Post on 12-Apr-2015
description
Overview of Microstrip Antennas
David R. JacksonDept. of ECE
University of Houston
1
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.
2
History of Microstrip Antennas
Invented by Bob Munson in 1972 (but earlier work by Dechamps goes back to1953).
Became popular starting in the 1970s.
G. Deschamps and W. Sichak, “Microstrip Microwave Antennas,” Proc. of Third Symp. on USAF Antenna Research and Development Program, October 18–22, 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 Phased Arrays,” IEEE Trans. Antennas Propagat., vol. AP-22, no. 1 (January 1974): 74–78.
3
Typical Applications
Single element Array
(Photos courtesy of Dr. Rodney B. Waterhouse)
4
Typical Applications (cont.)
Microstrip Antenna Integrated into a System: HIC Antenna Base-Station for 28-43 GHz
filter
MPA
diplexer
LNAPD
K-connector
DC supply Micro-D
connector
microstrip antenna
fiber input with collimating lens
(Photo courtesy of Dr. Rodney B. Waterhouse) 5
Geometry of Rectangular Patch
x
y
h
L
W
Note: L is the resonant dimension. The width W is usually chosen to be larger than L (to get higher bandwidth). However, usually W < 2L. W = 1.5L is typical.
r
6
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 (minimizes higher-order modes and cross-pol.)
x
surface current
7
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 moderate directivity (about 6-8 dB is typical).
8
Disadvantages of Microstrip Antennas Low bandwidth (but can be improved by a variety of
techniques). 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 more severe for thinner substrates.
** Surface-wave losses become more severe for thicker substrates (unless air or foam is used).
9
Basic Principles of Operation The patch acts approximately as a resonant cavity (short
circuit (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 also increases, making the patch currents stronger at resonance. These two effects cancel, allowing the patch to radiate well even for small substrate thicknesses. 10
Thin Substrate Approximation
On patch and ground plane, 0tE ˆ ,zE z E x y
Inside the patch cavity, because of the thin substrate, the electric field vector is approximately independent of z.
Hence ˆ ,zE z E x y
h
,zE x y
11
Thin Substrate Approximation
1
1ˆ ,
1ˆ ,
z
z
H Ej
zE x yj
z E x yj
Magnetic field inside patch cavity:
12
Thin Substrate Approximation (cont.)
1ˆ, ,zH x y z E x y
j
Note: The magnetic field is purely horizontal.(The mode is TMz.)
h
,zE x y
,H x y
13
Magnetic Wall Approximation
On edges of patch,
ˆ 0sJ n
ˆ 0botsJ z H
Hence,
0tH
x
y
n̂ L
WsJ
t̂Also, on bottom surface of patch conductor we have
ˆ nH n H
n̂h
H
(Js is the sum of the top and bottom surface currents.)
14
Magnetic Wall Approximation (cont.)
n̂h
0 ( )tH PMC
Since the magnetic field is approximately
independent of z, we have an approximate PMC condition on the entire vertical edge.
PMC
x
y
n̂ L
WsJ
t̂
15
n̂h
x
y
n̂
L
W t̂Hence,
PMC0zE
n
1ˆ, ,zH x y z E x y
j
ˆ , 0n H x y
ˆ ˆ , 0zn z E x y
ˆˆ , 0zz n E x y
Magnetic Wall Approximation (cont.)
ˆ ˆ ˆˆ ˆ ˆ, , ,z z zn z E x y z n E x y E x y n z
16
Resonance Frequencies2 2 0z zE k E
cos cosz
m x n yE
L W
2 22 0z
m nk E
L W
Hence
2 22 0
m nk
L W
x
y
L
W
From separation of variables:
(TMmn mode)
PMC
,zE x y
17
Resonance Frequencies (cont.)2 2
2 m nk
L W
0 0 rk Recall that
2 f
Hence2 2
2 r
c m nf
L W
0 01/c
x
y
L
W
18
2 2
2mn
r
c m nf
L W
Hence mnf f
(resonance frequency of (m, n) mode)
Resonance Frequencies (cont.)
x
y
L
W
19
(1,0) Mode
This mode is usually used because the radiation pattern has a broadside beam.
10
1
2 r
cf
L
cosz
xE
L
0
1ˆ 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 the x direction.
x
y
L
W
current
20
Basic Properties of Microstrip Antennas
The resonance frequency is controlled by the patch length L 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 length L is one-half of a wavelength in the dielectric:
0 / 2/ 2d
r
L
kL
2 22 m n
kL W
(1,0) mode:
21
The calculation can be improved by adding a “fringing length extension” L to each edge of the patch to get an “effective length” Le .
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.22
Resonance Frequency (cont.)
Hammerstad formula:
0.3 0.264/ 0.412
0.258 0.8
effr
effr
Wh
L hWh
1/ 21 1
1 122 2
eff r rr
h
W
23
Resonance Frequency (cont.)
Note: 0.5L h
This is a good “rule of thumb.”
24
0 0.01 0.02 0.03 0.04 0.05 0.06 0.07
h / 0
0.75
0.8
0.85
0.9
0.95
1
NO
RM
AL
IZE
D F
RE
QU
EN
CY 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
25
Basic Properties of Microstrip Antennas
The bandwidth is directly proportional to substrate thickness h.
However, if h is greater than about 0.05 0 , the probe inductance (for a coaxial feed) becomes large enough so that matching is difficult.
The bandwidth is inversely proportional to r (a foam substrate gives a high bandwidth).
Bandwidth: Substrate effects
26
Basic Properties of Microstrip Antennas
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 = 1.5 L is typical.
27
Basic Properties of Microstrip Antennas
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.
28
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
BA
ND
WID
TH
(%
)
r
2.2
= 10.8
W/ L = 1.5r = 2.2 or 10.8
Results: Bandwidth
The discrete data points are measured values. The solid curves are from a CAD formula.
29
Basic Properties of Microstrip Antennas
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 edges x = 0 or x = L, zero at center of patch.
Resonant Input Resistance
L
W(x0, y0)
Lx
y
30
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
31
Resonant Input Resistance (cont.)
For a given mode, it can be shown that the resonant input resistance is proportional to the square of the cavity-mode field at the feed point.
20 0,in zR E x y
For (1,0) mode:
2 0cosin
xR
L
Lx
W(x0, y0)
y
32
Resonant Input Resistance (cont.)
Hence, for (1,0) mode:
2 0cosin edge
xR R
L
The value of Redge depends strongly on the substrate permittivity. For
a typical patch, it may be about 100-200 Ohms.
Lx
W(x0, y0)
y
33
0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08
h /
0
50
100
150
200
INP
UT
RE
SIS
TAN
CE
(
2.2
r = 10.8
r = 2.2 or 10.8
W/L = 1.5x0 = L/4
Results: Resonant input resistance
The discrete data points are from a CAD formula.
L x
W
(x0, y0)
y
y0 = W/234
Basic Properties of Microstrip Antennas
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
Pe
P
35
Radiation Efficiency (cont.)
surface wave
TM0
cos () pattern
x
y
36
Radiation Efficiency (cont.)
r r
rtot 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,
37
Radiation Efficiency (cont.)
Conductor and dielectric loss is more important for thinner substrates.
Conductor loss increases with frequency (proportional to f ½) due to the skin effect. Conductor loss is usually more important than dielectric loss.
2
1
sR
Rs is the surface resistance
of the metal. The skin depth of the metal is .
38
Radiation Efficiency (cont.)
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.)
39
Radiation Efficiency (cont.)
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.
40
r = 2.2 or 10.8 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 / 0
0
20
40
60
80
100E
FF
ICIE
NC
Y (
%)
exact
CAD
Results: Conductor and dielectric losses are neglected
2.2
10.8
Note: CAD plot uses Pozar formulas41
0 0.02 0.04 0.06 0.08 0.1
h / 0
0
20
40
60
80
100E
FF
ICIE
NC
Y (
%)
= 10.8
2.2
exact
CAD
r
r = 2.2 or 10.8 W/L = 1.5
Results: Accounting for all losses
Note: CAD plot uses Pozar formulas42
Basic Properties of Microstrip Antenna
Radiation Patterns The E-plane pattern is typically broader than the H-
plane pattern.
The truncation of the ground plane will cause edge diffraction, which tends to degrade the pattern by introducing:
rippling in the forward direction back-radiation
Note: Pattern distortion is more severe in the E-plane, due to the angle dependence
of the vertical polarization E and the SW pattern. Both vary as cos ().
43
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” and tends to zero at the horizon due to the presence of the infinite substrate.
44
H-plane pattern
Red: infinite substrate and ground plane
Blue: 1 meter ground plane
-90
-45
0
45
90
135
180
225
-40
-30
-30
-20
-20
-10
-10
Radiation Patterns (cont.)
45
Basic Properties of Microstrip Antennas
Directivity
The directivity is fairly insensitive to the substrate thickness.
The directivity is higher for lower permittivity, because the patch is larger.
46
0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.1
h / 0
0
2
4
6
8
10D
IRE
CT
IVIT
Y
(dB
)
exact
CAD
= 2.2
10.8
r
r = 2.2 or 10.8 W/ L = 1.5
Results: Directivity
47
Approximate CAD Model for Zin
Near the resonance frequency, the patch cavity can be approximately modeled as an RLC circuit.
A probe inductance Lp is added in series, to account for the “probe inductance” of a probe feed.
LpR C
L
Zin
probe patch cavity
48
Approximate CAD Model (cont.)
LpR C
L
01 2 / 1in p
RZ j L
j Q f f
0
RQ
L 1
2BW
Q BW is defined here by
SWR < 2.0.
0 0
12 f
LC
49
Approximate CAD Model (cont.)
in maxR R
Rin max is the input resistance at the resonance of the patch cavity (the frequency that maximizes Rin).
LpR C
L
50
4 4.5 5 5.5 6
FREQUENCY (GHz)
0
10
20
30
40
50
60
70
80
Rin
(
)
CADexact
Results : Input resistance vs. frequency
r = 2.2 W/L = 1.5 L = 3.0 cm
frequency where the input resistance is maximum (f0)
51
Results: Input reactance vs. frequency
r = 2.2 W/L = 1.5
4 4.5 5 5.5 6
FREQUENCY (GHz)
-40
-20
0
20
40
60
80X
in (
)
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
52
Approximate CAD Model (cont.)
0.577216
00
0
2ln
2f
r
X k hk a
(Euler’s 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.
53
Approximate CAD Model (cont.)
00
0
2ln
2f
r
X k hk a
Feed (probe) reactance increases proportionally with substrate thickness h.
Feed reactance increases for smaller probe radius.
54
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
55
CAD Formulas
In the following viewgraphs, CAD formulas for the important properties of the rectangular microstrip antenna will be shown.
56
CAD Formula: Radiation Efficiency
0 0 1 0
1 3 11
/ 16 /
hedr
r
hed s rr d
ee
R Le
h p c W h
where
tand loss tangent of substrate
1
2sR
surface resistance of metal
57
CAD Formula: Radiation Efficiency (cont.)
where
Note: “hed” refers to a unit-amplitude horizontal electric dipole.
2 20 12
0
180hed
spP k h c
1
1
hedsphed
r hedhed hedswsp swhed
sp
Pe
PP PP
3
3 30 12
0
1 160 1hed
swr
P k h c
58
CAD Formula: Radiation Efficiency (cont.)
3
01
1
3 1 11 1
4
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.)
59
1 2
1 2 / 51
r r
c
2 4 2220 2 4 0 2 0
2 2
2 2 0 0
3 11 2
10 560 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
CAD Formula: Radiation Efficiency (cont.)
60
CAD Formula: Radiation Efficiency (cont.)
1
1
hedr hed
swhed
sp
eP
P
Improved formula (due to Pozar)
3/ 22200 0
2 21 0 0 1
1
4 1 ( ) 1 1
rhedsw
r r
xkP
x k h x x
20
1 20
1
r
xx
x
2 2 20 1 0 1 0
0 2 21
21 r r r
r
x
2 20 12
0
180hed
spP k h c
61
CAD Formula: Radiation Efficiency (cont.)
Improved formula (cont.)
0 0tans k h s
0
1 0 20
1tan
cos
k h sk h s
s k h s
1rs
62
CAD Formula: Bandwidth
1
0 0 0
1 1 16 1
/ 32s
d hedr r
R p c h WBW
h L e
BW is defined from the frequency limits f1 and f2 at which SWR = 2.0.
2 1
0
f fBW
f
(multiply by 100 if you want to get %)
63
CAD Formula: Resonant Input Resistance
2 0cosedge
xR R
L
(probe-feed)
00
1
0 0 0
4
1 16 1/ 3
edge
sd hed
r r
L hW
RR p c W h
h L e
64
CAD Formula: Directivity
tanc tan /x x x
where
212
1 1
3tanc
tanr
r
D k hpc k h
65
CAD Formula: Directivity (cont.)
1
3D
p c
For thin substrates:
(The directivity is essentially independent of the substrate thickness.)
66
CAD Formula: Radiation Patterns(based on electric current model)
The origin is at the center of the patch.
L
rεh
infinite GP and substrate
x
The probe is on the x axis.
coss
πxˆJ x
L
æ ö÷ç= ÷ç ÷çè ø
y
L
W E-plane
H-plane
x
(1,0) mode
67
CAD Formula: Radiation Patterns (cont.)
22
sin cos2 2
( , , ) , ,2
2 2 2
y x
hexi i
y x
k W k LWL
E r E rk W k L
0 sin cosxk k
0 sin sinyk k
The “hex” pattern is for a horizontal electric dipole in the x direction, sitting on top of the substrate.
ori
The far-field pattern can be determined by reciprocity.
68
0, , coshexE r E G
0, , sinhexE r E F
where
0
0
2 tan1
tan secTE
k h NF
k h N j N
0
0
2 tan coscos 1
tan cos
TM
r
k h NG
k h N jN
2sinrN
000 4
jk rjE e
r
CAD Formula: Radiation Patterns (cont.)
69
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 due to cancellation effect, but requires more space).
70
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
71
Circular Polarization: Single Feed
Design equations:
0 CPf f
0
11
2xf fQ
0
11
2yf fQ
Top sign for LHCP, bottom sign for RHCP.
in x yR R R Rx and Ry are the resonant input resistances of the two LP (x and y)
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.)
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
73
Circular Polarization: Dual Feed
L
L
x
y
P
P+g/4RHCP
Phase shift realized with delay line
74
Circular Polarization: Dual Feed
Phase shift realized with 90o hybrid (branchline coupler)
L
L
x
y
g/4
LHCP
g/4
0Z0 / 2Z
feed
50 Ohm load
0Z
0Z
75
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
x
y
h
a
r
77
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.)
mnka xa PMC
(nth root of Jm Bessel function)
2mn mn
r
cf x
Dominant mode: TM11
11 112 r
cf x
a
11 1.842x
79
Fringing extension: ae = a + a
11 112 e r
cf x
a
“Long/Shen Formula”:
a PMC
a + a
ln 1.77262r
h aa
h
2
1 ln 1.77262e
r
h aa a
a h
or
Circular Patch: Resonance Frequency (cont.)
80
Circular Patch: Patterns(based on magnetic current model)
1
1
1, cosz
J kE
J ka h
(The edge voltage has a maximum of one volt.)
a
yx
E-plane
H-plane
In patch cavity:
The probe is on the x axis.
2a
rεh
infinite GP and substrate
0 rk k ε=
x
The origin is at the center of the patch.
81
Circular Patch: Patterns (cont.)
01 1 0
0
, , 2 tanc cos sinRz
EE r a k h J k a Q
1 001
0 0
sin, , 2 tanc sin
sinR
z
J k aEE r a k h P
k a
0
2cos 1 cos
tan secTE jN
Pk hN jN
0
2 cos
1tan cos
r
TM
r
jN
Qk h N j
N
( ) ( )tanc tanx x / x=where
2sinrN 82
Circular Patch: Input Resistance
21 0
21
in edge
J kR R
J ka
a
0
83
Circular Patch: Input Resistance (cont.)
1
2edge rsp
R eP
/ 22 2
0 00 0
2 22 21 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 maximum
edge voltage of one volt.
er = radiation efficiency
84
Circular Patch: Input Resistance (cont.)
20
0
( )8sp cP k a I
CAD Formula:
4
3c cI p 6
2
0 20
k
c kk
p k a e
0
2
4
36
48
510
712
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
Feeding Methods
Some of the more common methods for feeding microstrip antennas are shown.
86
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 0cosedge
xR R
L
87
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.)
Recent work has shown that the resonant input resistance varies as
2 02cos
2in
xR A B
L
L
WWf
S
x0
The coefficients A and B depend on the notch width S but (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 Input Impedance of the Inset-Fed Rectangular Microstrip Antenna,” IEEE Trans. Antennas and Propagation, Vol. 56, No. 10, pp. 3314-3318, Oct. 2008. 89
Results for a resonant patch fed on three different substrates.
h = 0.254 cmL / W = 1.5S / Wf = 3
0 / / 2nx x L
L
WWf
S
x0
L
WWf
S
x0
Feeding Methods: Inset Feed (cont.)
0 0.2 0.4 0.6 0.8 10
50
100
150
200
250
300
350
400
450
Rin (
Ohm
s)
xn
r = 1.0
2.42
10.2
εr = 1.00
Wf = 0.616 cm
εr = 2.42
Wf = 0.380 cm
εr = 10.2
Wf = 0.124 cm
Solid lines: CADData points: Ansoft Designer
90
Feeding Methods: Proximity (EMC) Coupling
Advantages: Allows for planar feeding 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
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) Advantages: Allows for planar feeding 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 optimize antenna and feed-circuit performance
Disadvantages: Requires multilayer fabrication Alignment is important for input match
patch
microstrip line
slot
93
Improving Bandwidth
Some of the techniques that have been successfully developed are illustrated here.
(The literature may be consulted for additional designs and modifications.)
94
Improving Bandwidth: Probe Compensation
L-shaped probe:
Capacitive “top hat” on probe:
top view
95
Improving Bandwidth: SSFIP
SSFIP: Strip Slot Foam Inverted Patch (a version of the ACP).
microstrip substrate
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
• 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.
microstrip substrate
driven patch
microstrip lineslot
patch substrates
parasitic patch
97
Improving Bandwidth: Stacked Patches (cont.)
-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
Ret
urn
Lo
ss (
dB
)
MeasuredComputed
98
Stacked patch with ACP feed
0
0.2
0.5 1 2 5 1018
017
016
015
014
0
130120
110 100 90 80 7060
50
4030
2010
0-10
-20-30
-40
-50-60
-70-80-90-100-110-120-1
30
-140
-150
-160
-170
4 GHz
13 GHz
Two extra loops are observed on the Smith chart.
Improving Bandwidth: Stacked Patches (cont.)
99
Improving Bandwidth: Parasitic Patches
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 work was pioneered by K. C. Gupta.
100
Improving Bandwidth: Direct-Coupled Patches
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
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
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
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
General Principle:
Introduce multiple resonance paths into the antenna. (The same technique can be used to increase bandwidth via multiple resonances, if the resonances are closely spaced.)
A multi-band antenna is often more desirable than a broad-band antenna, if multiple narrow-band channels are to be covered.
105
Multi-Band Antennas: Examples
Dual-Band E patch
high-band
low-band
low-band
feed
Dual-Band Patch with Parasitic Strip
low-band
high-band
feed
106
Miniaturization
• 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
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
L
W E-plane
H-plane
Ez = 0
It has about one-half the bandwidth of the regular patch.
0s
r
UQ
P
Neglecting losses:/ 2s sU U
/ 4r rP P2Q Q
W E-plane
H-plane
short-circuit vias
L´=L/2
109
Miniaturization: Smaller Quarter-Wave Patch
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
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)
A single shorting plate or via is used.
This antenna can be viewed as a limiting case of the quarter-wave patch, or as an LC resonator.
feedshorting plate or via top view
/ 4dL
112
PIFA with Capacitive Loading
The capacitive loading allows for the length of the PIFA to be reduced.
feedshorting plate top view
113
Miniaturization: Circular Patch Loaded with Vias
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 LC resonator
114
Example: Circular Patch Loaded with 2 Vias
Unloaded: Resonance frequency = 5.32 GHz.
0.5 1 1.5 2 2.5
Frequency [GHz]
-20
-15
-10
-5
0
S11
[db
]
0
45
90
13
5
18
0
22
5
27
0
31
5
-40
-30
-30
-20
-20
-10
-10
E-thetaE-phi
(miniaturization factor = 4.8)115
Miniaturization: Slotted Patch
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
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:
Reducing Surface-Wave Excitation and Lateral Radiation
Reduced Surface Wave (RSW) Antenna
SIDE VIEW
zb
h
x
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
Reducing surface-wave excitation and lateral radiation reduces edge diffraction.
RSW: Improved Patterns
space-wave radiation (desired)
lateral radiation (undesired)
surface waves (undesired)
diffracted field at edge
119
RSW: Principle of Operation
0 cosz
VE
h
0 coss
VM
h
ˆˆ ˆs 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.)
0 coss
VM
h
Surface-Wave Excitation:
ax
y
sM
0 0
0 0
21cos zTM jk z
z TM TME A H e
0 01TM TMA AJ a
(z > h)
01 0TMJ a Set
121
ax
y
sM
0 1TM na x
11 1.842x For TM11 mode:
01.842TM a
Patch resonance: 1 1.842k a
Note: 0 1TM k (The RSW patch is too big to be resonant.)
RSW: Principle of Operation (cont.)
122
01.842TM b
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
zb
h
x
shorted annular ring
ground plane feed
a
TOP VIEW
a
o
b
feed
SIDE VIEW
zb
h
x
shorted annular ring
ground plane feed
a
TOP VIEW
a
o
b
feed
RSW: Principle of Operation (cont.)
123
RSW: Reducing Lateral Wave
0 coss
VM
h
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
0 coss
VM
h
Space-Wave Field:
ax
y
sM
01
cos jkSPz 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
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 no substrate outside of the patch).
126
-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
conventionalconventional RSWRSW
Measurements were taken on a 1 m diameter circular ground plane at 1.575 GHz.
RSW: E-plane Radiation Patterns
Measurement Theory
127
Reducing surface-wave excitation and lateral radiation reduces mutual coupling.
RSW: Mutual Coupling
space-wave radiation
lateral radiation
surface waves
128
Reducing surface-wave excitation and lateral radiation reduces mutual coupling.
-100
-90
-80
-70
-60
-50
-40
-30
-20
-10
0
0 1 2 3 4 5 6 7 8 9 10
Separation [Wavelengths]
S12
[d
B]
RSW - Measured
RSW - Theory
Conv - Measured
Conv - Theory
RSW: Mutual Coupling (cont.)
“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
ReferencesGeneral 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 Designer’s 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, Imperial College Press, 2011.
Microstrip and Patch Antennas Design, 2nd Ed., R. Bancroft, Scitech Publishing, 2009.
130
References (cont.)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. James and 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
Computer-Aided Design of Rectangular Microstrip Antennas, D. R. Jackson, S. A. Long, J. T. Williams, and V. B. Davis, Ch. 5 of Advances in Microstrip and Printed Antennas, K. F. Lee, Editor, John Wiley, 1997.
More information about the CAD formulas presented here for the rectangular patch may be found in:
References (cont.)
Microstrip Antennas, D. R. Jackson, Ch. 7 of Antenna Engineering Handbook, J. L. Volakis, Editor, McGraw Hill, 2007.
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.
References (cont.)
133