An Overview Of Microstrip Antenna

Abstract

An astonishing growth in the area of broadband communication has been opened a access in the wide extent research and application of microstrip antenna. The adaptability of microstrip antennas added new aspect to it. The research article presents an introduction and overview of the Microstrip antenna. The features, advantages, limitations and the different types of feeding techniques are also discussed. The methods for the modelling of the microstrip antenna are also conversed. It also presents idea about the classification of antennas.

Full text

HCTL Open International Journal of Technology Innovations and Research (IJTIR)
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Dr. Ranjan Mishra, Page 1
An Overview of Microstrip Antenna.
An Overview of Microstrip
Antenna
Dr. Ranjan Mishra1
rmishra@ddn.upes.ac.in
Abstract
An astonishing growth in the area of broadband communication has been opened a
access in the wide extent research and application of microstrip antenna. The
adaptability of microstrip antennas added new aspect to it. The research article
presents an introduction and overview of the Microstrip antenna. The features,
advantages, limitations and the different types of feeding techniques are also
discussed. The methods for the modelling of the microstrip antenna are also
conversed. It also presents idea about the classification of antennas.
Keywords
Microstrip Antenna; Feeding techniques; Introduction; Overview.
Introduction
The magic and mystery of Radio waves have captured imaginations from the earliest
speculations of thinkers [1] to the present day. The marvel of Radio is taken for granted in
the world of pervasive and instantaneous wireless communications. All around us quiver
vibrations in the conveying voices, images, data and information. The magic of radio
plucks these vibrations out of air and recover the original data. The wand responsible for
this wizardry is the antenna. An antenna makes a Radio wireless device possible. A
transmit antenna takes signal from a transmission line, converts them into
electromagnetic waves, and broadcasts them into free space. On the other end of the
link, a receive antenna collects the incident electromagnetic waves and converts them
back to the signals.
Overview
An antenna can be classified in number of ways [2]. The simplest of the antenna
configuration is the wire antenna. This oldest structure is regarded as the simpler and
cheaper. It is also the most versatile antenna for many applications.
1Associate Professor, Department of Electronics, Instrumentation and Control Engineering, College
of Engineering Studies, University of Petroleum and Energy Studies, Dehradun, India.

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An Overview of Microstrip Antenna.
Classification in Terms of Wire Structure
In wire antenna there are three further classifications.
i. Dipole or Linear Wire Antenna - An antenna in the form of straight wire is
termed as dipole or linear wire antennas.
ii. Loop Antenna - An antenna where the single wire is used to form a loop it is
termed as loop antenna. The loop can take any form, but generally circular and
square loop is mostly used for the ease of analysis and construction.
iii. Helical Antenna - An antenna where the wire is bend in a helical shape. It is
also termed as helix antenna.
Classification in Terms of Aperture
One more classification is based on the aperture antenna, where the surface is a two
dimensional one. The different antenna comes under it are as follows:
i. Horn Antenna - An antenna constructed from the waveguides are termed as
horn antenna. A waveguide is a hollow metallic tube used. The shape of the
horn antenna depends on the cross section of this waveguide through which
the waves propagates when one end of the waveguide is trapped to a large
opening and its acts as an antenna. The shape of the antenna is either
rectangular or circular in shape.
ii. Parabolic Disc Antenna - An antenna having a shape of parabolic or disc
shape. It is mostly used for very long distance reception or space reception.
They are also termed as reflector antenna.
iii. Microstrip Antenna - An antenna having radiating patch etched on the
substrate. The other side of the substrate is a metallic ground plane.
iv. Array Antenna - An antenna formed by multi elements. They are used at times
when a single element is unable to give the desired features. The desired
features can be achieved by using multi-element in the antenna structure.
Classification in Terms of Frequency
An antenna is also classified in terms of frequency, aperture, polarization and radiation.
Out of these classes the frequency is the most important one. Therefore in term of
frequency specific class, the antenna classification is as follow:
i. Very High Frequency (VHF) & Ultra High Frequency (UHF) Antennas: Yagi-
Uda antennas, log periodic antennas, Helical antennas, Panel antennas,
Corner reflector antennas, parabolic antennas, discone antennas are some of
the antenna who are operating in this frequency. The range of VHF is from 30

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An Overview of Microstrip Antenna.
MHz to 300 MHz, and that of UHF is from 300 MHz to 3 GHz.
ii. Super High Frequency (SHF) & Extremely High Frequency (EHF)
Antennas: In this range the frequency is in excess of 3 GHz. They are also
called microwave antenna. The different antenna covers are parabolic antenna,
pyramidal horn antennas, discone antennas, monopoles and dipoles antennas,
microstrip patch antennas, fractal antennas.
Microstrip Antenna
Microstrip antenna (also known as a patch antenna) is one of the latest technologies in
antennas and electromagnetic applications. It is widely used now days in the wireless
communication system due to its simplicity and compatibility with printed circuit
technology. Microstrip geometries which radiate electromagnetic waves were originally
contemplated in the 1950s.
The concept of microstrip antenna was first proposed by Deschamps [3] in the year 1953.
Gutton and Baissinot presented a patent in on the microstrip in the year 1955. Early
microstrip lines and radiators were specialized devices developed in laboratories. No
commercially available printed circuit boards with controlled dielectric constants were
developed during this period. So this antenna didn't become practical till 1970s when it
was developed further by Robert E. Munson [4, 5].
Development during this decade was accelerated by other researchers’ by the availability
of low-loss tangent substrate materials. Others factors for the development include
improved photo-lithographic techniques, better theoretical modelling and attractive
thermal and mechanical properties of the substrate. The first practical antenna was
developed by Munson [6] and Howell [7]. Since then extensive research and development
of microstrip antennas and their arrays have led to diversified application within the broad
field of microwave antennas.
Microstrip or printed patch antennas are used in almost all wireless systems with recent
advancements in printed circuit technology. The purpose of microstrip or patch antenna is
to radiate and receive electromagnetic energy in microwave range and it plays an
important role in wireless communication applications. The performance and operation of
a microstrip antenna is dependent on the geometry of the printed patch [8] and the
material characteristics of the substrate onto which the antenna is printed.
Characteristics of Microstrip Antenna
The Microstrip antenna has proved to be an excellent radiator for many applications
because of its several advantages [9] as compared to conventional microwave antennas.
This result its many applications over the broad frequency range from around 100 MHz to
100 GHz.

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Some of the principal advantages are:
i. They are light in weight.
ii. They occupy low volume.
iii. They are of low profile planer configuration.
iv. They can be made conformal to planar and non-planar surfaces.
v. Their ease of mass production leads to a low fabrication cost.
vi. They are easy to implement on the device.
vii. They are easier to integrate with other MICs on the same substrate.
viii. They allow both linear polarization and circular polarization with simple
feed.
ix. They can be made compact for use in personal mobile communication.
x. They allow for dual- and triple-frequency operations.
xi. They act as an efficient radiator.
xii. They have low scattering cross section.
xiii. They are mechanically robust when mounted on rigid surfaces.
xiv. They are Resistant to shock and vibration.
xv. They are well compatible for embedded antennas in handheld wireless
devices.
xvi. The feed line can be easily fabricated at the same time on the substrate.
However, these antennas also have some limitations as compared to conventional ones.
These are:
i. They have narrow bandwidth.
ii. The efficiency is low.
iii. They have low gain.
iv. They have low power handling capacity.
v. They have low isolation between radiating elements and feed.
vi. Complex feed structure required for high performance arrays.
vii. Large ohmic loss in the feed structure of arrays.
viii. Extraneous radiation from feed and junctions.
ix. Excitation of surface waves.
Size of microstrip antenna comes in both advantages and disadvantages but there are
some applications where the size of microstrip antenna is outsized for any use. The size
of a microstrip antenna is inversely proportional to its frequency. At frequencies lower
than microwave, microstrip patches don't make sense because of the sizes required. The
narrow bandwidth is one of the main drawbacks of these types of antennas. A straight
forward method of improving the bandwidth is increasing the substrate thickness [10].
However, surface wave power increases and radiation power decreases with the
increasing substrate thickness, which leads to poor radiation efficiency. Therefore various
other techniques are presented to provide wide-impedance bandwidths of microstrip
antennas. Some of the techniques in principle are suitable feeding techniques and
impedance matching networks, insertion of slot, slit and notches on the microstrip
antennas. Feeding technique has a large number of adjustable parameters like length,
width and shape. Other ways to overcome these limitations are decreasing dielectric
constant of the substrate, increasing thickness of the substrate and width of the patch.
Another problem to be solved is the low gain for conventional microstrip antenna element.

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Cavity backing and lens covering [11] are the two ways to improve the gain. Cavity
backing has been used to eliminate the bidirectional radiation and thereby providing
higher gain compared with conventional microstrip antenna. The integrated lens
microstrip antenna can be treated as composite antenna combined by microstrip radiator
elements and dielectric lens, which is very useful for high frequencies applications.
Antenna array [12] is another effective means for improving the gain of the microstrip
antennas.
Applications of Microstrip Antenna
Numerous commercial requirements are fulfilled by the use of microstrip or printed patch
antenna. Out of many shapes, rectangular shaped patch antennas are the most widely
used antennas. Microstrip patch antenna fulfils most requirements for mobile and satellite
communication system [13] and many kinds of microstrip antennas is designed for this
purpose. Air-craft, spacecraft, satellite, and missile are others dominant applications,
where the use of microstrip antenna is most suitable due to its size, weight, cost,
performance, ease of installation, and low-profile nature. Also, there are other government
and commercial applications in the area of mobile radio and wireless communications
where the requirement of this antenna is suitable.
Some notable applications for which microstrip antennas are developed and found
suitable are:
i. Satellite Communication Direct Broadcast Service.
ii. Mobile Communication Systems.
iii. Doppler and other Radars.
iv. Missile and Telemetry.
v. Remote Sensing and Environmental Instrumentation.
vi. Satellite Navigation Receivers.
vii. Radio Altimeter.
viii. Biomedical Radiators and Intruder Alarms.
ix. Personal Wireless Communication Systems and Service.
A large number of commercial needs are met by the use of these antennas [14]. The
various application include the ubiquitous Global Positioning System (GPS), ZigBee,
Bluetooth, WiMax, Wireless Fidelity (WiFi) and wireless communication systems.
Navigational applications, such as asset tracking of vehicles as well as marine uses, have
created a large demand for antennas. It finds extensive use in radio frequency
identification (RFID) and radar system [15] in the area of manufacturing, transportation
and medical. In short, microstrip antennas fulfil the demands of a flexible, less weight
antenna system. In recent years printed monopole microstrip antenna finds use in
Satellite Digital Audio Radio Services which is an alternative to audio commercial
broadcasts in automobiles. The advantages of using antennas in communication systems
will continue to generate new applications which require their use. They are the device
which enables all the wireless systems that have become so ubiquitous in our society.
The material costs for wired infrastructure also encourage the use of antennas in many
modern communication systems. With the increase in the awareness of the possibilities of
Microstrip antenna [16], particularly due to its radiation mechanism and functional

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An Overview of Microstrip Antenna.
performance, the number of applications will continue to grow. Wide bandwidth is required
for certain applications in communications, electronic support and counter measures,
radar and radiometry [17].
Structure of Microstrip Antenna
Most commonly used microstrip antenna is the rectangular and circular patches. These
patches can be used for the simplest and the most demanding applications. For example,
characteristics such as dual-frequency operations, circular and dual polarizations, broad
bandwidth, beam scanning and so on are easily obtained in these patches. Any new
numerical or analytical technique is standardized by first applying to these geometries.
Undoubtedly the simplest microstrip antenna configuration is rectangular microstrip patch
antenna. Hence this article deals with rectangular microstrip antenna.
The most basic form, of a Microstrip patch antenna consists of a radiating patch [18] on
one side of a dielectric substrate; it has a ground plane on the other side as shown in
Figure 1.
Figure 1: Geometry of a Microstrip Antenna
In its simplest form a microstrip antenna is a dielectric substrate panel sandwiched [19] in
between two conductors. The lower conductor is called ground plane and upper
conductor is known as patch. Commonly used frequencies for microstrip antenna is in
between 1 GHz to 100 GHz. The patch is selected to be very thin. Patches are normally
made of material such as gold or copper and design in to any shapes. These conducting
metals are the main choice because of their low resistivity, resistance to oxidation, and
ease in soldering and adhere well to substrate. The feed line and radiating patch is etched
on the dielectric substrate. The radiating patch can be design in various shapes according
to the desired characteristics but circular, square and rectangular shapes are common
due to ease of fabrication and analysis. Their radiation characteristics are similar, despite
the difference in the geometrical shape, because they behave like a dipole. If the
thickness of the dielectric substrate is large the surface waves and spurious feed radiation
increases, this will reduce the bandwidth of the antenna The feed radiation also leads to
undesired cross polarized radiation.

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Microstrip patch antennas radiate chiefly due to the fringing fields between the edges of
the patch and the ground plane. A thick dielectric substrate with low dielectric constant is
desirable for a better performance. This will delivers better efficiency and radiation. But
this property has a tendency towards increasing the size of the antenna. For the design of
a compact shape, substrate dielectric constants should be high. However such design will
be less efficient and has narrower bandwidth. Impedance matching is required between
antenna and feed line to ensure the maximum transfer of energy from source to radiating
elements. An approximately selected port location will provide matching between antenna
and feed line.
Through decades of research it was identified that the performance and operation [20] of
a microstrip antenna is driven mainly by the geometry of the printed patch and the
material characteristics on to which the antenna is printed. The patch geometries are
generally rectangular but square, circular and triangular patches are also possible.
Depending upon the characteristics of the transmitted electromagnetic energy, the
radiating element may be square, rectangular, triangular elliptical or circular in shape and
must be separated by a finite distance from the ground plane. A sheet of dielectric
substrate is introduced between these two conducting layers.
Feeding Techniques for Microstrip Antenna
The feed guides the electromagnetic energy from the source to the region under the
patch. Some of this energy crosses the boundary of the patch and radiated into space.
The signal in a microstrip patch antennas is fed by a variety of methods. The methods of
feeding are categorized into two categories namely contacting and non-contacting. In the
contacting method, the input radio frequency power is fed directly to the patch by a
connecting element such as a microstrip line. In the non-contacting scheme, often
regarded as indirect one, electromagnetic field coupling is done to transfer the power
between them microstrip line and the radiating patch.
The four most popular feeding techniques are the microstrip line feed, coaxial feed,
aperture coupling feed, proximity coupled microstrip feed, and coplanar waveguide feed.
Different feeding methods influence different antenna properties such as bandwidth,
radiation pattern, polarization, gain and impedance. In practice, the coaxial and microstrip
feed is the most commonly used feeding method. Matching is usually required between
the antenna and the feed lines because input impedance differ from customary 50 ohm
line impedance. An appropriately selected port location will provide matching between the
antenna and its feed line. A brief description of each of these feeding methods is given in
the section below.
Microstrip Line Feed
In this type of feed technique, as shown in Figure 2, a conducting strip is connected
directly [21] to the edge of the Microstrip patch. The conducting strip is smaller in width as
compared to the size of the patch.

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Figure 2: Microstrip Antenna fed by Microstrip Line Feed
This method is the easiest to fabricate as this feeding arrangement and radiating patch
can be printed on same dielectric substrate. This arrangement provides a planar
structure. Due to this advantage a large arrays may be designed using edge-fed patches.
The drawback is the radiation from the feed line, which leads to an increase in the cross-
polar level. Also, in the mm wave region of spectrum, the dimension of the feed line is
equivalent to the dimension of the patch size, leading to increased undesired radiation.
The feed arrangement to the patch may also have an inset cut in the patch. 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. This is an easy feeding scheme, because it provides ease of fabrication
and simplicity in modelling as well as impedance matching.
Coaxial Probe Feed
The coaxial feed or probe feed [22] is the most common techniques used to feed printed
patch antennas. It is shown in Figure 3. This feed can be given at any desired location
within the patch to achieve impedance matching. The inner conductor of the coaxial
connector extends through the dielectric and is soldered to the radiating patch, while the
outer conductor is connected to the ground plane.
Figure 3: Microstrip Antenna fed by Coaxial Probe Feed

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The coaxial feed or probe feed method gives low radiation loss. The main advantage of
this feed is that it can be placed at any desired location inside the patch to match with its
input impedance. This feed method is easy to fabricate and has low spurious radiation.
However, its major disadvantage is that it gives less bandwidth and is not so easy to use
since a hole has to be introduced into the dielectric substrate. Also the hole has to be
drilled in the substrate and that the connector protrudes outside the bottom ground plane.
It is not completely planar and this feeding arrangement makes the configuration
asymmetrical. For thicker dielectric substrates, the increased coaxial feed or probe feed
length gives the input impedance more inductive, resulting impedance matching
problems. For thick substrates, which are generally employed to achieve broad band both
the above methods of direct feeding the microstrip antenna have problems. In the case of
a coaxial feed, increased probe length makes the input impedance more inductive,
leading to the matching problem. For the microstrip feed, an increase in the substrate
thickness increases its width, which in turn increases the undesired feed radiation. In
large thickness dielectric substrate the microstrip line feed and the coaxial feed suffer
from problem with probe reactance and surface wave excitation. The indirect feed, as
discussed below solves these problems.
Aperture Coupled Microstrip Feed
In an aperture coupling [23] the field is coupled from the feed line to the resonating patch
through slot in the ground structure, which is placed in between the two substrate. On
bottom substrate feed line is there and on top substrate radiating patch. In the aperture-
coupled microstrip antenna configuration, the field is coupled from the microstrip line feed
to the radiating patch through an electrically small aperture or slot cut in the ground plane,
as shown in Figure 4. The coupling aperture is generally centred under the patch. This
help in lowering the cross-polarization because of configuration symmetry.
Figure 4: Microstrip Antenna fed by Aperture Coupled Microstrip Feed
The amount of coupling from the feed line to the patch is decided by the shape, size and
location of the aperture. The slot aperture can be either resonant or non-resonant. The
resonant slot provides another resonance in addition to the patch resonance thereby
increasing the bandwidth, but this lead to an increase in back radiation too. As a result, a
non-resonant aperture is normally used. The performance is moderately insensitive to

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An Overview of Microstrip Antenna.
small errors in the alignment of the different layers. Different substrate parameters can be
chosen for the two layers for getting an optimum antenna performance. This feeding
method gives increased bandwidth.
Proximity Coupled (Electromagnetically) Microstrip Feed Technique
A configuration of this non-contacting microstrip feed used two-layer substrate with the
microstrip line on the lower layer and the patch antenna on the upper layer as shown in
Figure 5. This feeding method [24] contains two dielectric layers i.e. one is a radiating
patch layer and on lower layer feed line is fabricated with a ground plane on back side.
The two substrates are separated by a common ground plane. The microstrip feed line on
the lower substrate is electromagnetically coupled to the patch through a slot aperture in
the common ground plane. The slot can be of any shape or size and these parameters
can be used to improve the bandwidth. The radiation from the open end of the feed line
does not interfere with the radiation pattern of the patch because of the shielding effect of
the ground plane.
Figure 5: Microstrip Antenna fed by Proximity Coupled Microstrip Feed
This feature also improves the polarization purity [25]. The coupling aperture is usually
centred under the patch, leading to lower cross polarization due to symmetry of the
configuration. Generally, a high dielectric material is used for the bottom substrate and a
thick, low dielectric constant material is used for the top substrate to optimize radiation
from the patch. The notable feature of this feed configuration is wider bandwidth, which is
primarily because of an inclusive increase in the thickness of the microstrip patch
antenna. This technique has the provision of choosing separate substrate for the patch
and the feed line in order to achieve an optimize performances. The major drawback of
this method is difficulty in fabrication due to multiple layers which need proper alignment.
Also in this method the thickness of the antenna increases.
Coplanar Wave Guide Feeding (CPW)
The coplanar waveguide feed [26] has also been used to excite the Microstrip antenna. In
this method, the coplanar waveguide is printed on the ground surface of the patch as
shown in Figure 6. The line is excited by a coaxial feed and is terminated by a slot whose
length is nearly one quarter of the slot wavelength.

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Figure 6: Microstrip Antenna fed by Coplanar Wave Guide Feed
This feeding method has been widely used for wireless communications due to its many
features such as wide band width, simple structure, a single metallic layer, less numbers
of soldering points, and easily compatible with other circuits etc. The main disadvantage
of this method is the high radiation from the relatively longer slot. This can be better by
reducing the slot dimension and adjusting its shape in the form of a loop.
A comparison between various types of feeding techniques [27] is made and shown in the
Table 1.
Characteristic
Microstrip
feed
line
Co
-
axial
Feed
Aperture
coupled
feed
line
Proximity
coupled
feed
CPW feed
Spurious feed
radiation
More More Less Minimum
Less
Reliability
Better Poor Good Good
Good
Ease of
fabrication
Easy Difficult Difficult Difficult
Difficult
Impedance
matching
Easy
Easy Easy Easy Easy
Bandwidth
achieved
2-3% 2-3% 3-5% 15%
3%
Table 1: Comparison of Different Types of Feeding Techniques

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Analysis of Rectangular Microstrip Antenna
The microstrip antenna has a thin dielectric substrate in between a two dimensional
radiating patch and a ground plane. For analysis purposes [28]it is categorized as a two-
dimensional planar component. The analysis method is divided into two groups. In the
first group of method, the analysis is done on the basis of equivalent current distribution
around the edges of the radiating patch. In this group the two widely used analytical
techniques are the transmission line model and the cavity model. This group presents
more physical insight but less accuracy. In the second group, the analysis is based on the
distribution of the electric current distribution in between the radiating patch and the
ground plane. This group is analysed by numerical methods based on the Method of
Moments (MoM) and the Finite Element Method (FEM). This group requires more
rigorous analysis and hence takes longer simulation time, but they give a more accurate
result. Besides these to popular methods other methods such as Finite Difference Time
Domain (FDTD) Method, Finite Integration Technique (FIT), Green Function Methods, etc.
are also well used for the analysis.
Transmission Line Model
This model is simple in analysis and also presents a good physical insight. It is helpful in
understanding the basic performance of the microstrip antenna. In this model [29] the
radiating patch element is viewed as a transmission line resonator without any transverse
field variations. The variation of the field is taken along the length. The radiating patch is
represented by two slots separated by the length of the resonator. Fringing fields at the
open circuited ends is the main source for the radiation. Originally this model was
proposed for rectangular patches but later on it has been extended for all generalized
shape of the patch. It is also most suitable for rectangular microstrip antenna. The
drawback of this model is its accuracy. It is less accurate. Although it is easy to use, but in
this case all types of configurations can't be analyzed as it does not take care of field
variation in the orthogonal direction of propagation. In this approach, the microstrip
antenna is represented as parallel plate transmission line, with no transverse field
variation, and connected by two radiating slot of dimension W and height h. The slots are
represented high impedance terminations of the transmission line. When excited, the two
open ends which are normal to the direction of propagation radiates only. The resonant
characteristic of it depends on the length L of the patch. The direction of the propagation
of the electromagnetic wave is along the z-direction. A representation of this structure is
shown in the Figure 7.

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Figure 7: Representation of Microstrip Antenna using Transmission Line Model
Fringing effect accounts for the radiation from the edge of the patch. The resonant length
is more than the physical length due to this fringing. The resonant length or the effective
length of the patch is represented as Leff .The electric field distribution along the patch is
shown in the Figure 8along with the two lengths stated.
Figure 8: Electrical field Distribution in Microstrip Antenna using Transmission Line Model
Cavity Model
This model [30] gives more accurate result as compared to transmission line mode. It also
gives good physical insight of the behaviour of the field on the radiating patch. Here the
portion in between the radiating patch and the ground plane is treated as a cavity. The
cavity is surrounded by magnetic walls around the periphery and from the top and bottom
side it is enclosed with the electric walls. The field inside the cavity is uniform since the
thickness of the substrate is small. The field underneath the radiating patch is expressed
as a sum of the various resonant modes of the two-dimensional resonator. The fringing
fields occurred around the periphery. The fringing fields and the radiated power are not
enclosed inside the cavity but they are distributed at the edges of the cavity. The radiation
loss from the antenna, the conductor loss, loss due the loss tangent of the dielectric
substrate and sky wave loss are responsible for the total radiation of the antenna. The
radiated power in the far field is computed from the magnetic current around the
periphery. An alternate way of computing the radiation effect [31] is the introduction of the
impedance boundary condition at the walls of the cavity. The only drawback of this model
is its complex analyses. The Transmission line method is inadequate in its description of

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the real processes when a patch is excited. It takes into account only the modes where
the energy propagates only in the longitudinal z- direction. The field distribution along the
x- and y- axes is assumed uniform. Though the dominant is prevalent but the
performance is also exaggerated by higher-order modes. The cavity model is a more
common model [32] in analyzing the microstrip antenna, which imposes open-end
conditions at the side edges of the patch. Here the patches are represented as a
dielectric-loaded cavity with the electrical walls at the top and bottom of the cavity and the
magnetic wall is around the periphery of the patch. A schematic diagram of the patch
using this model is shown in the Figure 9.
Figure 9: Representation of Microstrip Antenna using Cavity Model
In a magnetic wall the H-field is purely normal and E-field is purely tangential. The
thickness of the substrate is assumed to be very small in this method of analysis. The
waves generated and propagated below the patch experience considerable reflection at
the edges of the patch. Merely a small fraction is being radiated. This model assumes that
the E- field is purely tangential to the slots formed between the ground plane and the
edges of the patch. Moreover, it considers only modes with no H- field component.
Assuming the material of the substrate is truncated and limited to the periphery of the
patch, the four side walls represent four narrow slots, and the radiation takes from these
slots. Here the field inside the cavity is assumed to be zero, and the influence of the field
in the outside infinite region is represented by the surface currents. These currents are on
the surface of the cavity. The field is concentrated under the patch region as the height of
the substrate is very small. The current density of the patch is maximum at its edges. The
equivalent magnetic current density for the dominant TM001 is shown in the Figure 10. It
reveals that the magnetic current densities are co-directed and are of equal magnitudes at
slot no 1and 2. They are also of constant values along x-direction and y-direction. There
are electrical walls at x = 0 and x = h. so at these two points the tangential Electric field
components are zero.

HCTL Open International Journal of Technology Innovations and Research (IJTIR)
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Volume 21, Issue 2, August 2016
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Dr. Ranjan Mishra, Page 15
An Overview of Microstrip Antenna.
Figure 10: Magnetic Field Distribution in Microstrip Antenna using Cavity Model
In the case of Microstrip antenna, the dominant mode is the mode with the lowest
resonant frequency. Since, the length of the patch is lower than the width of the patch i.e.
L < W. Hence, the lowest resonant frequency mode is TM001. This is the same mode
resulted in the both the transmission line model and cavity model. The resonant
frequency, f, for any given substrate of permittivity
ℰ
r, corresponding to this is given as:
= 
.
Conclusion
The article is a concise summary of the antenna in general and microstrip antenna in
particular. It briefly describes the microstrip antenna advantages and disadvantages in a
crisp manner, the various techniques used in the feeding from the source. Along with the
description of the feeding technique; a small note on the analysis method is also cited.
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HCTL Open International Journal of Technology Innovations and Research (IJTIR)
http://ijtir.hctl.org
Volume 21, Issue 2, August 2016
e-ISSN: 2321-1814, ISBN (Print): 978-81-932623-1-3
Dr. Ranjan Mishra, Page 16
An Overview of Microstrip Antenna.
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HCTL Open International Journal of Technology Innovations and Research (IJTIR)
http://ijtir.hctl.org
Volume 21, Issue 2, August 2016
e-ISSN: 2321-1814, ISBN (Print): 978-81-932623-1-3
Dr. Ranjan Mishra, Page 17
An Overview of Microstrip Antenna.
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