Embed Size (px)
Citation preview
EELE 5333
Antenna & Radio
Propagation
Part II:
Antenna families
Winter 2020
Re-Prepared by
Dr. Mohammed Taha El Astal
Chapter 14:
Microstrip Antennas
Session 1
Acknowledgment
This PPT is prepared based mainly on Dr.Talal Skaik’s PPT, David Jackson’s short course, Balanis Antenna Book
Also called “patch antennas” One of the most useful antennas at microwave frequencies (f > 1 GHz).
It usually consists of a metal “patch” on top of a grounded dielectricsubstrate.
The patch may be in a variety of shapes, but rectangular and circular are themost common.
3
Overview of MicroStrip Antennas
Invented by Bob Munson in 1972 (but earlier work by Deschamps goes back to 1953).
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.
4
History of MicroStrip Antennas
Low profile (light weight, low volume, and caneven be “conformal,” i.e. flexible to conform to asurface).
Easy to fabricate (use etching andphotolithography, can be manufactured in largequantities in low cost).
Support both linear and circular polarization, anddual or triple frequency operations
Easy to feed (coaxial cable, microstrip line, etc.).
Easy to incorporate with other microstrip circuitelements (i.e MICs) and integrate into systems.
Patterns are somewhat hemispherical, with amoderate directivity (about 6-8 dB is typical).
Easy to use in an array to increase the directivity. 5
Advantages of MicroStrip Antennas
Low bandwidth (but can be improved by a varietyof techniques). Bandwidths of a few percent aretypical. Bandwidth is roughly proportional to thesubstrate thickness and inversely proportional tothe substrate permittivity.
Efficiency may be lower than with otherantennas. Efficiency is limited by conductor anddielectric losses*, and by surface-wave loss**.
Only used at microwave frequencies and above(the substrate becomes too large at lowerfrequencies).
Cannot handle extremely large amounts ofpower (dielectric breakdown).
6
DisAdvantages of MicroStrip Antennas
Satellite communications
Microwave communications
Cell phone antennas
GPS antennas
7
Applications include:
Applications use MicroStrip Antennas
Microstrip Antenna Integrated into a System: HIC Antenna Base-Station for 28-43 GHz
Filter
Diplexer
LNA
PD
K-connector
DC supply Micro-D
connector
Microstrip
antenna
Fiber input with
collimating lens
(Photo courtesy of Dr. Rodney B. Waterhouse)8
Microstrip Antenna Integrated into a system
• Microstrip antennas, consist of a very thin (t <<λ0,
where λ0 is the free-space wavelength) metallic
strip (patch) placed above a ground plane a
distance h (h <<λ0). Usually:
0.003λ0 ≤ h ≤ 0.05λ0
• For a rectangular patch, the length L of the
element is usually:
λ0/3 < L < λ0/2
• The strip (patch) and the ground plane are
separated by a dielectric sheet (referred to as the
substrate)
• There are numerous substrates that can be used
for the design of microstrip antennas, and their
dielectric constants are usually in the range:
2.2 ≤ ϵr ≤ 129
Microstrip (Patch) Antenna, Basic Characteristics
• The substrates that are most desirable for good antenna performance are
thick substrates whose dielectric constant ϵr is in the lower end of the range
because they provide better efficiency, larger bandwidth, but at the expense
of larger element size.
• In thicker dielectric substrates surface waves are excited and they
deteriorate the antenna efficiency and generate cross-polarized fields which
spoil the antenna characteristics and polarization purity.
• In order to design a compact Microstrip patch antenna, higher dielectric
constants must be used which are less efficient and result in narrower
bandwidth.
• Hence a compromise must be reached between antenna dimensions and
antenna performance.
10
Cont.’s
11
• The waves transmitted slightly downward, having elevation angles θ between
π/2 and π –sin-1 (1/√εr), meet the ground plane, which reflects them, and then
meet the dielectric-to-air boundary, which also reflects them (total reflection
condition).
• Surface waves reaching the outer boundaries of an open microstrip structure
are reflected and diffracted by the edges. The diffracted waves provide an
additional contribution to radiation, degrading the antenna pattern by raising
the side lobe and the cross polarization levels.
Surface Waves
Space-wave radiation (desired)
Lateral radiation (undesired)
Surface waves (undesired)
Diffracted field at edge
12
• The radiating patch may be square, rectangular, thin strip, circular, elliptical,
triangular, or any other configuration.
• Square, rectangular, and circular are the most common because of ease of
analysis and fabrication, and their attractive radiation characteristics.
13
Common Shapes
Rectangular Square Circular
Elliptical
Annular ring
TriangularDipoleDisk sector Ring sector
The most popular feeding methods are: Microstrip line, coaxial probe,
aperture coupling and proximity coupling.
Microstrip Line Feed
Easy to fabricate, simple to match by controlling the inset position.
Narrow bandwidth (typically 2–5%).
14
(Inset Feed)
Feeding Methods
15
• The inner conductor of the coax is
attached to the radiation patch
while the outer conductor is
connected to the ground plane.
• The coaxial probe feed is also easy
to fabricate and match. However,
it also has narrow bandwidth and it
is more difficult to model.
• For thicker substrates, the increased
probe length makes the input
impedance more inductive, leading
to matching problems.
Feeding Methods: Coaxial Line Feed
16
Advantages:
Simple
Directly compatible with coaxial cables
Easy to obtain input match by adjusting feed position
Disadvantages:
Significant probe (feed) radiation for thicker substrates
Significant probe inductance for thicker substrates (limits
bandwidth)
Not easily compatible with arrays
2 0cosedge
xR R
L
xr h
z
x
y
L
W 0 0,x y
(The resistance varies as the square of the modal field shape.)
Feeding Methods: Coaxial Line Feed
17
Feeding Methods: Coaxial Line Feed
EELE 5333
Antenna & Radio
Propagation
Part II:
Antenna families
Winter 2020
Re-Prepared by
Dr. Mohammed Taha El Astal
Chapter 14:
Microstrip Antennas
Session 1
• Two substrates with ground plane in middle.
• Microstrip feed line and radiating patch are on both sides of the ground
plane, the coupling aperture is in the ground plane.
20
Feeding Methods: Aperture Coupling Feed
• The energy of the micro-strip feed line is
coupled to the patch through a slot
(aperture) on the ground plane separating
the two substrates.
21
Feeding Methods: Aperture Coupling Feed
• The amount of coupling from the feed line
to the patch is determined by the shape,
size and location of the aperture.
• The ground plane between the substrates also
isolates the feed from the radiating element
and minimizes spurious radiation.
Advantages:
Allows for planar feeding
Feed-line radiation is isolated from patch radiation
Higher bandwidth is possible since probe inductance is
eliminated (allowing for a thick substrate), and also 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
22
Aperture-coupled Patch (ACP)
Top view
Slot
Microstrip
line
Feeding Methods: Aperture Coupling Feed
• Two dielectric substrates are used such that the feed line is between the two
substrates and the radiating patch is on top of the upper substrate.
• Matching can be achieved by controlling the length of the feed line and the
width-to-line ratio of the patch.
23
Feeding Methods: Proximity Coupling Feed
Advantages:
Allows for planar feeding
it eliminates spurious feed radiation and provides very high bandwidth (as high as 13%).
Less line radiation compared to microstrip feed (the line is closer to the ground plane)
Can allow for higher bandwidth (no probe inductance, so substrate can be thicker)
Disadvantages:
it is difficult to fabricate because of the two dielectric layers which need proper alignment in addition to
the increase in the overall thickness of the antenna, requires multilayer fabrication
Alignment is important for input match
Patch
Microstrip line
24
(Electromagnetically-coupled Feed)
Top viewMicrostrip
line
Feeding Methods: Proximity Coupling Feed
25
Feeding Methods
26
Feeding Methods
The microstrip antenna can also be matched to a transmission line of
characteristic impedance Z0 by using a quarter-wavelength transmission line
of characteristic impedance Z1.
27
Feeding Methods: Quarter Wavelength Transmission Line Feed
• The input impedance viewed from the
beginning of the quarter-wavelength
line is:
Zin=Z0=Z12/ZA
The parameter Z1 can be altered by changing
the width of the quarter-wavelength strip. The
wider the strip is, the lower the
characteristic impedance (Z0) is for that
section of line.
28
Feeding Methods: Quarter Wavelength Transmission Line Feed
Microstrip Line Design (For microstrip feed line and λ/4 –Line)
Microstrip line consists of a conductor of width W printed on a grounded
dielectric substrate of thickness h and relative permittivity εr.
29
Microstrip transmission line. (a) Geometry. (b) Electric and magnetic field lines.
MicroStrip Transmission Line Design
30
MicroStrip Transmission Line Design
31
• The most popular models are the
transmission-line,
cavity,
and full wave (which include primarily
integral equations/Moment Method).
• Since they are the most popular and practical, in
this chapter the only two patch configurations that
will be considered are the rectangular and
circular.
MicroStrip Antenna- Methods of Analysis
Most complex, most accurate
Easiest, less accurate
32
• Basically the transmission-line model represents the microstrip antenna by two
slots, separated by a low-impedance Zc transmission line of length L.
• A microstrip line is a nonhomogeneous line of two dielectrics; typically the
substrate and air.
• Most of the electric field lines reside in the substrate and parts of some lines
exist in air.
Transmission Line Model
33
• Since some of the waves travel in the substrate
and some in air, an effective dielectric constant
ϵreff is introduced to account for fringing and
the wave propagation in the line.
• ϵeff can be interpreted as the dielectric constant
of a homogeneous medium that replaces the air
and dielectric regions of the microstrip
• Effective dielectric constant has values in the
range of 1 < ϵreff < ϵr .
Transmission Line Model
34
• Because of the fringing effects, the patch of the microstrip antenna looks
(electrically ) greater than its physical dimensions.
• The dimensions of the patch along its length have been extended on each end by a
distance ∆L, which is a function of the effective dielectric constant ϵreff and the
width-to-height ratio (W/h).
Transmission Line Model
35
Since the length of the patch has been extended by ∆L on each side, the
effective length of the patch is now (L = λ/2 for dominant TM010 mode
with no fringing):
For the dominant TM010 mode, the resonant frequency of the microstrip
antenna is a function of its length:
where vo=3x108 is speed of light.
Transmission Line Model
Specified information: The dielectric constant of the substrate (ϵr ), the resonant
frequency (fr ), and the height of the substrate h.
(1)-A practical width that leads to good radiation efficiencies:
(2)- Determine the effective dielectric constant of the microstrip antenna using
(3)- Determine the extension of the length ∆L using
(4)- The actual length of the patch can now be determined by
36
Design Procedures
(5)- The width of a microstrip feed line is:
37
0 r
2
For a given characterestic impedance Z and dielectric constant ,
the / ratio can be found as:
8 / 2
2
12 0.611 ln(2 1) ln( 1) 0.39 / 2
2
A
A
r
r r
W h
efor W h
eW
hB B B for W h
where
0
0
1 1 0.11A= 0.23
60 2 1
377
2
r r
r r
r
Z
BZ
Design Procedures
38
Design Procedures/Example
39
Design Procedures/Example
40
Design Procedures/Example continued
41
Recall that there are two radiating slots.
Recall….
Conductance
Each radiating slot is represented by a parallel equivalent admittance Y (with conductance G and susceptance B)
42
Conductance:
43
The resonant input resistance can be
changed by using an inset feed, recessed a
distance y0 from slot #1.
This technique can be used effectively to
match the patch antenna using a microstrip-
line.
Input resistance
44
