EELE 5333
Chapter 14:
Microstrip Antennas
Session 1
Re-Prepared by
Dr. Mohammed Taha El Astal
Antenna &
Radio
Propagation
Part II:
Antenna
families
Winter 2020
Acknowledgment
This PPT is prepared based mainly on Dr.Talal Skaik’s PPT, David
Jackson’s short course, Balanis Antenna Book
Overview of MicroStrip Antennas
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 dielectric
substrate.
 The patch may be in a variety of shapes, but rectangular and circular are the
most common.
3
History 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
Advantages of MicroStrip Antennas
 Low profile (light weight, low volume, and can
even be “conformal,” i.e. flexible to conform to a
surface).
 Easy
to
fabricate
(use
etching
and
photolithography, can be manufactured in large
quantities in low cost).
 Support both linear and circular polarization, and
dual or triple frequency operations
 Easy to feed (coaxial cable, microstrip line, etc.).
 Easy to incorporate with other microstrip circuit
elements (i.e MICs) and integrate into systems.
 Patterns are somewhat hemispherical, with a
moderate directivity (about 6-8 dB is typical).
 Easy to use in an array to increase the directivity.
5
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 and inversely proportional to
the substrate permittivity.
 Efficiency may be lower than with other
antennas. Efficiency is limited by conductor and
dielectric losses*, and by surface-wave loss**.
 Only used at microwave frequencies and above
(the substrate becomes too large at lower
frequencies).
 Cannot handle extremely large amounts of
power (dielectric breakdown).
6
Applications use MicroStrip Antennas
Applications include:
 Satellite communications
 Microwave communications
 Cell phone antennas
 GPS antennas
7
Microstrip Antenna Integrated into a system
Microstrip
antenna
Filter
DC supply Micro-D
connector
K-connector
LNA
PD
Fiber input with
collimating lens
Diplexer
Microstrip Antenna Integrated into a System: HIC Antenna Base-Station for 28-43 GHz
(Photo courtesy of Dr. Rodney B. Waterhouse)
8
Microstrip (Patch) Antenna, Basic Characteristics
• 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 ≤ 12
9
Cont.’s
• 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
Surface Waves
• 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.
11
Space-wave radiation (desired)
Lateral radiation (undesired)
Diffracted field at edge
Surface waves (undesired)
12
Common Shapes
• 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.
Rectangular
Elliptical
Square
Triangular
Circular
Disk sector
Annular ring
Ring sector
13 Dipole
Feeding Methods
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%).
(Inset Feed)
14
Feeding Methods: Coaxial Line Feed
• 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.
15
Feeding Methods: Coaxial Line Feed
x 
R  Redge cos2  0 
 L 
(The resistance varies as the square
of the modal field shape.)
z
r
h
Advantages:
x
y
 Simple
 Directly compatible with coaxial cables
 Easy to obtain input match by adjusting feed position
 x0 , y0 
W
Disadvantages:
 Significant probe (feed) radiation for thicker substrates
 Significant probe inductance for thicker substrates (limits
bandwidth)
 Not easily compatible with arrays
x
L
16
Feeding Methods: Coaxial Line Feed
17
Dr. Mohammed Taha El Astal
mtastal@iugaza.edu.ps
Dr.mastal@gmail.com
11/2020
EELE 5333
Chapter 14:
Microstrip Antennas
Session 1
Re-Prepared by
Dr. Mohammed Taha El Astal
Antenna &
Radio
Propagation
Part II:
Antenna
families
Winter 2020
Feeding Methods: Aperture Coupling Feed
• 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.
• 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.
21
Feeding Methods: Aperture Coupling Feed
Aperture-coupled Patch (ACP)
Advantages:
 Allows for planar feeding
Slot
 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
Top view
Microstrip
line
 Allows for use of different substrates to optimize
antenna and feed-circuit performance
Patch
Disadvantages:
Slot
 Requires multilayer fabrication
 Alignment is important for input match
Microstrip line
22
Feeding Methods: Proximity 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
(Electromagnetically-coupled 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)
Patch
Microstrip line
Top view
Microstrip
line
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
24
Feeding Methods
25
Feeding Methods
26
Feeding Methods: Quarter Wavelength Transmission Line Feed
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.
• 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.
27
Feeding Methods: Quarter Wavelength Transmission Line Feed
28
MicroStrip Transmission Line Design
 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.
Microstrip transmission line. (a) Geometry. (b) Electric and magnetic field lines.
29
MicroStrip Transmission Line Design
30
MicroStrip Antenna- Methods of Analysis
• The most popular models are the
Easiest, less accurate
 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.
Most complex, most accurate
31
Transmission Line Model
• 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.
32
Transmission Line Model
• 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 .
33
Transmission Line Model
• 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).
34
Transmission Line Model
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.
35
Design Procedures
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:
For a given characterestic impedance Z0 and dielectric constant  r ,
the W / h ratio can be found as:
 8e A
 2A
W e  2

 r 1 
h  2
0.61  
ln( B  1)  0.39 
 B  1  ln(2 B  1) 


 
2


r 
r 

where
for W / h  2
for W / h  2
Z0  r  1  r  1 
0.11 
A=

 0.23 

60
2
r 1 
r 
B
377
2Z 0  r
37
Design Procedures/Example
38
Design Procedures/Example
39
Design Procedures/Example continued
40
Recall….
Recall that there are
two radiating slots.
41
Conductance:
Conductance
Each radiating slot is represented by a parallel equivalent admittance
Y (with conductance G and susceptance B)
42
Input resistance
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 microstripline.
43
44
Dr. Mohammed Taha El Astal
mtastal@iugaza.edu.ps
Dr.mastal@gmail.com
11/2020