5 Microstrip Antennas

8/13/2019 5 Microstrip Antennas

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Overview of Microstrip Antennas

Also called patch antennas

One of the most useful antennas at microwave frequencies

(f > 1 GHz).

It consists of a metal patch on top of a grounded

dielectric substrate.

The patch may be in a variety of shapes, but rectangular

and circular are the most common.

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History of Microstrip Antennas

Invented by Bob Munson in 1972 (but earlier workby Dechamps goes back to1953).

Became popular starting in the 1970s.

G. Deschamps and W. Sichak, Microstrip Microwave Antennas, Proc. ofThird Symp. on USAF Antenna Research and Development Program,October 1822, 1953.

R. E. Munson, Microstrip Phased Array Antennas, Proc. of Twenty-

Second Symp. on USAF Antenna Research and Development Program,October 1972.

R. E. Munson, Conformal Microstrip Antennas and Microstrip PhasedArrays, IEEE Trans. Antennas Propagat., vol. AP-22, no. 1 (January1974): 7478.

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Typical Applications

Single element Array

(Photos courtesy of Dr. Rodney B. Waterhouse)

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Typical Applications (cont.)

Microstrip Antenna Integrated into a System: HIC Antenna Base-Station for 28-43 GHz

filter

MPA

diplexer

LNA

PD

K-connector

DC supply

Micro-D

connector

microstrip

antenna

fiber input with

collimating lens

(Photo courtesy of Dr. Rodney B. Waterhouse) 5


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Geometry of Rectangular Patch

x

y

h

L

W

Note:L is the resonant dimension. The width Wis usually

chosen to be larger thanL (to get higher bandwidth).

However, usually W< 2L. W= 1.5L is typical.

r

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Geometry of Rectangular Patch (cont.)

View showing coaxial feed

x

y

L

W

feed at (x0,y0)

A feed along the

centerline is the most

common (minimizeshigher-order modes

and cross-pol.)

x

surface current

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Advantages of Microstrip Antennas

Low profile (can even be conformal).

Easy to fabricate (use etching and phototlithography).

Easy to feed (coaxial cable, microstrip line, etc.) .

Easy to use in an array or incorporate with other

microstrip circuit elements.

Patterns are somewhat hemispherical, with a moderatedirectivity (about 6-8 dB is typical).

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Disadvantages of Microstrip Antennas

Low bandwidth (but can be improved by a variety oftechniques). Bandwidths of a few percent are typical.

Bandwidth is roughly proportional to the substrate

thickness.

Efficiency may be lower than with other antennas.Efficiency is limited by conductor and dielectric

losses*, and by surface-wave loss**.

* Conductor and dielectric losses become moresevere for thinner substrates.

** Surface-wave losses become more severe for

thicker substrates (unless air or foam is used).9


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Basic Principles of Operation

The patch acts approximately as a resonant cavity (shortcircuit (PEC) walls on top and bottom, open-circuit (PMC)

walls on the sides).

In a cavity, only certain modes are allowed to exist, at

different resonant frequencies. If the antenna is excited at a resonance frequency, a strong

field is set up inside the cavity, and a strong current on the

(bottom) surface of the patch. This produces significant

radiation (a good antenna).

Note: As the substrate thickness gets smaller the patch current radiates less,due to image cancellation. However, the Q of the resonant mode alsoincreases, making the patch currents stronger at resonance. These two effects

cancel, allowing the patch to radiate well even for small substrate thicknesses.10


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Thin Substrate Approximation

On patch and ground plane, 0tE ,zE z E x y

Inside the patch cavity, because of the thin substrate, the

electric field vector is approximately independent ofz.

Hence ,zE z E x y

h

,zE x y

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Thin Substrate Approximation

1

1 ,

1 ,

z

z

H Ej

zE x yj

z E x yj

Magnetic field inside patch cavity:

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Thin Substrate Approximation (cont.)

1 , ,zH x y z E x yj

Note: The magnetic field is purely horizontal.(The mode is TMz.)

h

,zE x y

,H x y

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Magnetic Wall Approximation

On edges of patch, 0sJ n

0botsJ z H

Hence,

0t

H

x

y

n L

Ws

JtAlso, on bottom surface of

patch conductor we have

nH n H

nh

H

(Js is the sum of the top and bottom surface currents.)

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Magnetic Wall Approximation (cont.)

nh

0 ( )tH PMC

Since the magnetic field is approximatelyindependent ofz, we have an approximatePMC condition on the entire vertical edge.

PMC

x

y

n L

Ws

Jt

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nh

x

y

n

L

W t

Hence,

PMC0z

E

n

1

, ,zH x y z E x yj

, 0n H x y

, 0zn z E x y

, 0zz n E x y

Magnetic Wall Approximation (cont.)

, , ,z z zn z E x y z n E x y E x y n z

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Resonance Frequencies2 2 0

z z

E k E

cos coszm x n y

E

L W

2 2

2

0zm n

k EL W

Hence

2 2

2 0m n

k

L W

x

y

L

W

From separation of variables:

(TMmn mode)

PMC

,z

E x y

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Resonance Frequencies (cont.)

2 2

2 m nkL W

0 0 rk Recall that

2 f

Hence2 2

2 r

c m nf

L W

0 01/c

x

y

L

W

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2 2

2mn

r

c m nfL W

Hence mnf f

(resonance frequency of

(m,n) mode)

Resonance Frequencies (cont.)

x

y

L

W

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(1,0) Mode

This mode is usually used because theradiation pattern has a broadside beam.

10

1

2 r

cf

L

cosz

x

E L

0

1 sins

xJ x

j L L

This mode acts as a wide

microstrip line (width W)that has a resonant length

of 0.5 guided wavelengths

in thex direction.

x

y

L

W

current

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Basic Properties of Microstrip Antennas

The resonance frequency is controlled by the patch

lengthL and the substrate permittivity.

Resonance Frequency

Note: A higher substrate permittivity allows for a smaller

antenna (miniaturization) but lower bandwidth.

Approximately, (assuming PMC walls)

Note: This is equivalent to

saying that the lengthL is

one-half of a wavelength in

the dielectric:

0/ 2/ 2dr

L

kL

2 2

2 m nkL W

(1,0) mode:

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The calculation can be improved by adding afringing length extension L to each edge of the

patch to get an effective lengthLe .

Resonance Frequency (cont.)

10

1

2 er

cf L

2eL L L

y

xL

Le

LL

Note: Some authors use effective permittivity in this equation.

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Hammerstad formula:

0.3 0.264

/ 0.412

0.258 0.8

eff

r

eff

r

W

hL hW

h

1/ 2

1 11 12

2 2

eff r rr

h

W

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Note: 0.5L h

This is a good rule of thumb.

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R lt R f


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0 0.01 0.02 0.03 0.04 0.05 0.06 0.07

h / 00.75

0.8

0.85

0.9

0.95

1

NORMALIZED

FREQUENC

Y Hammerstad

Measured

W/L = 1.5

r= 2.2The resonance frequency has been normalized

by the zero-order value (without fringing):

fN=f /f0

Results: Resonance frequency

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The bandwidth is directly proportional to substrate

thickness h.

However, if h is greater than about 0.050 , the probe

inductance (for a coaxial feed) becomes large enough so

that matching is difficult.

The bandwidth is inversely proportional to r (a foamsubstrate gives a high bandwidth).

Bandwidth: Substrate effects

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The bandwidth is directly proportional to the width W.

Bandwidth: Patch geometry

Normally W< 2L because of geometry constraints

and to avoid (0, 2) mode:

W= 1.5L is typical.

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Bandwidth: Typical results

For a typical substrate thickness (h /0 = 0.02), and a

typical substrate permittivity (r= 2.2) the bandwidth is

about 3%.

By using a thick foam substrate, bandwidth of about

10% can be achieved.

By using special feeding techniques (aperture coupling)and stacked patches, bandwidths of 100% have been

achieved.

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R lt B d idth


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0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.1

h /

0

5

10

15

20

25

30

BAN

DWIDTH(%) r

2.2

= 10.8

W/L = 1.5r= 2.2 or10.8

Results: Bandwidth

The discrete data points are measured values. The solid curves are

from a CAD formula.

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The resonant input resistance is almost independent

of the substrate thickness h (the variation is mainly

due to dielectric and conductor loss)

The resonant input resistance is proportional to r.

The resonant input resistance is directly controlled by

the location of the feed point. (maximum at edgesx =

0 orx =L, zero at center of patch.

Resonant Input Resistance

L

W

(x0,y0)

L x

y

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Resonant Input Resistance (cont.)

Note: The patch is usually fed along the centerline (y0 =W/ 2)

to maintain symmetry and thus minimize excitation of

undesirable modes (which cause cross-pol).

Desired mode: (1,0)

Lx

W

feed: (x0,y0)y

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For a given mode, it can be shown that the resonant inputresistance is proportional to the square of the cavity-mode

field at the feed point.

2 0 0,in zR E x y

For (1,0) mode:

2 0cosinx

RL

L

x

W

(x0,y0)

y

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Hence, for (1,0) mode:

2 0

cosin edgex

R R L

The value ofRedge depends strongly on the substrate permittivity. Fora typical patch, it may be about 100-200 Ohms.

Lx

W

(x0,y0)

y

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R lt R t i t i t


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0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08

h /

0

50

100

150

200

INPUTRE

SISTANCE

(

2.2

r= 10.8

r= 2.2 or10.8

W/L = 1.5

x0 = L/4

Results: Resonant input resistance

The discrete

data points are

from a CADformula.

L x

W

(x0,y0)

y

y0 = W/2 34


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Radiation Efficiency

The radiation efficiency is less than 100% due to

conductor loss

dielectric loss

surface-wave power

Radiation efficiency is the ratio of power radiated

into space, to the total input power.

rr

tot

PeP

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Radiation Efficiency (cont.)

surface wave

TM0

cos () pattern

x

y

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r rr

tot r c d sw

P Pe

P P P P P

Pr= radiated power

Ptot= total input power

Pc

= power dissipated by conductors

Pd= power dissipated by dielectric

Psw = power launched into surface wave

Hence,

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Conductor and dielectric loss is more important for

thinner substrates.

Conductor loss increases with frequency (proportionaltof) due to the skin effect. Conductor loss is usually

more important than dielectric loss.

2

1s

R

Rs is the surface resistance

of the metal. The skin depth

of the metal is .

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Surface-wave power is more important for thicker substrates

or for higher substrate permittivities. (The surface-wave

power can be minimized by using a foam substrate.)

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For a foam substrate, higher radiation efficiency is

obtained by making the substrate thicker (minimizing the

conductor and dielectric losses). The thicker the better!

For a typical substrate such as r = 2.2, the radiation

efficiency is maximum for h / 0 0.02.

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r= 2.2 or10.8 W/L = 1.5

0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.1

h / 00

20

40

60

80

100

EFFICIE

NCY(%)

exact

CAD

Results: Conductor and dielectric losses are neglected

2.2

10.8

Note: CAD plot uses Pozar formulas41


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Radiation Patterns (cont.)

-90

-60

-30

0

30

60

90

120

150

180

210

240

-40

-30

-30

-20

-20

-10

-10

E-plane pattern

Red: infinite substrate and ground plane

Blue: 1 meter ground plane

Note: The E-plane

pattern tucks in andtends to zero at the

horizon due to the

presence of the infinite

substrate.

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Directivity

The directivity is fairly insensitive to the substrate

thickness.

The directivity is higher for lower permittivity, because

the patch is larger.

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R lt Di ti it


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0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.1

h / 00

2

4

6

8

10

DIRECT

IVITY

(dB)

exact

CAD

= 2.2

10.8

r

r= 2.2 or10.8 W/L = 1.5

Results: Directivity

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Approximate CAD Model forZin

Near the resonance frequency, the patch cavity can be

approximately modeled as anRLCcircuit.

A probe inductanceLp is added in series, to account for the

probe inductance of a probe feed.

LpR C

L

Zin

probe patch cavity

48

A i t CAD M d l ( t )


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Approximate CAD Model (cont.)

LpR C

L

01 2 / 1in pR

Z j L j Q f f

0

RQ

L

1

2BW

Q BWis defined here by

SWR < 2.0.

0 0

12 f

LC

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in maxR R

Rin max is the input resistance at the resonance of the

patch cavity (the frequency that maximizesRin).

LpR C

L

50

R lt I t i t f


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4 4.5 5 5.5 6

FREQUENCY (GHz)

0

10

20

30

40

50

60

70

80

Rin(

) CAD

exact

Results : Input resistance vs. frequency

r

= 2.2 W/L = 1.5 L = 3.0 cm

frequency where theinput resistance is

maximum (f0)

51

Results: Input reactance vs. frequency


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p q y

r= 2.2 W/L = 1.5

4 4.5 5 5.5 6

FREQUENCY (GHz)

-40

-20

0

20

40

60

80

Xin

(

)

CADexact

L = 3.0 cm

frequency where the input

resistance is maximum (f0)

frequency where the

input impedance is real

shift due to probe reactance

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0.577216

0

0

0

2ln2

f

rX k h k a

(Eulers constant)

Approximate CAD formula for feed (probe) reactance (in Ohms)

f pX L

0 0 0

/ 376.73

a = probe radius h = probe height

This is based on an infinite parallel-plate model.

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0

0

0

2ln

2f

r

X k h

k a

Feed (probe) reactance increases proportionally with

substrate thickness h.

Feed reactance increases for smaller probe radius.

54

Results: Probe reactance (X =X = L )


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0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

Xr

0

5

10

15

20

25

30

35

40

Xf

()

CADexact

Results: Probe reactance (Xf=Xp= Lp)

xr= 2 (x0 /L) - 1

xr is zero at the center of the patch, and is

1.0 at the patch edge.

r= 2.2

W/L = 1.5

h = 0.0254 0

a = 0.5 mm

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CAD Formulas

In the following viewgraphs, CAD formulas for the

important properties of the rectangular microstrip

antenna will be shown.

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CAD Formula: Radiation Efficiency

0 0 1 0

1 3 11

/ 16 /

hed

rr

hed s rr d

ee

R Le

h pc W h

where

tand loss tangent of substrate

1

2sR

surface resistance of metal

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CAD F l R di i Effi i ( )


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CAD Formula: Radiation Efficiency (cont.)

where

Note: hed refers to a unit-amplitude horizontal electric dipole.

2 2

0 12

0

180

hed

spP k h c

1

1

hed

sphedr hedhed hed

swsp sw

hed

sp

Pe PP P

P

3

3 3

0 12

0

1 160 1hed

sw

r

P k h c

58

CAD F l R di ti Effi i ( t )


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3

0

1

13 1 1

1 14

hedr

r

e

k hc

Hence we have

(Physically, this term is the radiation efficiency of a

horizontal electric dipole (hed) on top of the substrate.)

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1 2

1 2 / 51

r r

c

2 4 2220 2 4 0 2 0

2 2

2 2 0 0

3 11 210 560 5

1

70

ap k W a a k W c k L

a c k W k L

The constants are defined as

2 0.16605a

4 0.00761a

2 0.0914153c


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1

1

hed

r hed

sw

hed

sp

eP

P

Improved formula (due to Pozar)

3/ 22200 0

2 2

1 0 0 1

1

4 1 ( ) 1 1

rhed

sw

r r

xkP

x k h x x

2

01 2

0

1

r

xx

x

2 2 2

0 1 0 1 0

0 2 2

1

21

r r r

r

x

2 2

0 12

0

180hedspP k h c

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Improved formula (cont.)

0 0tans k h s

0

1 0 2

0

1tan

cos

k h sk h s

s k h s

1rs

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CAD Formula: Bandwidth

1

0 0 0

1 1 16 1

/ 32

sd hed

r r

R pc h W BW

h L e

BWis defined from the frequency limitsf1 andf2 at which

SWR = 2.0.

2 1

0

f fBW

f

(multiply by 100 if you want to get %)

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CAD Formula: Resonant Input Resistance

2 0cosedgex

R RL

(probe-feed)

00

1

0 0 0

4

1 16 1

/ 3

edge

sd hed

r r

L h

WR

R pc W h

h L e

64

CAD F l Di ti it


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CAD Formula: Directivity

tanc tan /x x x

where

2 12

1 1

3tanc

tan

r

r

D k hpc k h

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CAD Formula: Directivity (cont.)

1

3D

p c

For thin substrates:

(The directivity is essentially independent of the

substrate thickness.)

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CAD F l R di ti P tt


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CAD Formula: Radiation Patterns

(based on electric current model)

The origin is at the

center of the patch.

L

rh

infinite GP and substrate

x

The probe is on thex axis.

cossx

J xL

y

L

W E-plane

H-plane

x

(1,0) mode

67

CAD Formula: Radiation Patterns (cont )


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CAD Formula: Radiation Patterns (cont.)

22

sin cos2 2

( , , ) , ,2

2 2 2

y x

hex

i iy

x

k W k L

WLE r E r

k W k L

0sin cosxk k

0sin sinyk k

The hex pattern is for a horizontal electric dipole in thex direction,sitting on top of the substrate.

ori

The far-field pattern can be determined by reciprocity.

68

CAD Formula: Radiation Patterns (cont )


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0, , coshexE r E G

0, , sinhexE r E F

where

0

0

2tan1

tan sec

TE k h N

Fk h N j N

0

0

2tan coscos 1

tan cos

TM

r

k h NG

k h N jN

2

sinrN

000

4

jk rjE e

r

CAD Formula: Radiation Patterns (cont.)

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Circular Polarization

Three main techniques:

1) Single feed with nearly degenerate eigenmodes (compact

but narrow CP bandwidth).

2) Dual feed with delay line or 90o hybrid phase shifter

(broader CP bandwidth but uses more space).

3) Synchronous subarray technique (produces high-quality CP dueto cancellation effect, but requires more space).

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Circular Polarization: Single Feed

L

W

Basic principle: the two modes are excited with equal

amplitude, but with a 45o phase.

The feed is on the diagonal.

The patch is nearly (but not

exactly) square.

L W

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Ci l P l i i Si l F d


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Circular Polarization: Single Feed

Design equations:

0 CPf f

0

11

2xf f

Q

011

2y

f fQ

Top sign for LHCP,

bottom sign for RHCP.

in x yR R R Rx andRy are the resonant input resistances of the two LP (x andy)

modes, for the same feed position as in the CP patch.

L

W

x

y

1

2BW

Q

(SWR < 2 )

The resonance frequency

(Rin is maximum) is the

optimum CP frequency.

0 0x y

At resonance:

72

Circular Polarization: Single Feed (cont )


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Circular Polarization: Single Feed (cont.)

Other Variations

Patch with slot Patch with truncated corners

Note: Diagonal modes are used as degenerate modes

L

L

x

y

L

L

x

y

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Circular Polarization: Dual Feed

L

L

x

y

P

P+g/4RHCP

Phase shift realized with delay line

74

Ci l P l i ti D l F d


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Circular Polarization: Dual Feed

Phase shift realized with 90o hybrid (branchline coupler)

g/4

LHCP

g/4

0

Z0/ 2Z

feed

50 Ohm load

0Z

0Z

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Circular Polarization: Synchronous Rotation


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Circular Polarization: Synchronous Rotation

Elements are rotated in space and fed with phase shifts

0o

-90o

-180o

-270o

Because of symmetry, radiation from higher-order modes (or probes)

tends to be reduced, resulting in good cross-pol.76

Circular Patch


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Circular Patch

x

y

h

a

r

77

Circular Patch: Resonance Frequency


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Circular Patch: Resonance Frequency

a PMCFrom separation of variables:

cosz mE m J k

Jm = Bessel function of first kind, order m.

0z

a

E

0mJ ka

78

Circular Patch: Resonance Frequency (cont )


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Circular Patch: Resonance Frequency (cont.)

mnka xa PMC

(nth root ofJm Bessel function)

2mn mn

r

cf x

Dominant mode: TM11

11 112 r

cf x

a

11 1.842x

79

Circular Patch: Resonance Frequency (cont )


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Fringing extension: ae = a + a

11 11

2 e r

cf x

a

Long/Shen Formula:

a PMC

a + a

ln 1.77262r

h aa

h

21 ln 1.7726

2e

r

h aa a

a h

or

Circular Patch: Resonance Frequency (cont.)

80

Circular Patch: Patterns


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(based on magnetic current model)

1

1

1, cosz

J kE

J ka h

(The edge voltage has a maximum of one volt.)

a

y

xE-plane

H-plane

In patch cavity:The probe is on thex axis.

2a

rh

infinite GP and substrate

0 rk k

x

The origin is at the

center of the patch.

81

Circular Patch: Patterns (cont )


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Circular Patch: Patterns (cont.)

0 1 1 00

, , 2 tanc cos sinR zE

E r a k h J k a Q

1 00 10 0

sin, , 2 tanc sin

sin

R

z

J k aEE r a k h P

k a

0

2cos 1 cos

tan sec

TE jN

Pk hN jN

0

2 cos

1

tan cos

r

TM

r

jN

Q

k h N jN

tanc tanx x / x

where

2

sinrN 82

Circular Patch: Input Resistance


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Circular Patch: Input Resistance

2

1 0

2

1

in edge

J kR R

J ka

a

0

83

Circular Patch: Input Resistance (cont )


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Circular Patch: Input Resistance (cont.)

12

edge r

sp

R eP

/ 22

20 0

0 0

2 22 2

1 0 0

tanc8

sin sin sin

sp

inc

P k a k hN

Q J k a P J k a d

1 /incJ x J x x

where

Psp = power radiated into space by circular patch with maximumedge voltage of one volt.

er= radiation efficiency

84

Circular Patch: Input Resistance (cont )


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Circular Patch: Input Resistance (cont.)

2

0

0

( )8

sp cP k a I

CAD Formula:

4

3c cI p

62

0 2

0

k

c k

k

p k a e

0

2

4

36

4

8

5

10

7

12

1

0.400000

0.0785710

7.27509 10

3.81786 10

1.09839 10

1.47731 10

e

e

e

e

e

e

e

85


85/132

Feeding Methods

Some of the more common methods for

feeding microstrip antennas are shown.

86

Feeding Methods: Coaxial Feed


86/132

Feeding Methods: Coaxial Feed

Advantages:

Simple

Easy to obtain input match

Disadvantages:

Difficult to obtain input match for thicker substrates,

due to probe inductance.

Significant probe radiation for thicker substrates

2 0cosedgex

R RL

87

Feeding Methods: Inset-Feed


87/132

Feeding Methods: Inset-Feed

Advantages:

Simple

Allows for planar feeding

Easy to obtain input match

Disadvantages:

Significant line radiation for thicker substrates

For deep notches, pattern may show distortion.

88

Feeding Methods: Inset Feed (cont )


88/132

Feeding Methods: Inset Feed (cont.)

Recent work hasshown that the

resonant input

resistance varies as

2 02cos2

in

xR A BL

L

W

WfS

x0

The coefficientsA andB depend on the notch width Sbut (to a good approximation) not on the line width Wf .

Y. Hu, D. R. Jackson, J. T. Williams, and S. A. Long, Characterization of the InputImpedance of the Inset-Fed Rectangular Microstrip Antenna, IEEE Trans. Antennasand Propagation, Vol. 56, No. 10, pp. 3314-3318, Oct. 2008.

89

Feeding Methods: Inset Feed (cont.)


89/132

Results for a resonant patch fed on three different substrates.

h = 0.254 cm

L/ W= 1.5

S/ Wf = 3

0/ / 2nx x L

L

W

WfS

x0

L

W

WfS

x0

g ( )

0 0.2 0.4 0.6 0.8 10

50

100

150

200

250

300

350

400

450

Rin

(Ohms)

xn

r= 1.0

2.42

10.2

r= 1.00

Wf

= 0.616 cm

r= 2.42

Wf

= 0.380 cm

r= 10.2

Wf

= 0.124 cm

Solid lines: CADData points: Ansoft Designer

90

Feeding Methods: Proximity (EMC) Coupling


90/132

Feeding Methods: Proximity (EMC) Coupling

Advantages:


Less line radiation compared

to microstrip feed

Disadvantages: Requires multilayer fabrication

Alignment is important for input match

patch

microstrip line

91

Feeding Methods: Gap Coupling


91/132

g g

Advantages:

Allows for planar feeding Can allow for a match with high edge

impedances, where a notch might be too large

Disadvantages: Requires accurate gap fabrication

Requires full-wave design

patch

microstrip line

gap

92

Feeding Methods:Aperture Coupled Patch (ACP)


92/132

g p p ( )

Advantages:


Feed-line radiation is isolated from patch radiation

Higher bandwidth, since probe inductance

restriction is eliminated for the substrate thickness,

and a double-resonance can be created.

Allows for use of different substrates to optimizeantenna and feed-circuit performance

Disadvantages:

Requires multilayer fabrication

Alignment is important for input match

patch

microstrip line

slot

93

I i B d idth


93/132

Improving Bandwidth

Some of the techniques that have been successfully

developed are illustrated here.

(The literature may be consulted for additional designsand modifications.)

94

Improving Bandwidth: Probe Compensation


94/132

p g p

L-shaped probe:

Capacitive top hat on probe:

top view

95

Improving Bandwidth: SSFIP


95/132

p g

SSFIP: Strip Slot Foam Inverted Patch (a version of the ACP).

microstripsubstrate

patch

microstrip line slot

foam

patch substrate

Bandwidths greater than 25% have been achieved.

Increased bandwidth is due to the thick foam substrate and

also a dual-tuned resonance (patch+slot).

96

Improving Bandwidth: Stacked Patches


96/132

p g

Bandwidth increase is due to thick low-permittivity antenna

substrates and a dual or triple-tuned resonance.

Bandwidths of 25% have been achieved using a probe feed.

Bandwidths of 100% have been achieved using an ACP feed.

microstripsubstrate

driven patch

microstrip lineslot

patch substrates

parasitic patch

97

Improving Bandwidth: Stacked Patches (cont.)


97/132

-10 dB S11 bandwidth is about 100%

Stacked patch with ACP feed3 4 5 6 7 8 9 10 11 12

Frequency (GHz)

-40

-35

-30

-25

-20

-15

-10

-5

0

ReturnLoss(dB)

Measured

Computed

98


98/132

Improving Bandwidth: Parasitic Patches


99/132

p g

Radiating Edges Gap Coupled

Microstrip Antennas(REGCOMA).

Non-Radiating Edges Gap

Coupled Microstrip Antennas

(NEGCOMA)

Four-Edges Gap Coupled

Microstrip Antennas

(FEGCOMA)

Bandwidth improvement factor:

REGCOMA: 3.0, NEGCOMA: 3.0, FEGCOMA: 5.0?

Most of this workwas pioneered byK. C. Gupta.

100

Improving Bandwidth: Direct-Coupled Patches


100/132

Radiating Edges Direct

Coupled Microstrip Antennas(REDCOMA).

Non-Radiating Edges Direct

Coupled Microstrip Antennas

(NEDCOMA)

Four-Edges Direct Coupled

Microstrip Antennas

(FEDCOMA)

Bandwidth improvement factor:

REDCOMA: 5.0, NEDCOMA: 5.0, FEDCOMA: 7.0

101

Improving Bandwidth: U-shaped slot


101/132

Improving Bandwidth: U shaped slot

The introduction of a U-shaped slot can give a

significant bandwidth (10%-40%).

(This is partly due to a double resonance effect.)

Single Layer Single Patch Wideband Microstrip Antenna, T. Huynh and K. F. Lee,

Electronics Letters, Vol. 31, No. 16, pp. 1310-1312, 1986.

102

Improving Bandwidth: Double U-Slot


102/132

p o g a d dt oub e U S ot

A 44% bandwidth was achieved.

Double U-Slot Rectangular Patch Antenna, Y. X. Guo, K. M. Luk, and Y. L. Chow,

Electronics Letters, Vol. 34, No. 19, pp. 1805-1806, 1998.

103

Improving Bandwidth: E-Patch


103/132

A modification of the U-slot patch.

A bandwidth of 34% was achieved (40% using a capacitive

washer to compensate for the probe inductance).

A Novel E-shaped Broadband Microstrip Patch Antenna, B. L. Ooi and Q. Shen,

Microwave and Optical Technology Letters, Vol. 27, No. 5, pp. 348-352, 2000.

104

Multi-Band Antennas


104/132

General Principle:

Introduce multiple resonance paths into the antenna. (The

same technique can be used to increase bandwidth viamultiple resonances, if the resonances are closely spaced.)

A multi-band antenna is often more desirable than abroad-band antenna, if multiple narrow-band channels

are to be covered.

105

Multi-Band Antennas: Examples


105/132

u t a d te as a p es

Dual-Band E patch

high-band

low-band

low-band

feed

Dual-Band Patch with Parasitic Strip

low-band

high-band

feed

106

Miniaturization


106/132

High Permittivity Quarter-Wave Patch

PIFA

Capacitive Loading

Slots

Meandering

Note: Miniaturization usually comes at a price of reduced bandwidth.

General rule: The maximum obtainable bandwidth is proportional to the

volume of the patch (based on the Chu limit.)

107

Miniaturization: High Permittivity


107/132

g y

It has about one-fourth the bandwidth of the regular patch.

L

W E-plane

H-plane

1r 4r

L=L/2

W =W/2

(Bandwidth is inversely proportional to the permittivity.)

108

Miniaturization: Quarter-Wave Patch


108/132

L

W E-plane

H-plane

Ez= 0

It has about one-halfthe bandwidth of the regular patch.

0s

r

UQ

P

Neglecting losses:/ 2

s sU U

/ 4r rP P 2Q Q

W E-plane

H-plane

short-circuit

vias

L=L/2

109

Miniaturization: Smaller Quarter-Wave Patch


109/132

L/2

W E-plane

H-plane

It has about one-fourth the bandwidth of the regular patch.

(Bandwidth is proportional to the patch width.)

E-plane

H-plane

L=L/2

W =W/2

110

Miniaturization: Quarter-Wave Patch with Fewer Vias


110/132

L

W E-plane

H-plane

Fewer vias actually gives more miniaturization!

(The edge has a larger inductive impedance.)

W E-plane

H-plane

L

L< L

111

Miniaturization: Planar Inverted F Antenna (PIFA)


111/132

A single shorting plate or via is used.

This antenna can be viewed as a limiting case of the quarter-wave patch, or as

anLCresonator.

feedshorting plate

or viatop view

/ 4dL

112

PIFA with Capacitive Loading


112/132

p g

The capacitive loading allows for the length of the PIFA to be reduced.

feedshorting plate top view

113

Miniaturization: Circular Patch Loaded with Vias


113/132

The patch has a monopole-like pattern

feed c

b

patch metal vias

2a

(Hao Xu Ph.D. dissertation, UH, 2006)

The patch operates in the (0,0) mode, as an LCresonator

114

Example: Circular Patch Loaded with 2 Vias


114/132

Unloaded: Resonance frequency = 5.32 GHz.

0.5 1 1.5 2 2.5

Freque ncy [GHz]

-20

-15

-10

-5

0

S11[db]

0

45

90

135

180

225

270

315

-40

-30

-30

-20

-20

-10

-10

E-theta

E-phi

(miniaturization factor = 4.8)

115

Miniaturization: Slotted Patch


115/132

The slot forces the current to flow through a longer path, increasing

the effective dimensions of the patch.

Top view

linear CP

0o 90o

116

Miniaturization: Meandering


116/132

Meandering forces the current to flow through a longer path,

increasing the effective dimensions of the patch.

feed

via

Meandered quarter-wave patch

feed

via

Meandered PIFA

117

Improving Performance:


117/132

Reducing Surface-Wave Excitation and

Lateral Radiation

Reduced Surface Wave (RSW) Antenna

SIDE VIEW

z

b

hx

shorted annular ring

ground plane feed

a

TOP VIEW

a

o

b

feed

D. R. Jackson, J. T. Williams, A. K. Bhattacharyya, R. Smith, S. J. Buchheit, and S. A.

Long, Microstrip Patch Designs that do Not Excite Surface Waves, IEEE Trans. Antennas

Propagat., vol. 41, No 8, pp. 1026-1037, August 1993.118

RSW: Improved Patterns


118/132

Reducing surface-wave excitation and lateral

radiation reduces edge diffraction.

space-wave radiation (desired)

lateral radiation (undesired)

surface waves (undesired)

diffracted field at edge

119

RSW: Principle of Operation


119/132

0

cosz

V

E h

0 cossV

Mh

s zM n E zE

TM11 mode:

0 11

1, coszE V J k

hJ ka

At edge:

,s zM E a

ax

y

sM

120

RSW: Principle of Operation (cont.)


120/132

0 cossV

Mh

Surface-Wave Excitation:

ax

y

sM

0 00 0

2

1cos zTM jk z

z TM TME A H e

0 01TM TM A AJ a(z > h)

01 0TMJ a Set

121



121/132

ax

y

sM

0 1TM na x

11 1.842x For TM11 mode:

01.842TMa

Patch resonance: 1 1.842k a

Note:0 1TM

k (The RSW patch is too big to be resonant.)122



122/132

01.842TMb

The radius a is chosen to make the patch resonant:

0

0

1 111

1 1

1 1 1 111

TM

TM

k xJ

kJ k a

Y k a k xY

k

SIDE VIEW

z

b

h

x


ground plane feed

a

TOP VIEW

a

o

b

feed

SIDE VIEW

z

b

h

x


ground plane feed

a

TOP VIEW

a

o

b

feed

123

RSW: Reducing Lateral Wave


123/132

0 cossV

Mh

Lateral-Wave Field:

bx

y

sM

0

2

1cos

jkLW

z LWE A e

1 0LWA BJ k b

(z = h)

1 0 0J k b Set124

RSW: Reducing Space Wave


124/132

0

cossV

M h

Space-Wave Field:

ax

y

sM

01

cos jkSP

z SPE A e

1 0SPA CJ k b

(z = h)

1 0 0J k b Set

Assume no substrate outside of patch:

125

RSW: Thin Substrate Result


125/132

For a thin substrate:a

x

y

sM

0 0TM k

The same design reduces both surface-wave and

lateral-wave fields (or space-wave field if there is nosubstrate outside of the patch).

126

Measurements were taken on a 1 m diameter circular ground plane at

RSW: E-plane Radiation Patterns


126/132

-90

-60

-30

0

30

60

90

120

150

180

210

240

-40

-30

-30

-20

-20

-10

-10-90

-60

-30

0

30

60

90

120

150

180

210

240

-40

-30

-30

-20

-20

-10

-10

conventional RSW

Measurements were taken on a 1 m diameter circular ground plane at

1.575 GHz.

Measurement

Theory

127

RSW: Mutual Coupling


127/132

Reducing surface-wave excitation and lateral radiation

reduces mutual coupling.

space-wave radiation

lateral radiation

surface waves

128

Reducing surface-wave excitation and lateral radiation reduces mutual coupling.

RSW: Mutual Coupling (cont.)


128/132

-100

-90

-80

-70

-60

-50

-40

-30

-20

-10

0

0 1 2 3 4 5 6 7 8 9 10

Separation [Wavelengths]

S

12

[dB]

RSW - MeasuredRSW - Theory

Conv - Measured

Conv - Theory

Mutual Coupling Between Reduced Surface-Wave Microstrip Antennas, M. A. Khayat, J.

T. Williams, D. R. Jackson, and S. A. Long, IEEE Trans. Antennas and Propagation, Vol.

48, pp. 1581-1593, Oct. 2000.

E-plane

129

ReferencesG l f b t i t i t


129/132

General references about microstrip antennas:

Microstrip Antenna Design Handbook, R. Garg, P. Bhartia, I. J. Bahl,

and A. Ittipiboon, Editors, Artech House, 2001.

Microstrip Patch Antennas: A Designers Guide, R. B. Waterhouse,

Kluwer Academic Publishers, 2003.

Advances in Microstrip and Printed Antennas, K. F. Lee, Editor, John

Wiley, 1997.

Microstrip Patch Antennas, K. F. Fong Lee and K. M. Luk, ImperialCollege Press, 2011.

Microstrip and Patch Antennas Design, 2ndEd., R. Bancroft, Scitech

Publishing, 2009.

130

References (cont.)


130/132

General references about microstrip antennas (cont.):

Millimeter-Wave Microstrip and Printed Circuit Antennas, P. Bhartia,Artech House, 1991.

The Handbook of Microstrip Antennas (two volume set), J. R. Jamesand P. S. Hall, INSPEC, 1989.

Microstrip Antenna Theory and Design, J. R. James, P. S. Hall, and

C. Wood, INSPEC/IEE, 1981.

Microstrip Antennas: The Analysis and Design of Microstrip Antennas

and Arrays, D. M. Pozar and D. H. Schaubert, Editors, Wiley/IEEE

Press, 1995.

CAD of Microstrip Antennas for Wireless Applications, R. A. Sainati,

Artech House, 1996.

131

References (cont.)


131/132

Computer-Aided Design of Rectangular Microstrip Antennas, D. R.

Jackson, S. A. Long, J. T. Williams, and V. B. Davis, Ch. 5 ofAdvances

in Microstrip and Printed Antennas, K. F. Lee, Editor, John Wiley, 1997.

More information about the CAD formulas presented herefor the rectangular patch may be found in:

Microstrip Antennas, D. R. Jackson, Ch. 7 of Antenna Engineering

Handbook, J. L. Volakis, Editor, McGraw Hill, 2007.

132

References (cont.)


132/132

References devoted to broadband microstrip antennas:

Compact and Broadband Microstrip Antennas, K.-L. Wong,

John Wiley, 2003.

Broadband Microstrip Antennas, G. Kumar and K. P. Ray,

Artech House, 2002.

Broadband Patch Antennas, J.-F. Zurcher and F. E. Gardiol,

Artech House, 1995.

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5 Microstrip Antennas