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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.

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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.

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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

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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.

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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

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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.

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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.

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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.

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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.

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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.

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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

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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

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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.

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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.

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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].

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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

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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]

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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

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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).

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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.