2

8412.1 0110

re

ra

vf

21

7726.12

ln2

1

h

a

a

haa

re

0200 sincos

2

0

Jr

eVakjE

rjke

Cavity model and

equivalent magnetic

current density


Circular Patch

E-plane (φ=0°, 18

0°)

H-plane (θ=90°, 270°)


Quality Factor, Bandwidth and Efficiency

Quality Factor

The quality factor is a figure-of merit that is representative of the antenna losses

Total quality factor :

Qt : total quality factor

Qrad : quality factor due to radiation (space wave) losses

Qc : quality factor due to conduction (ohmic) losses

Qd : quality factor due to dielectric losses

Qsw : quality factor due to surface waves

Bandwidth

swdcradt QQQQQ

11111

VSWRQ

VSWR

f

f

t

1

0


Quality Factor, Bandwidth and Efficiency

BW ~ volume = area · height = lengt

h · width · height ~

Efficiency

The radiation efficiency of an antenn

a can be expressed in terms of the q

uality factors

Typical variations of the efficiency as

a function of the substrate height

r1

rad

t

t

radcdsw Q

Q

Q

Qe

1

1

Efficiency and

bandwidth versus

substrate height


Circular Polarization

Circular polarization

Two orthogonal modes are excited with a 90° phase difference

between them

Dual-feeding

Square patch

Circular patch


Circular Polarization

Single feeding CP for square patch with thin

slots on patch

LHCP RHCP

LHCP RHCP


Arrays and Feed networks

Feed arrangement

To match 100-ohm patch elements to a 50-ohm line

14. Micro Strip Antennas

Documents

resonant frequency

microstrip

antenna lab

microwave

determine