Lecture 10
Microstrip Antenna
Antenna
Engineering and
Design
Assoc. Prof. Anwer Sayed
2024 – 2025
2nd Semester
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Lecture 10 Outcomes
By the end of the lecture you will learn how to extract the
microstrip antenna parameters.
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Let’s enjoy discovering the secrets of Antenna
Course!
I’m hoping to generate insight and interest – not pages of equations!
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Microstrip Antennas
• A microstrip antenna (also known as a printed antenna) usually means an antenna fabricated using microstrip
techniques on a printed circuit board (PCB).
• They are mostly used at microwave frequencies (300 MHz-300 GHz).
• An individual microstrip antenna consists of a patch of metal foil of various shapes (a patch antenna) on the
surface of a PCB, with a metal foil ground plane on the other side of the board.
• Microstrip or patch antennas are becoming increasingly useful because they can be printed directly onto a
circuit board. Microstrip antennas are becoming very widespread within the mobile phone market.
• Patch antennas are:
o Low cost
o Low weight
o Low profile
o Easily to be fabricated and installed.
• In high-performance aircraft, spacecraft, mobile communications, satellite, and missile applications,
Microstrip antennas may be required.
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Microstrip Antennas
• These antennas are low profile, conformable to planar and nonplanar surfaces, simple and inexpensive to
manufacture using modern printed-circuit technology, mechanically robust when mounted on rigid surfaces,
compatible with MMIC designs, and when the particular patch shape and mode are selected, they are very
versatile in terms of resonant frequency, polarization, pattern, and impedance.
• In addition, by adding loads between the patch and the ground plane, such as pins and varactor diodes, adaptive
elements with variable resonant frequency, impedance, polarization, and pattern can be designed.
• Major operational disadvantages of microstrip antennas are
 Low power
 High Q (sometimes in excess of 100),
 Poor polarization purity
 Poor scan performance
 Spurious feed radiation and very narrow frequency bandwidth, which is typically only a fraction of
a percent or at most a few percent (3-5%)
 Low Efficiency at higher frequencies (MMW frequencies) due to dielectric and conductor loss
• In some applications, such as in government security systems, narrow bandwidths are desirable.
• However, there are methods, such as increasing the height of the substrate, that can be used to extend the
efficiency (to as large as 90 percent if surface waves are not included) and bandwidth (up to about 35 percent)
• Stacking, as well as other methods, of microstrip elements can also be used to increase the bandwidth
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Basic Characteristics of Microstrip Antennas
• Microstrip antennas received considerable attention
starting in the 1970s, although the idea of a microstrip
antenna can be traced to 1953 [1] and a patent in 1955 [2].
• Microstrip antennas, consist of a very thin (t << λo, where
λo is the free-space wavelength) metallic strip (patch)
placed a small fraction of a wavelength (h << λo, usually
0.003λo ≤ h ≤ 0.05λo) above a ground plane.
• The microstrip patch is designed so its pattern maximum is
normal to the patch (broadside radiator).
• This is accomplished by properly choosing the mode (field
configuration) of excitation beneath the patch.
• End-fire radiation can also be accomplished by judicious
mode selection.
• For a rectangular patch, the length L of the element is
usually λo/3 < L < λo/2. The strip (patch) and the ground
plane are separated by a dielectric sheet (referred to as the
substrate).
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Basic Characteristics of Microstrip Antennas
• There are numerous substrates that can be used for the design of microstrip antennas, and their dielectric
constants are usually in the range of 2.2 ≤ ϵr ≤ 12.
• The ones that are most desirable for good antenna performance are thick substrates whose dielectric constant
is in the lower end of the range because they provide better efficiency, larger bandwidth, loosely bound fields
for radiation into space, but at the expense of larger element size.
• Thin substrates with higher dielectric constants are desirable for microwave circuitry because they require
tightly bound fields to minimize undesired radiation and coupling, and lead to smaller element sizes;
however, because of their greater losses, they are less efficient and have relatively smaller bandwidths.
• Often microstrip antennas are also
referred to as patch antennas.
• The radiating elements and the feed lines
are usually photoetched on the dielectric
substrate.
• The radiating patch may be square,
rectangular, thin strip (dipole), circular,
elliptical, triangular, or any other
configuration.
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Feeding Methods
• There are many configurations that can be used to feed microstrip antennas.
• The four most popular are the microstrip line, coaxial probe, aperture coupling, and proximity coupling.
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Feeding Methods
• The microstrip feed line is also a conducting strip, usually of much smaller width compared to the patch. The
microstrip-line feed is easy to fabricate, simple to match by controlling the inset position and rather simple to
model.
• However as the substrate thickness increases, surface waves and spurious feed radiation increase, which for
practical designs limit the bandwidth (typically 2–5%).
• Coaxial-line feeds, where the inner conductor of the coax is attached to the radiation patch while the outer
conductor is connected to the ground plane, are also widely used. The coaxial probe feed is also easy to fabricate
and match, and it has low spurious radiation.
• However, it also has narrow bandwidth and it is more difficult to model, especially for thick substrates (h >
0.02λo).
• The aperture coupling is the most difficult of all four to fabricate and it also has narrow bandwidth. However, it
is somewhat easier to model and has moderate spurious radiation.
• The aperture coupling consists of two substrates separated by a ground plane. On the bottom side of the lower
substrate there is a microstrip feed line whose energy is coupled to the patch through a slot on the ground plane
separating the two substrates.
• The proximity coupling has the largest bandwidth (as high as 13 percent), is somewhat easy to model and has
low spurious radiation. However its fabrication is somewhat more difficult. The length of the feeding stub and the
width-to-line ratio of the patch can be used to control the match.
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RECTANGULAR PATCH
• The rectangular patch is by far the most widely used configuration.
• It is very easy to analyze using both the transmission-line and cavity models, which are most accurate for thin
substrates.
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.
• Because the dimensions of the patch are finite along the
length and width, the fields at the edges of the patch
undergo fringing. This is illustrated along the length for
the two radiating slots of the microstrip antenna. The
same applies along the width.
• The amount of fringing is a function of the dimensions of
the patch and the height of the substrate. For the principal
E-plane (xy-plane) fringing is a function of the ratio of
the length of the patch L to the height h of the substrate
(L/h) and the dielectric constant ϵr of the substrate.
• Fringing in this case makes the microstrip line look wider
electrically compared to its physical dimensions.
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RECTANGULAR PATCH
• Fringing in this case makes the microstrip line look wider electrically compared to its physical dimensions.
• 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.
• The effective dielectric constant are referred to as the static values, and they are given by
• A very popular and practical approximate relation for the
normalized extension of the length is
• Since the length of the patch has been extended by 3L on each side, the effective length of the patch is now (L = λg/2
for dominant TM010 mode with no fringing)
𝝀𝒈 = 𝝀𝒐 / 𝝐𝒓𝒆𝒇𝒇
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RECTANGULAR PATCH
• For the dominant TM010 mode, the resonant frequency of the microstrip antenna is a function of its length.
Usually it is given by
Design procedure
• For an efficient radiator, a practical width that leads to good radiation of efficiencies
where υ0 is the free-space velocity of light.
• The actual length of the patch can now be determined by
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RECTANGULAR PATCH
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RECTANGULAR PATCH
• Each radiating slot is represented by a parallel equivalent admittance Y (with conductance G and susceptance B).
• The slots are labeled as #1 and #2. The equivalent admittance of slot #1, based on an infinitely wide, uniform slot
is given by
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RECTANGULAR PATCH
where for a slot of finite width W
• Since slot #2 is identical to slot #1, its equivalent admittance is
• The total admittance at slot #1 (input admittance) is obtained by transferring the admittance of slot #2 from the
output terminals to input terminals using the admittance transformation equation of transmission lines.
• Ideally the two slots should be separated by λ/2 where λ is the wavelength in the dielectric (substrate).
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RECTANGULAR PATCH
• Therefore the total resonant input admittance is real and is given by
• Since the total input admittance is real, the resonant input impedance is also real, or
• The resonant input resistance does not take into account mutual effects between the slots. This can be
accomplished by modifying
• Using modal expansion analysis, the input resistance for the inset feed is given approximately
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RECTANGULAR PATCH
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RECTANGULAR PATCH
Cavity Model
• Microstrip antennas resemble dielectric-loaded
cavities, and they exhibit higher order resonances.
• The normalized fields within the dielectric substrate
(between the patch and the ground plane) can be found
more accurately by treating that region as a cavity
bounded by electric conductors (above and below it)
and by magnetic walls (to simulate an open circuit)
along the perimeter of the patch.
• the mode with the lowest frequency (dominant mode)
is the TMx010 whose resonant frequency is given by:
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RECTANGULAR PATCH
• The presence of the ground plane can be taken into account by image theory which will double the equivalent
magnetic current density. Therefore the final equivalent is doubling magnetic current density or
• The two slots form a two-element array with a spacing of λ/2
between the elements.
• It undergoes a phase reversal along the length but it is
uniform along its width. The phase reversal along the length
is necessary for the antenna to have broadside radiation
characteristics.
• It will be shown here that in a direction perpendicular to the
ground plane the components of the field add in phase and
give a maximum radiation normal to the patch; thus it is a
broadside antenna.
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RECTANGULAR PATCH
• Since the current densities on each wall are of the same magnitude but of opposite direction, the fields
radiated by these two slots cancel each other in the principal H-plane.
• Also since corresponding slots on opposite walls are 180◦ out of phase, the corresponding radiations cancel
each other in the principal E-plane.
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RECTANGULAR PATCH
Radiation Patterns of TM010
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Any Questions ?