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CHAPTER 1
INTRODUCTORY OVERVIEW
Antenna forms the front end of a transmitter or a receiver in a radio communication
system. It is termed as a conformal antenna if it conforms to the shape of the parent
body surface. The design and development of antennas using state of the art
technologies has led to advancement in the communication, navigation and
electronics warfare systems. New approaches for the integration to the aircraft surface
of antenna systems have evolved with the advancement in fighter aircraft
technologies. Future fighter aircraft systems must have the ability to fend for itself in
a rapidly changing threat scenario. Design of antennas must have the important
characteristics incorporated to assist in the defence of the aircraft systems. The design
of the antenna and optimization of its characteristics will lead to considerable
improvements in the overall system performance like better accuracy, superior
aerodynamics, and lighter weight, etc. Structurally integrated, efficient antenna
systems, designed for the aircraft systems will be capable of multi role operations.
When under Electronic Attack the aim of these designs is towards tackling of the
threat dynamically.
This thesis is devoted to the design of microstrip antenna conformable to both planar
and cylindrical surfaces. Efforts have been made to design such antennas for the
aircraft system to ensure protection in an electronic warfare environment [Defence
Research & Development Organization, Ministry of Defence, Government of India
has developed the capability in designing antennas for various ground-based and
airborne radar systems, communication systems, electronic warfare, and underwater
scenarios. A lightweight, broadband planar antenna with an aperture size of 0.7 m x
0.4 m and a gain of 35 dB has been developed for UAV. To meet the gain
requirement, dimension and weight, operating frequency of the antenna is in Ku band.
Two approaches have been followed, viz., parabolic reflector and planar microstrip
patch antenna]. The effect of mutual coupling on the planar antenna arrays has been
considered appropriately in the design.
2
The work has also been carried out to design multidielectric microstrip patch antennas
with a cover layer. Design suitable for specific high-performance airborne
applications is based on conformal mapping technique. Impedance bandwidth
enhancement, frequency agility and importance of cover layer are discussed in detail.
Impedance bandwidth is an important characteristic of the microstrip patch antenna.
The same has been significantly improved by using multilayer dielectric configuration
designed for X-band applications. The conformal mapping based on Wheelers
transformation of multidielectric microstrip antenna has been employed for
performance analysis. This approach leads to mapping of the complex permittivity of
a multilayer substrate to a single layer.
The shape of an aircraft wing, fuselage or external pods is approximated as a singly
curved surface. For large angular coverage in the azimuthal plane, low profile
conformal arrays of rectangular antennas are mounted on singly curved cylindrical
surface of an aircraft. Such a design will facilitate the use of antenna in defence
applications in radar and communication systems to avoid detection by the enemy.
The thesis presents the design and performance analysis by taking into account the
effect of mutual coupling of antenna and arrays conformable on singly curved
cylindrical surface.
An antenna is designed to conform to a shape that is some part of an aircraft. It is
required to conform to a prescribed shape be it fuselage, nosecone, wings or
externally carried pods. For the purpose of ease of analysis it can be defined as a part
of regular shape viz. cylinder, cone or sphere. The purpose is to build the antenna so
that it becomes integrated with the structure and does not cause extra drag. Further,
the antenna’s integration makes it less vulnerable to optical detection for intentional
physical damage. Such an embedded antenna on aircraft surface will have stealth
property. It will also aid in avoidance of backscatter antenna radiation when
illuminated by unwanted electromagnetic sources.
In the following sections, the necessity of using microstrip planar antenna and antenna
conformable to non planar surface, historical review and chapter wise contribution of
the thesis has been presented.
3
1.1 Advent of Microstrip Antennas
Microstrip antennas are suitable for aircraft and missile applications due to their low
profile, small size, less weight and ease of installation. These antennas are structurally
reliable because of mechanical robustness and can withstand shock and vibration. In
addition they are conformable to a curved surface of parent body, compatible with
MMIC design, versatile in terms of antenna parameters such as pattern and impedance
bandwidth and can be easily designed to produce linear or circular polarization with
significant range of gain.
Advent of microstrip structures as radiator of electromagnetic energy goes back as
early as 1950. Grieg and Englemann realized microstrip transmission line compatible
radiators in 1952 [1]. In 1953 Deschamps realized a microstrip antenna integrated
with microstrip transmission line [2]. For the first time microstrip antenna design was
patented by Gutton and Baissinot in 1955 [3]. Early microstrip lines and antennas
were restricted to theoretical study [4]. Wheeler [5] and Purcel et al. [6], [7]
contributed towards the methods of design and development of microstrip
transmission line and analysis up to late 1960s. Earlier researchers attributed to loss in
the form of radiation as high as 50% of the power in a microstrip resonator. Denlinger
first realized that rectangular and circular microstrip resonators could efficiently
radiate with radiation mechanism due to discontinuities at each end of a truncated
microstrip transmission line [8]. In late 1969, fields and currents of the resonant
modes of circular microstrip structures were described by Watkins [9].
Aerospace applications, such as spacecraft and missiles, gave momentum for research
to explore the efficacy of conformal antenna designs in early 1970s. Howell in 1972
explained the basic rectangular microstrip radiator fed with microstrip transmission
line at a radiating edge [10]. Earlier the antenna designers could hardly think of
design of a resonator to radiate efficiently with efficiency greater than 90%. Further,
thin patch microstrip antenna designs suffered from poor antenna efficiency due to
dielectric and conductor losses, besides being sensitive to environmental factors such
as temperature and humidity. Also the applications of these antennas remained limited
to narrow bandwidth (5% to 10% for VSWR 2:1) designs. Many of these limitations
4
derailed the use of microstrip antennas in numerous aerospace applications.
Microstrip antennas had become so omnipresent and studied extensively that in 1981
they were the subject of a special issue of the IEEE Transactions on Antennas and
Propagation [11].
Theoretical and experimental research work on microstrip antenna from later part of
1970s has been related to exploit its advantages of low profile, compatibility with
integrated circuit technology, and conformability to a shaped surface. The research
work thus contributed to the successful military applications of these antennas in
aircraft, precision guided munitions (PGM) and missiles. The designs evolved from
basic microstrip antenna configuration to cases where the metallic patch could be
embedded in a multilayered dielectric media with a superstrate or dielectric cover
used to protect the patch against environmental hazards. The cover layer thus
introduced helps in frequency agility and enhancement of the bandwidth.
In many microstrip antenna applications, systems requirements can be met with a
single patch element. In other cases, however, systems require higher antenna gains
while maintaining a low-profile structure, which calls for the development of
microstrip antenna arrays. Microstrip arrays, due to their extremely thin profiles
(0.01-0.05 free-space wavelength), offer three outstanding advantages relative to other
types of antennas such as low weight, low profile with conformability, and low
manufacturing cost. Because of these attractive features, many military, space, and
commercial applications are employing microstrip arrays instead of conventional
high-gain antennas, such as arrays of horns, helices, or parabolic reflectors. However,
advantages of the microstrip array can be offset by three inherent drawbacks: small
bandwidth (generally less than 5%), relatively high feed line loss, and low power-
handling capability. To minimize these effects, accurate analysis techniques, optimum
design methods, and innovative array concepts are imperative to the successful
development of a microstrip array antenna. For example, accurate analysis and a
correct design approach can often overcome deficiencies in such performance factors
as mutual coupling, beam scanning effect, pattern shaping, power divider
configuration, and impedance matching.
5
1.2 Models of Microstrip Antennas and Feeding Techniques
Two models – transmission line model and cavity model which are adopted by the
researchers for the design of microstrip antennas and the associated transmission
modes are discussed in the following subsections.
1.2.1 Transmission Line Model
A microstrip antenna with a rectangular metal patch of width a and length b,
separated by a distance h with a dielectric material between the ground plane and the
patch is shown in Figure 1.1. The radiation originates from the fringing electric field
at either end of the antenna. These edges are called radiating edges, the other two
sides (parallel to ŷ axis) are non-radiating edges. Fringing fields due to two radiating
edges can be viewed as the two ends of the antenna of width lying between 0 and a
and non-radiating edges lying between 0 and b. The patch antenna is fed with the feed
point located such that it is chosen to match the antenna with desired impedance.
Figure 1.1: Transmission Line Model of Rectangular Microstrip Patch Antenna.
The transmission line model of a rectangular microstrip antenna is the simplest to
implement as shown in Figure 1.2. In this model the rectangular microstrip antenna
consists of a microstrip transmission line with a pair of loads i.e. edge conductance Ge
and edge susceptance Be at either end. [12], [13]. As shown in Figure 1.2(a) the
resistive loads at each end of the transmission line represent loss due to radiation.
Two transmission line sections, consisting of antenna length L divided into parts L1
and L2, may be considered to contribute to the driving point impedance in the
equivalent circuit. Analysis may then be carried out using a pair of edge admittances
6
Ye separated by two sections with characteristic admittance Y0, shown in Figure
1.2(b). At resonance, the imaginary components of the input impedance seen at the
driving point cancel, and therefore the driving point impedance becomes exclusively
real. Figure 1.2(c) shows the transmission line model with a feed line of characteristic
admittance Yf of length Lf connected to a radiating edge. The driving point admittance
Ydrv may then computed at the end of the feed line.
Figure 1.2: Transmission Line Model
1.2.2 The Cavity Model
According to one school of thought the transmission line model is conceptually
simple, but it is often inaccurate for determining the impedance bandwidth of a
rectangular microstrip antenna for thin substrates. The transmission line model of a
rectangular microstrip antenna considers currents flowing only in one direction along
the line. It does not account for the transverse currents that really exist in the assumed
directions. Further, in the transmission line TEM mode, approximation accounts for
the radiation loss, and the combined dielectric and copper losses as increased
dielectric loss spread along the length of the line. All these drawbacks are overcome
in the cavity model shown in Figure 1.3. The cavity model, introduced in the late
1970s by Lo et al.[14], [15], is conceptually simple and can be easily implemented.
7
Figure 1.3: Cavity Model of Rectangular Microstrip Patch
These researchers proposed that the rectangular microstrip antenna may be considered
as a cavity model, with electric walls at the top and bottom, and magnetic walls on
four sides which are orthogonal to electrical walls [14] [15]. The radiation in two
dimensions may be considered due to the superposition of the resonant modes.
Several refinements of the cavity model have since been introduced [16], [17]. As
shown in Figure 1.3 for h << λ0 in the cavity model, vertical electric field will be
perpendicular to the patch plane and magnetic field component will be horizontal.
Further, the fields in the lossy cavity may be assumed to correspond to those existing
in the short cavity of the model.
Certain assumptions and approximations limit the accuracy of the cavity model based
on electrically thin substrates. For a substrate thickness of 0.02λ0 or less, the
impedance prediction is generally accurate for a very narrow bandwidth within 3% of
measured resonant frequency. For thicker substrates the impedance predictions in
cavity model provides inconsistent results [18]. Further, in the cavity model the self
inductance of a coaxial probe used to feed the rectangular microstrip antenna is not
included.
1.2.3 Transmission Modes
If the feed in the rectangular microstrip antenna is along the centreline of the width b
(for the length a > b) as shown in Figure 1.4, the TM10 is the lowest order
transmission mode with the corresponding cut-off frequency. The next highest
available mode is the TM01 mode (for the length a < b) with the feed located at a/2.
8
The TM01 is the dominant mode, it becomes the mode with the lowest resonant
frequency and TM10 has the next lowest resonant frequency. For a square patch
(a =b), both the orthogonal modes TM10 and TM01 exist with identical resonant
frequencies.
Figure 1.4: Transmission of TM10 and TM01 modes for a > b and b > a respectively
1.2.4 Common Feed Methods
Figure 1.5 shows four common methods to directly feed a rectangular microstrip
antenna. The first method shown in Figure 1.5(a) is referred as a coaxial probe feed.
In the coaxial feed method the outer shield is connected to the ground plane of the
microstrip patch antenna. With the metal of the ground plane and the dielectric of the
substrate removed, inner core is connected to the patch. The excitation of a mode
along the width of the antenna is suppressed by feeding the antenna in the centre (i.e.
at a/2). With this symmetrical feed linear polarization is realized along the length of
the patch. Figure 1.5(b) shows the feeding of the microstrip antenna with a
transmission line along a non-radiating edge. Using transmission line model, the feed
is modelled in line with coaxial probe feed. To radiate elliptical polarized wave, we
need to excite a mode along the width of the patch with a ≈ b. Impedance matching is
relatively simpler, as the 50 Ω transmission line can be directly connected to driving
point impedance of 50 Ω. A feeding technique using microstrip transmission line to
drive at one of the radiating edge of the patch is shown in Figure 1.5(c). Slight
changes in the radiation pattern introduced with this feed are attributable to the field
distribution affected along radiating edges. A rectangular patch with b>a/2, the
impedance at radiating edge is 200 Ω.
9
Figure 1.5: Common methods used to feed a rectangular microstrip antenna.
At resonance Rin= 1/(2Ge), where Rin is the edge resistance and Ge is the
transconductance. Impedance transformation to 50 Ω needs to be provided. The same
is achieved by using quarter wave transformer due to its property of having bandwidth
larger than the antenna. In case of a>b, microstrip patch antenna can dispense with a
quarter wave transformer since the edge resistance at resonance itself will be 50 Ω.
In the fourth type of feed, shown in Figure 1.5 (d) a notch is cut to achieve a driving
point impedance of 50 Ω. This feed, referred to as Inset Feed, affects the fields
slightly. Transmission line based modelling of this feed helps in identifying the
driving point location which is close to the measured value [19]. To increase the
antenna gain, patch width is increased which in turn increases the edge conductance
and may result in resonance if the edge impedance is 50 Ω.
1.3 Characteristics of Microstrip Antennas
Transmission line model may be used to analyze and study the characteristics of a
rectangular microstrip patch antenna operating in TM10 mode. To compute the
radiation pattern of the antenna this model takes into account both the thermal plots
and the fringing fields at the edges. The radiation pattern also gets affected due to the
ground plane and the substrate. Using the two slots-model the radiation patterns for
the TM10 mode can also be determined. Radiation pattern based on this model
replaces the metallic patch with surface current. Further taking into account the
grounded substrate, the electric field extending outward from the edges is also along
the thickness of the dielectric substrate.
10
A microstrip antenna array may be modelled as one formed by two parallel slots,
placed at λ/2 apart at the edges of the patch antenna. The slots radiating on a substrate
of given permittivity and thickness will have its length equal to half the guide
wavelength gλ . If the dielectric medium is air then the resonant length will be half the
free space wavelength 0λ with a maximum directivity. The increase in dielectric
constant of the medium results in a decrease in the resonant length and spacing
between the radiating slots. The fields at the patch end can be divided into tangential
and normal components with reference to the ground plane. The field components
normal to the ground plane are out of phase because the length of the patch is
approximately λ/2. The resultant contributions to the far field in broadside direction
cancel each other. The tangential field components, which are in phase, combine to
give the maximum radiated field normal to the surface of the patch. The rectangular
patch excited in its fundamental mode has a maximum directivity in the direction
perpendicular to the patch (broadside). The directivity decreases when moving away
from broadside towards lower elevations.
For each individual mode in the cavity model, the electric field distribution in the
cavity can be determined. From the electric field distribution for each mode, an
equivalent electric circuit is defined and impedance of this circuit is determined. The
impedances of all the modes (including the higher-order mode) are placed in series to
determine the total input impedance. Impedance bandwidth requirement of a
microstrip patch antenna can be met with, single mode driven cavity model of a linear
rectangular microstrip patch antenna. The impedance bandwidth, which is related to
the total quality factor QT, can be determined using this model. A commonly used
measure of bandwidth of an antenna is estimated in terms of VSWR lying between 1
and 2 [20].
The impedance bandwidth of a microstrip antenna can be increased by using a thick
substrate with a low dielectric permittivity. Selection of such a substrate for
bandwidth enhancement should not significantly increase the surface wave
propagation. A reasonably thick substrate should be considered for the antenna design
and enhancement of the bandwidth can be achieved using additional techniques.
11
Figure 1.6 shows effect of the variation of the permittivity and substrate thickness on
impedance bandwidth. Though the change in permittivity and thickness of the
substrate layer affects the impedance bandwidth characteristics, these factors also
contribute to losses and hence the efficiency of the antenna.
Figure 1.6: Plot showing the normalized bandwidth of a square microstrip antenna based on
the cavity model.
1.4 Multidielectric Layer Microstrip Antennas
A microstrip antenna with a multidielectric layer has a distinct advantage over the
single dielectric layer microstrip antenna in terms of impedance bandwidth
enhancement. Addition of cover layer i.e. superstrate layer enhances the impedance
bandwidth further and can provide frequency agility to the antenna as discussed in
Chapter 5. Significant literature exist those studies, using transmission line model, the
effect of the cover layer on the performance analysis of the multidielectric antenna. A
quasi-static analysis of a microstrip transmission line with a dielectric cover forms the
basis of this analysis [21], [22], [23], [24], [25]. We will also utilize the transmission
line model to analyze the performance of a rectangular microstrip antenna with a
dielectric cover. The design considerations may therefore be based on the
characteristics of the substrate, the patch geometry, the location of the feed and proper
choice of thickness of the superstrate layer. In such a design it is essential to study the
effect of effective permittivity effε of the multilayer structure on the resonant
frequency. An important aspect of the analysis is to use a technique where patch
antenna with multidielectric layers but without superstrate layer can be effectively
12
carried out. Chapter 3 discusses a number of analytical methods that have been
suggested for the analysis of such a structure.
One of the limitations of microstrip patch is its inherent narrow bandwidth that is
typically in the range of a 5 percent of the radiating frequency without a matching
network. Broadbanding can be achieved with an impedance matching network for the
feed geometry. Within a limit, an increase in the thickness of dielectric substrate
results in the broadbanding of the microstrip patch antenna. The limiting factor is the
series inductance that is produced by the higher order modes which result in a
mismatch of the driving point impedance. Alternatively by using a matching network
to overcome the impedance mismatch, the impedance bandwidth of a microstrip patch
antenna can be increased.
Some of the conventional methods used to enhance the impedance bandwidth are (i)
Increase the thickness of the dielectric substrate, alternatively addition of resonators.
(ii) External impedance matching network and (iii) Introducing externally short-
circuits or gaps using photonic gap materials. A considerable number of microstrip
antenna design variations which utilize these approaches have been compiled by
Kumar and Ray [26] as has Wong [27].
1.5 Microstrip Antenna Arrays 1.5.1 Planar Array
Some critical application related to defence, more specifically those related to radar or
communications may need narrow beamwidth, which can be met by an antenna array.
Other applications where there is need of high gain/bandwidth or operational
requirement of a scanning beam. For such a requirement it is the antenna array which
needs to be considered instead of a single microstrip patch antenna. Depending on the
nature of the application and the limitations of the parent structure, the antenna
designer may choose the nature of array to be a linear, planar, or conformal.
In 1960s Elliot’s contribution on linear and planar arrays, helped in the analysis of
rectangular microstrip antenna arrays [28], [29], [30]. As shown in Figure 1.7, N
rectangular microstrip patch antennas are located in the plane of the paper viz. x–y
plane, with z axis pointing out from the plane. Modelling of each patch antenna is
13
done as a pair of radiating slots in a ground plane. For the TM01 mode, the antennas
are polarized along the y axis.
Figure 1.7: N set of rectangular microstrip antennas with centre of each patch used to locate pairs of equivalent slots.
1.5.2 Array Feeding Techniques
Figure 1.8: Corporate feed network for four microstrip patch linear array.
14
Figure 1.9: 4x4 patch of 16 elements planar array with corporate feed network.
Feed methods for the microstrip antenna arrays can be broadly classified as series and
parallel feeds. Figure 1.8 shows a linear array of four elements with a corporate feed
network. The corporate feed corresponds to the parallel feed with one input and
number of parallel outputs. A corporate feed network for a planar array of 16
elements is shown in Figure 1.9. As depicted in this figure the 4x4 array has been
divided into four 2x2 sub arrays. The arrangement of sub arrays in two dimensions
will create a planar array.
Series feed is into individual patch element arranged in a continuous line. In series
feed, energy is coupled progressively from one patch element to the next. Energy can
be coupled in many ways and it includes proximity, direct, aperture or probe coupling.
The resistance of each square patch antenna element is matched to the connecting
transmission line impedances Z1, Z2, Z3, and Z4. This will provide the desired power
split and is accomplished with a number of quarter wave transformers Z1q, Z2
q, Z3q,
and Z4q.
15
1.5.3 Effect of Mutual Coupling between Square Patch Antennas
The effect of mutual coupling between the square patch antennas may be studied by
using the cavity model. The resonant frequency fr gets affected by about 1% and
corresponds to a frequency shift of 10 MHz at 1 GHz. Further, the mutual coupling
also affects the input impedance Rr by about 50% and the far field radiation pattern
gets affected by about 30% [31].
The inter element spacing d effects the mutual coupling which is frequency sensitive
as well. The effect of mutual coupling is reduced with an increase in the spacing d.
At the first resonant frequency the effect on coupling peaks sharply and in between
the first and second resonance it is down by about 30 dB approximately [32]. The
mutual coupling between microstrip patches is mainly due to both space wave and
surface wave. The effect of mutual coupling due to surface wave is significant in E-
plane compared to effect in the H-plane [33].
1.5.4 Impact of Patch dimension on Microstrip Antennas
As explained above, suitability of the microstrip antenna in aerospace related
applications is primarily due to the limitation of space available in the parent
structure. It is well known that larger patch width results in generation of grating
lobes in addition to space requirements. Cross polarization is also an important
characteristic related to the patch width. Therefore in addition to achieving good
radiation efficiency, the patch width selection should be based on the space
requirement, the suppression of grating lobes and the avoidance of cross polarization.
1.6 Necessity of Conformal Antennas
A modern aircraft has many antennas protruding from its structure like the antennas
for navigation, various communication systems, the instrument landing systems, the
radar altimeter, and so on. There can be as many as 20 or more different antennas as
depicted in Figure 1.10. It was reported that increased number of antennas (up to 70
antennas on a typical military aircraft results in a considerable drag and hence
increased fuel consumption [34]. Solution lies in integrating these antennas into the
aircraft skin [35].
16
Figure1.10: At least 20–30 antennas protrude from the skin of a modern aircraft. (From Hopkins et al.
1997) (Courtesy American Institute of Aeronautics and Astronautics, Inc.) [36].
Array antennas with radiating elements on the surface of a cylinder, sphere, a cone, or
a similar shape without the shape being dictated by aerodynamic or other reasons, are
usually called conformal arrays [36]. Though strictly speaking they are not conformal
array antennas according by the IEEE definition, we have followed the common
practice today. A paper on conformal arrays for radar systems in aircraft has presented
a very bright perspective for the development [37]. For large-sized apertures
involving functions like satellite communication and military airborne surveillance
radars the necessity of conformal antennas is even more evident. Conformable to non
planar surface antennas may have their shape determined by a particular
electromagnetic requirement such as antenna beam shape and/or angular coverage.
Significant work on conformal antennas and both on cylindrical and conical arrays as
well as feeding systems was done at the U.S. Naval Electronics Laboratory Center
(NELC) in San Diego around 1974 [36].
The conformal arrays were first introduced by Chireix [38] using a circular
arrangement of dipole elements. Later in 1950s, several contributions on conformal
arrays were reported e.g. Knudsen [39]. The circular array found extensive
applications in broadcasting, communication, navigation and direction finding due to
its ability to scan all around in azimuth. Second World War had led to the
development of HF circular arrays for radio signal intelligence gathering and direction
17
finding in Germany. A large circular array the French RIAS experimental radar
system is an advanced, more recent application [40], [41].
Thomas contributed an approach in the development of conformal arrays for nose
radar systems in aircraft [42]. The conformal nose-mounted arrays have an increased
field of view compared to the traditional ±60° coverage of planar antennas. For about
two decades no substantial interest in research was taken in the area of conformal
arrays. Later with the advent of Monolithic Microwave Integrated Circuits (MMIC),
reliable design for low cost very complex microstrip antenna arrays got the much
needed impetus. Further, digital processing techniques aided better design and
development of cost effective phased array microstrip antenna systems.
Several circular arrays placed on top of each other could be used to enhance the
directivity and reduce the beam width in the vertical plane. A fundamental
contribution on radiation from apertures in metallic circular cylinders and effect of
mutual coupling has been due to Jim Wait, Hessel and Pathak et al. [43], [44], [45].
A cylindrical or circular array of elements has a potential of 360° coverage, either
with an omnidirectional beam, multiple beams, or a narrow beam that can be steered
over 360°. Today, the common solution is to employ three separate antennas, each
covering a 120° sector. Instead, one cylindrical array could be used could be used to
cover 3600, resulting in a much more compact installation and at a lower cost [36].
Figure 1.11: A single planar array covered by a dome with a passive lens for hemispherical coverage
[36]
18
A radome (A domelike shell transparent to radio-frequency radiation, used to house a
radar antenna) on the nose cone of the missiles or aircraft protects the antenna
elements. A typical reported design is a monopulse tracking antenna array consisting
of four radiating microstrip patches mounted on the surface of a cone with a triplate
feed network placed below the patches. Such a design is used as a guided-weapon
seeker antenna, operating at a centre frequency of 10 GHz, for high-speed missiles.
This type of conical microstrip array can be a promising candidate for the
employment on curved bodies with conical or nearly conical surfaces. Alternatively
the antenna elements can be placed on the radome itself [46]. An example of dome
radar antenna is conformal spherical antennas [47], [48].
Figure 1.12: Vision of a smart-skin antenna [36]
Preferably, some of the antenna functions should be combined in the same unit if the
design can be made broadband enough. Significant importance has been attached to
the study of conformal microstrip antenna arrays for applications related to aircraft
systems. Subsequently the vision of a future “smart skin” conformal antenna [36] was
introduced in 1967. A structure of “smart skin” conformal antenna is shown in
19
Figure1.12. This antenna constitutes a complete RF system, including not only the
radiating elements but also feed networks, amplifiers, control electronics, power
distribution, cooling system, filters, and so on, all in a multilayer design that can be
tailored to various structural shapes [49], [50]. The vision for conformal antennas has
not been fulfilled due to the difficulties in the analysis and design of conformal
antennas.
It has been shown that in the case of a relatively small conical ground plane, the front-
to-back ratio of the elevation-plane radiation pattern can be improved by at least 8 dB
compared to the ratio using a same-size planar ground plane. The radiation in the
lower hemisphere can be enhanced significantly in comparison to that of a planar
microstrip antenna. Nonplanar ground plane also affects the resonant frequency of a
conical microstrip antenna.
1.7 Conformal Arrays versus Planar Arrays
The conformal arrays and planar arrays mainly differ in their geometry. The elements
are typically located in a symmetrical regular shape such as a rectangular or triangular
grid in a planar array while the elements of the grid lattice follow the shape of the
curved surface of the conformal array. For most applications a planar array can be
analyzed as an infinite array using known methods. Thus, the design of planar array is
simpler and it is cheaper to manufacture.
A planar array cannot be arranged practically to the desired aerodynamic shape,
which is generally non planar. For example, radar antenna placed in the nose cone of
an aircraft or navigational antenna placed in the wing tips as shown in Figure 1.10 is
not planar. A planar array can be embedded on an aerodynamic surface if the
curvature effect can be neglected like for example the tail fin of an aircraft. However,
in most aerodynamic applications requiring a scanning array, a planar array can be
placed inside the nose cone covered by the aerodynamic radome. Often a lot of space
is wasted in order to fit in the requirements of the radome. Further, the space occupied
by the radome around the periphery of the planar array may limit the dimension of the
array.
The performance of the planar array is adversely affected on account of the mutual
coupling if the elements of the array are closely spaced (less than half a wavelength).
20
The element spacing of planar arrays affects the operational bandwidth and the
maximum scan angle. Increased spacing of the elements of the array will result in
increased directive gain and narrow half power beamwidth. But the disadvantage of
increase in the inter-element spacing is that maximum scan angle coverage is limited
by grating lobes.
Aerodynamic restrictions make the placement of the conventional planar antenna
difficult where location of the antenna is mandatory, for example, placement of
antennas for navigational aids. Thus, in aircraft structures a conformal antenna array
can be located in such an active sector and still meet the operational requirements. By
employing a conformal array the scan angle can be increased and the grating lobe
phenomenon reduced. Switching of the antenna elements during operation may help
in eliminating the pointing of grating lobes in undesired directions.
Since a conformal array can be shaped to match the parent structure and shape and
can be designed for “stealth” operation, that is make them undetectable by enemy
radar. A lower radar cross section would be projected by the conformal array vis-à-vis
a planar array. Further, the conformal shapes have the advantage from an
electromagnetic point of view in that when a plane wave is incident on a curved
surface the energy will be diffracted along the surface and the reflected energy will be
defocused. As a result the reflected energy intensity will be lower than that available
on account of reflection from a planar surface.
1.8 Scope of the Thesis
The scope of the thesis is devoted to an introductory overview of microstrip antenna
on a planar, multidielectric and cylindrical surface. It first deals with the advent of
microstrip antenna and their characteristics. Subsequent sections of the chapter
present the multidielectric layer microstrip planar antennas with superstrate (cover)
layer. Finally the chapter is devoted to necessity of conformal antennas and conformal
arrays for defence applications and meeting the shaping requirements of the
aerodynamic structure.
Designs of symmetrical 2×2 and asymmetrical 2×3 patch array configurations along
with effect of mutual coupling have been analyzed in Chapter 2. First the basic
characteristics and the structure of a microstrip antenna, modelling and analysis of a
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2x2, i.e. four elements rectangular microstrip patch antenna array are discussed.
Mutual coupling in both E and H plane due change in inter element spacing and its
effect on antenna parameters viz. directivity, gain, efficiency; resonant frequency and
power output are studied. A symmetrical 2×2 microstrip antenna array at 10 GHz is
fabricated to validate the results. At 0.5 λ identical antenna parameters is observed in
both E and H plane. At 0.55λ the antenna resonating frequency matches, both the
directivity and gain in E and H plane are of the order of approximately 11 dB. Inter
element spacing at 0.6λ results in antenna radiating 1.9 mW powers in both the
planes. At 0.7λ element spacing the antenna efficiency is 97.5% in both the planes.
An optimum inter-element spacing of 0.55 λ in both E and H plane for the
symmetrical configuration for frequency range of the patch antenna array has been
arrived at. Next the chapter 2 discusses the performance analysis of a six elements
array in a 2×3 configuration for obtaining an optimum frequency range of the patch
antenna array. With linear spacing of 0.55 λ in H-plane and 0.5 λ in E-plane and vice
versa, the performance parameters is closest to the designed operating frequency with
a good return loss, directivity, gain and power output. In comparison to symmetrical
array design with spacing of 0.55 λ in both E and H plane, in the asymmetrical array
propose, optimum result has been arrived at, which corresponds to 0.55 λ and 0.5 λ for
E and H plane respectively and vice versa.
The third chapter presents the conformal mapping technique for the design with
improved accuracy in the performance of multidielectric layer microstrip antenna.
Design of the multidielectric layer planar antennas using an algorithm developed by
the author provides frequency close to the designed operating frequency with an
acceptable directivity and gain. Performance of the antenna designed for the given
resonant frequency has been found to be in correspondence to the patch dimension
with accuracy to sixth decimal place. The resonant frequency of patch operating at 2
.7010 GHz, with variation in its length at 4th, 5th, at 6th decimal place resulted at
corresponding changes of frequency 8MHz, 0.8MHz and 0.1MHz respectively.
Therefore the compounding effects of the inaccuracies from the design stage to the
fabrication of multidielectric layer microstrip antenna get eliminated with the
developed algorithm. Accuracy is achieved in both simulation and fabricated antenna
using the algorithm and is found to conform to the design. With the addition of the
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superstrate (cover) layer system performance is altered and significant changes in its
properties like resonance frequency, directivity, gain and bandwidth are observed.
The algorithm has been successfully tested on both thin and thick dielectric
superstrates having low permittivity.
Plots for a multilayer microstrip antenna subjected to the thin and thick superstrate
layer of thickness 0.254mm and 2.54mm respectively is used to predict the antenna
parameters including resonant frequency, return loss, power radiated, directivity and
gain. Gain of a multilayered structure increased as the height of the cover layer was
decreased. As regard thin cover layer dielectric, conductor losses are dominant while
for thicker cover layer surface wave losses are significant. It is found, choice of low
permittivity dielectric both thin and thick as cover layer is suitable for applications
requiring high antenna efficiency.
Chapter 4 discusses bandwidth enhancement procedure, emphasizing the importance
of quality factor optimization as a parameter to realize broadband communication. As
discussed in the chapter, the effective loss tangent δec is related to the total quality
factor QT for the patch. The total quality factor accounts for radiation, conductor and
dielectric losses. Further, the total quality factor may also be expressed in terms of the
average energy stored and average power loss per second. Certain quality factors have
significant impact on bandwidth for given permittivity and substrate thickness. The
antenna losses are contained by controlling these quality factors in the design of
microstrip patch antenna. While ensuring desired radiation pattern, the design
provides an improved bandwidth of the patch antenna. The design also considers the
effect of cover layer on impedance matching, Q factor hence bandwidth and
frequency. The results presented in the chapter are based on the Method of Moments
and Finite Difference Time Domain approach.
Evaluation of reflection and surface wave losses for low permittivity substrate based
on FDTD analysis has been carried out. Realization of improved bandwidth with
minimization of surface wave losses is attributed to contribution of offset impedance
matching employed in feeding technique. A series of analytical and graphical study of
a multilayer microstrip antenna, an impedance bandwidth of 5.8% has been obtained
with low permittivity substrates. The bandwidth improves to 7.5% by using a cover
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layer which is an improvement over a conventional multidielectric layer antenna by
approximately 30%. The designed antenna at 7.5 GHz based on the algorithm is also
fabricated and the result achieves the desired bandwidth conforming to the simulated
result.
The design of antenna of future aircraft systems to fend for itself from rapidly
changing threat situations has been addressed to in Chapter 5. It has been emphasized
that to overcome the threat in the form of Electronic Attack airborne antenna systems
need to be reconfigurable and to overcome intentional and unintentional
electromagnetic disturbances. This chapter therefore presents a novel design of
frequency agile reconfigurable multidielectric microstrip patch antenna with a cover
layer placed directly on the surface of the aircraft. Such a design can be suitably
utilized for the realization of frequency hopping specifically in high-performance
airborne applications.
A graph depicting linear relationship between the resonant frequency and the
permittivity of the cover layer has been obtained with the proposed design. Changes
in characteristics of the antenna to achieve frequency agility are shown in a
conformably mapped and fabricated microstrip antenna. Author proposes frequency
agility of the multidielectric microstrip antenna replacement of cover layer with
materials of different permittivity. The desired frequency is achieved by selecting the
permittivity of cover layer from this graph.
Appropriate choice of the cover layer parameters resulted in a significant increase in
gain and antenna efficiency, thus enabling the cover to act as the part of the antenna.
This will facilitate the use of the antenna in defence applications to avoid detection by
enemy radars. In the event of the enemy detecting the target, to prevent the jamming
of signals due antenna operating frequency detection, we can switch over to another
frequency just by replacing the original cover layer by a new cover layer with
different permittivity. Thus, the cover layer apart from shielding is utilized to make
the antenna reconfigurable and hence frequency agile. The proposed design achieves
frequency agility ranging from 0.5% to 18% with centre frequency at 2.718 GHz.
A radiating microstrip patch antenna mounted, as considered in Chapter 6, on a
cylindrical surface is chosen because major real world shapes can be approximated by
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cylindrical surface or cylindrical sector and uniformity in a plane provides an ease of
analysis. This chapter presents the necessity of conformal microstrip antenna and
arrays to suit the curved aerodynamic surfaces of supersonic aircraft or missiles and
modelled approximately in the shape of a cylinder. This Chapter is devoted to the
conformal mapping to the planar surface of the antenna arrays mounted on cylindrical
surface, Full-wave analysis of cylindrical microstrip using moment-method, design
and simulation of microstrip patch antenna and arrays on cylindrical surface, finally
analysis of the effect of mutual coupling in both planar and cylindrical surface.
Microstrip antenna on cylindrical surface designed to operate at 10 GHz is
transformed onto a planar surface by using conformal mapping technique. The feed to
the cylindrical antenna is also transformed and an impedance match is obtained. The
transformed antenna shows a return loss (S11) of -20dB at resonant frequency of
10GHz. The radiation pattern obtained is devoid of side lobes. The antenna is
radiating 2.3mW of power having directivity and gain of 7.22 dB and 6.93 dB
respectively. Resonant frequencies, fconformal, of the rectangular microstrip antenna
conformal on cylindrical surface are compared to resonant frequencies, fplanar, of the
transformed planar patch antenna. An empirical relationship from the curve showing
the variation of the ratios fconformal/fplanar versus Cylindrical Radius (r1)/Transformed
Length ( L′ ) is obtained. Similarly rectangular microstrip patch antenna array
operating at 10 GHz is transformed using conformal mapping technique. Analysis
for single patch as stated above is carried out for the array too. The antenna array is
seen radiating a power of 1.97mW, with directivity of 7.979 dB and gain of 6.988 dB.
The efficiency of the microstrip array antenna obtained is 87.6%. It is inferred that the
performance and the radiation pattern of the microstrip antenna array are affected due
to the mutual coupling between the antenna elements. Antennas mounted on singly
curved surfaces can be used in radar and communication systems due to its large
(azimuthal) angular coverage. The microstrip elements used for these investigations
employ dual patch antennas fed by two coaxial probes and then the mutual coupling
effect on combined quadratic patch antennas are studied. The antenna array is
designed at a frequency of 10 GHz. The resonating frequency is seen changing due to
mutual coupling, and at 0.6 λ it is resonating closest to the designed frequency. It is
observed that at 0.7 λ all antenna parameters provide best results except poor return
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loss with considerable shift in the resonant frequency from the designed frequency of
10 GHz. It can therefore be concluded that coupling in E-plane with spacing S= 0.7λ,
the return loss is -21.963 dB, whereas the gain, directivity and efficiency of the
antenna is maximum and the resonant frequency is at 10.21 GHz. Next best results of
antenna parameters are seen at 0.3λ and 0.4λ spacing. It is necessary that the effect of
the inter element spacing on the resonant frequency and antenna parameters must be
kept in mind while designing conformal arrays. In H plane with increase in inter
element spacing, the return loss improves up to λ/2, and thereafter it decays. The
antenna gain varies with the inter element spacing in H plane and it is limited but the
directivity drops linearly between 0.4λ to 0.7λ. Antenna efficiency improves at 0.4λ
spacing, and thereafter it drops, with a marginal increase at 0.8λ. Antenna parameters
in the H plane at 0.4λ spacing are the most optimum with the frequency deviation of
0.2841 GHz from the designed frequency of 10 GHz. At spacing of 0.5λ the resonant
frequency is at 10.24 GHz, the return loss is at -26.63dB, and the antenna parameters
are close to the best result. With four patches i.e. patch elements combined in E & H-
plane, effect of mutual coupling are studied. With S= 0.7λ & 0.5λ in E & H-plane
respectively, it is observed that the antenna characteristics shows best results. A
comparative study of the performance involving the planar array with the array
conformal to cylindrical surface has been carried out. Antenna parameters in the E
plane and for the H plane have been analysed. In the E plane best results are seen at
S=0.8λ, whereas for H plane corresponding value is at S=0.5λ.
Finally in the full-wave analysis, using Basis functions, the cylindrical surface
excitation has produced radiation pattern in both θ and φ plane. The cylindrical patch
is excited with impulse function in φ plane and cosine function in z plane. Field plots
shows, in the φ plane the plot is identical, however in the θ plane, the radiation pattern
shows significant side lobe. The radiation pattern has directional beam which is
symmetrical in all the four quadrants.
Lastly Chapter 7 deals with the main contributions and results of the thesis and scope
of further work.