ANTENNA LECTURES
BY Mohammed Kamil
MICROSTRIP ANTENNAS
INTRODUCTION
In high-performance aircraft, spacecraft, satellite, and missile applications, where size,
weight, cost, performance, and ease of installation are constraints, low-profile antennas may
be required. Nowadays, there are many other governments and commercial applications, such
as mobile radio and wireless communications that have similar specifications. To meet these
requirements, the microstrip antennas (MSA) can be used.
1. Basic Microstrip Antenna
A microstrip antenna (MSA) basically consists of radiating patch, dielectric substrate,
feed and ground plane. Patch and ground plane made of material such as copper or gold. The
structure of the MSA and its electric field distribution excited in its fundamental mode is
shown in figure below.
𝑡≪ 𝜆
0.003 𝜆 ≤ ℎ ≤ 0.05 𝜆
𝜆/3 < 𝐿 < 𝜆/2
i- Patch Shapes
The radiator should be a material with low ohmic loss and high conductivity at the operating
frequency, such as copper, which can be fixed to a dielectric substrate. The shape can be an
ordinary rectangle, square, ellipse, circle, triangle, ring, fractal, or their variations. More
complex variations on the basic shapes are frequently used to meet particular design demands.
The selection of a particular shape is contingent on specific requirements in terms of
polarization, bandwidth, gain, etc. Generally, the antenna characteristics are defined by the
excited operating modes, which depend on the shape and dimensions of the patch, the
thickness and dielectric constant of the substrate, as well as the feed arrangement.
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ANTENNA LECTURES
BY Mohammed Kamil
ii- Substrate
There are several types of dielectric materials available for substrates. Important parameters
are the dielectric constant (2.2 ≤ 𝜖𝑟 ≤ 16 in RF or microwave bands), the dielectric loss
tangent (0.0001 ≤ 𝑡𝑎𝑛 𝛿 ≤ 0.06), and the cost. Another point which must be taken into
consideration when choosing the substrate material is the effect of the dielectric constant on
the radiation characteristics. A high dielectric constant usually results in low radiation from a
microstrip patch antenna.
2- Microstrip Antenna Feed Techniques
Microstrip patch antennas can be fed by a variety of methods. These methods can be
classified into two categories- contacting and non-contacting. In the contacting method, the
RF power is fed directly to the radiating patch using a connecting element such as a
microstrip line. In the non-contacting scheme, electromagnetic field coupling is done to
transfer power between the microstrip line and the radiating patch.
The four most popular feed techniques used are the microstrip line, coaxial probe
(both contacting schemes), aperture coupling and proximity coupling (both noncontacting schemes).
2-1 Contacting Schemes
i- Microstrip Feed Line
In this type of feed technique, a conducting strip is connected directly to the edge
of the microstrip patch as shown in figure below. The conducting strip is smaller in width
as compared to the patch. This kind of feed arrangement has the advantage that the feed
can be etched on the same substrate to provide a planar structure.
Microstrip line
The purpose of the inset cut in the patch is to match the impedance of the feed line to the
patch without the need for any additional matching element. This is achieved by properly
controlling the inset position. Hence this is an easy feeding scheme since it provides ease of
fabrication and simplicity in modeling as well as impedance matching. However, as the
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ANTENNA LECTURES
BY Mohammed Kamil
thickness of the used dielectric substrate increases, the surface waves and the spurious feed
radiation also increase, which hamper the bandwidth of the antenna. The feed radiation also
leads to undesired cross-polarized radiation.
The impedance of microstrip line can be calculated by:
𝑍0 =
87
√𝜖𝑟 + 1.41
∗ 𝑙𝑛 [
5.98 ∗ ℎ
]
0.8 ∗ 𝐹 + 𝑇
where:
Z0: characteristic impedance of microstrip line (Ω)
h: substrate height
Microstrip line
F: width of microstrip line
Substrat
e
T: microstrip line thickness
Ground
plane
Side view of patch antenna with micro strip feed line
ii- Coaxial Probe
The coaxial feed or probe feed is a very common technique used for feeding Microstrip patch
antennas. The inner conductor of the coaxial cable extends through the dielectric and is
soldered to the radiating patch, while the outer conductor is connected to the ground plane.
(b)
(a)
Probe feed for rectangular patch antenna (a) side view (b) top view
The main advantage of this type of feeding scheme is that the feed can be placed at
any desired location inside the patch in order to match its input impedance. This feed method
is easy to fabricate and has low spurious radiation.
However, the major disadvantage of this type of feeding scheme is that it provides narrow
bandwidth and is difficult to model since a hole has to be drilled in the substrate and the
connector protrudes outside the ground plane, thus not making it completely planar for thick
substrates. Also, for thicker substrates, the increased probe length makes the input impedance
more inductive and leads to match problems.
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ANTENNA LECTURES
BY Mohammed Kamil
2-2 Non-Contacting Schemes
i- Aperture Coupled Feed
Aperture coupled feed is more complex and difficult to fabricate than to other types. It
consists of two substrates separated by ground plane. On the bottom of the lower substrate,
there is a microstrip fed line whose energy is coupled to the patch through slot on the ground
plane. High dielectric material is used for bottom substrate and thick and low dielectric
constant material is used for the top substrate. A model of Aperture Coupled is shown in
figure below.
In this type of feeding the substrate electrical parameters, feed line width and especially slot
size and position can be used to optimize the design and it has the spurious radiation. On the
other side, it provides the narrow bandwidth.
ii- Proximity Coupled Feed:
This type of feed technique is also called the “electromagnetic coupling scheme”.
Figure below shows that two dielectric substrates are used in such a way that the feed line is
between the two substrates and the radiating patch is on the top of the upper substrate.
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ANTENNA LECTURES
BY Mohammed Kamil
The main advantage of this feed technique is that it eliminates the spurious feed
radiation and provides high bandwidth due to the overall increase in the thickness of the
microstrip patch antenna. This scheme also provides choices between two different dielectric
media; one for the patch and the other for the feed line to optimize the individual
performances. Matching can be made by controlling the length of the feed line and the widthto-line ratio of the patch. The main disadvantage of this feed scheme is that it is difficult to
be fabricated because of the two dielectric layers that need proper alignment. Also, there is an
increase in the overall thickness of the antenna.
3- 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. We begin with the transmission-line model because it is easier to illustrate.
A. Fringing Effects
Because the dimensions of the patch are finite along the length and width, the fields at
the edges of the patch undergo fringing.
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. Since for microstrip antennas 𝐿/ℎ ≫ 1, fringing is reduced.
The initial values of the effective dielectric constant are given by:
𝝐𝒓 𝒆𝒇𝒇
𝝐𝒓 + 𝟏 𝝐𝒓 − 𝟏
𝒉 −𝟏/𝟐
=
+
[𝟏 + 𝟏𝟐 ]
𝟐
𝟐
𝑾
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𝑾/𝒉 > 𝟏
ANTENNA LECTURES
BY Mohammed Kamil
B. Effective Length, Resonant Frequency, and Effective Width
Because of the fringing effects, electrically the patch of the microstrip antenna looks
greater than its physical dimensions.
𝑾
(𝝐𝒓 𝒆𝒇𝒇 + 𝟎. 𝟑) ( 𝒉 + 𝟎. 𝟐𝟔𝟒)
∆𝑳
= 𝟎. 𝟒𝟏𝟐
𝑾
𝒉
− 𝟎. 𝟐𝟓𝟖) ( + 𝟎. 𝟖)
(𝝐
𝒓 𝒆𝒇𝒇
𝒉
𝑾
∆𝑳 = 𝟎. 𝟒𝟏𝟐 𝒉
(𝝐𝒓 𝒆𝒇𝒇 + 𝟎. 𝟑) ( 𝒉 + 𝟎. 𝟐𝟔𝟒)
𝑾
(𝝐𝒓 𝒆𝒇𝒇 − 𝟎. 𝟐𝟓𝟖) ( 𝒉 + 𝟎. 𝟖)
The length of the patch has been extended by ∆𝑳 on each side; the effective length of the
patch is:
𝑳𝒆𝒇𝒇 = 𝑳 + 𝟐∆𝑳
The resonant frequency of the microstrip antenna is a function of its length. Usually it is given
by:
𝒇𝒓 =
𝒗𝟎
𝟐𝑳√𝝐𝒓
The resonant frequency includes edge effects and should be computed using:
𝒇𝒓 =
𝒗𝟎
𝟐𝑳𝒆𝒇𝒇 √𝝐𝒓 𝒆𝒇𝒇
→ 𝑳𝒆𝒇𝒇 =
𝒗𝟎
𝟐𝒇𝒓 √𝝐𝒓 𝒆𝒇𝒇
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ANTENNA LECTURES
BY Mohammed Kamil
C. Design
1- For an efficient radiator, a practical width that leads to good radiation efficiencies is:
𝑾=
𝒗𝟎
𝟐
√
𝟐𝒇𝒓 𝝐𝒓 + 𝟏
2- Determine the effective dielectric constant of the microstrip antenna
3- Once W is found to determine the extension of the length ∆L
4- The actual length of the patch can now be determined by:
𝑳 = 𝑳𝒆𝒇𝒇 − 𝟐∆𝑳 =
𝒗𝟎
− 𝟐∆𝑳
𝟐𝒇𝒓 √𝝐𝒓 𝒆𝒇𝒇
Example: Design a rectangular microstrip antenna using a substrate (RT/duroid 5880) with
dielectric constant of 2.2, h = 0.1588 cm so as to resonate at 10 GHz.
Solution:
The width W of the patch is found by:
v0
2
3 ∗ 108
2
√
√
W=
=
= 0.01186 m = 1.186 cm (0.467 in)
2fr ϵr + 1 2 ∗ 10 ∗ 109 2.2 + 1
The effective dielectric constant of the patch is found by:
ϵr eff
ϵr + 1 ϵr − 1
h −1/2 2.2 + 1 2.2 − 1
0.1588 −1/2
=
+
[1 + 12 ]
=
+
[1 + 12
]
= 1.972
2
2
W
2
2
1.186
The extended incremental length of the patch ∆𝐿 is found by:
W
∆L = 0.412 h
(ϵr eff + 0.3) ( + 0.264)
h
W
(ϵr eff − 0.258) ( h + 0.8)
∆L = 0.412 ∗ 0.1588
1.186
(1.972 + 0.3) (
+ 0.264)
0.1588
1.186
(1.972 − 0.258) (
+ 0.8)
0.1588
= 0.081 cm (0.032 in)
The effective length of the patch is found by:
Leff =
v0
2fr √ϵr eff
=
3 ∗ 108
2 ∗ 10 ∗ 109 √1.972
= 0.01068 m = 1.068 cm (0.421 in)
The actual length L of the patch is found by:
L = Leff − 2∆L = 1.068 − 2 ∗ 0.081 = 0.906 cm (0.375 in)
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ANTENNA LECTURES
BY Mohammed Kamil
H.W
Design a rectangular microstrip antenna so that it will resonate at 2 GHz. The idealistic
lossless substrate (RT/Duroid 6010.2) has a dielectric constant of 10.2 and a h = 0.127 cm.
4- Advantages and Disadvantages of MSA
Microstrip patch antennas are increasing in popularity for use in wireless applications
due to their low-profile structure. Therefore, they are extremely compatible for embedded
antennas in handheld wireless devices such as cellular phones, pagers and others. The
telemetry and communication antennas on missiles need to be thin and conformal and are
often Microstrip patch antennas. Another area where they have been used successfully is in
satellite communication.
Some of their principal advantages are given below:
•
Light weight and low volume.
•
Low profile planar configuration which can be easily made conformal to host surface.
•
Low fabrication cost.
•
Support both, linear and circular polarization.
•
Can be easily integrated with microwave integrated circuits.
•
Capable of dual and triple frequency operation.
•
Mechanically robust when mounted on rigid surfaces.
Microstrip patch antennas suffer from a number of disadvantages as compared to
conventional antennas.
Some of their major disadvantages are given below:
•
Narrow bandwidth.
•
Low efficiency.
•
Low gain.
•
Low power handling capacity.
•
Surface wave excitation.
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