Design and Simulation of Dual Frequency E-Shaped Microstrip Patch Antenna for Wireless Communication

 TABLE OF CONTENT

Table of Content………………………………………………………………………………….v

List of Figure………………………………………………………………………………........viii

List of Tables……………………………………………………………………………….........ix

List of Symbols and Abbreviations……………………………………………………………....x

CHAPTER 1: INTRODUCTION ................................................................................................1

1.1Background ................................................................................................................................1

1.2Problem Statement .....................................................................................................................1

1.3Motivation ……………………………………………………………………………………..1

1.4Research Objective ....................................................................................................................2

1.5Organization of this thesis .........................................................................................................2

CHAPTER 2: LIERATURE REVIEW ……………………………………………………………4

2.1 Introduction ……………………………………………………………………............................4

2.2 Antenna Miniaturization………………………………………………………………………….4

2.3 Miniaturization Techniques………………………………………………………………………4

2.3.1 High permittivity Material……………………………………………………………………...5

2.3.2 Quarter wavelength patch……………………………………………………………………….6

2.3.3 Planner Inverted F Antenna……………………………………………………………………..7

2.2.4 Capacitive Loading of the Antenna…………………………………….....................................8

2.3.5 Slots in Anteena…………………………………………………...............................................7

2.6 Top view of different slot antennas in circular shape……………………………………………9

2.3.6 Meandering n Antenna…………………………………………………………………….9

2.4 selected Miniaturization Technique………………………………………………………....9

2.5Antenna perfomence perameters…………………………………………………………….10

2.6 Gain Considerations…………………………………………………………………………12

2.7 Feeding Techniques…………………………………………………………………………13

2.7.1 Microtrap Line Feed…………………………………………………………………….. 13

2.7.2 Coaxial Feed…………………………………………………………………………….. 14

2.7.3 Aperture Coupled Feed………………………………………………………………… 17

2.7.4 Proximity Coupled Feed………………………………………………………….......... 16

2.8 Considerations for Biomedical Application……………………………………………… 17

2.8.1 Biomaterial……………………………………………………………………………… 18

2.8.2 Biocompatibility………………………………………………………………………… 18

CHAPTER 3: METHODOLOGY……………………………………………………….. 19

3.1 Frequency Selection…………………………………………………………………….. 19

3.2 Antenna Design Equations……………………………………………………………… 19

3.2.1 Circular Antenna……………………………………………………………………… 19

3.2.2 Rectangular Antenna ………………………………………………………………….. 20

3.3 Design Process and Simulation Setting………………………………………………… 23

3.4 Antenna Feeding……………………………………………………………………….. 24

3.5.1 Rectangular Antenna………………………………………………………………..…… 25

3.5.1.3 Antenna Simulation in Biological Tissue…………………………………………… 25

3.5.1.4 Simulation with Biocompatible Material Coating…………………………………. 26

3.5.1.5 Simulation with Biological Tissue…………………………………………………. 26

3.5.1.6 Performance Analysis of the Rectangular Antenna………………………………... 37

3.5.2 Circular antenna………………………………………………………………… ...... 28

3.5.2.3 Antenna Simulation in Biological Tissue…………………………………………... 28

3.5.1.6 Performance Analysis for Circular Antenna……………………………………… 30

3.5.3 Discussion……………………………………………………………………………. 30

CHAPTER 4: RESULT AND DISCUSSION…………………………………………… 32

4.1 Radiation pattern directivity…………………...……………………………………… 32

4.2. Directivity (theta)……………………………………………………………………… 32

4.3 Far Field Gain (Phi)……………………………………………………………………… 33

4.4 Far field directivity (phi)…………………………………………………………………….34

4.5 Gain………………………………………………………………………………………….35

4.6 S parameter………………………………………………………………………………….36

4.7 Voltage standing wave ratio (VSWR)……………………………………………………….37

CHAPTER 5: CONCLUSION………………………………………………………………..38

5.1 Conclusion…………………………………………………………………………………..38

5.2 Limitations of the Work……………………………………………………………………..38

5.3 Future Work…………………………………………………………………………………38

REFFERENCES………………………………………………………………………………39

LIST OF FIGURES

Figure 2.1: Miniaturization using higher dielectric permittivity material [22]…………………4

Figure 2.2: Quarter wavelength patch technique [22]…………………………………………..5

Figure 2.3: Smaller quarter wavelength patch technique [22]………………………………….5

Figure 2.4: Top view and side view of PIFA antenna [22]……………………………………..6

Figure 2.5: Current distributions on the patch layer (a) without slots (b) with slots……………8

Figure 2.6: Top view of different slot antennas in circular shape [22]…………………………8

Figure 2.7: Meandering for (a) quarter wave patch and (b) PIFA antenna [22]………………..9

Figure 2.8: Microstrip Line Feed [18]…………………………………………………………14

Figure 2.9: Coaxial Feed [39]………………………………………………………………….14

Figure 2.10: Aperture Coupled Feed [40]……………………………………………………..16

Figure 2.11: Proximity Coupled Feed [40]……………………………………………………17

Figure 3.1 (a) 2D view of the conventional antenna (b) 3D view of the conventional antenna…20

Figure 3.2 parameter distribution……………………………………………………………..22

Figure 3.3 Rectangular miniaturized antenna 2D and 3D view……………………………….25

Figure 3.4 circularly miniaturized antenna 2D and 3D view………………………………….28

Figure 4.1: radiation pattern directivity (3D)………………………………………………….32

Figure 4.2: directivity (Theta)…………………………………………………………………33

Figure 4.3: Far field directivity (theta)…………………………………………………………34

Figure 4.4: Far field directivity (phi)……………………………………………………………35

Figure 4.6: S-parameters………………………………………………………………………..36

LIST OF TABLE

Table 2.1 Comparison of Different Feeding Methods…………………………………………17

Table 3.1 Different Parameters for Conventional Antenna……………………………………22

CHAPTER 1: INTRODUCTION

1.1 Background

The microstrip patch antenna first took form in the early 1970’s [1], and interest was renewed in the first microstrip antenna proposed by Deschamps in 1953 [2]. Some of the benefits of microstrip patch antennas include [3] small profile, low weight and inexpensive fabrication. Additionally, by changing the shape of the structure, versatility in resonant frequency, polarization, pattern, and impedance can be achieved. Many feeding mechanisms are possible for feeding the microstrip patch structure, such as probe feeds, aperture feeds, microstrip line feeds and proximity feeds, where each method has advantages depending on the application [3]-[4]. The compact size of the microstrip patch antenna is advantageous for the biomedical application by personal communication devices since it is planar, and does not extend vertically from its mounting surface. The radiation pattern of the microstrip antenna has broad coverage in the E-plane with a maximum at broadside [4], which allows good coverage of signals from broadside down to near the horizon.

1.2 Problem Statement

Problem statement is a concise description of an issue to be addressed or a condition to be improved upon. It identifies the gap between the current (problem) state and desired (goal) state of a process or product. Focusing on the facts, the problem statement should be designed to address. The first condition of solving a problem is understanding the problem, which can be done by way of a problem statement.

1.3 Motivation

Miniaturization becomes one of the greatest challenges in implantable-antenna design, with the aim of new technological developments in IMD electronics, leading to ultra-small antennas. Antennas used in retinal prosthesis implantable medical devices, for instance, are small enough to be inserted inside the human ocular orbit. The dimensions ofthe traditional half-wavelength (λ/2) antennas at the frequency bands allocated for medical implants – and especially at the ISM band, make them impractical for implantable applications.Another purpose of this work is to use the miniaturized antenna for biomedical applications such as, Pacemaker, Intracranial Pressure Sensor

System, and Cochlear Implant, Retinal Implant, Wireless medicine and many heath care systems using wireless implantable devices.The motivation for this study evolved from the desire to

design miniaturized antenna with smaller in size than existing work having bandwidth greater than 5MHz at resonant frequency when matched to a source impedance Z0 of 50Ω.

1.4 Research Objective

Research objectives describe concisely what the research is trying to achieve. They summarize the accomplishments a researcher wishes to achieve through the project and provides direction to the study. A research objective must be achievable, i.e., it must be framed keeping in mind the available time, infrastructure required for research, and other resources. Before forming a research objective, you should read about all the developments in your area of research and find gaps in knowledge that need to be addressed. This will help you come up with suitable objectives for your research project.

1.5 Organization of this thesis

An organizational statement is a map that tells your reader what h/she should expect to read in your essay. It introduces the two or three main pieces of evidence that you will use to support your position. While not required in a thesis, organizational statements can make for stronger thesis statements.

2.1 Introduction

In order to perform a successful design, the necessary steps are, Miniaturization, since the dimensions of the traditional antennas in MICS make these antennas unfeasible for implantable applications, Biocompatibility, in order to preserve patient safety and prevent rejection of the implant, Restricted power incident, on the implantable antenna due to patient safety related issue, Far-Field Gain, which indicates the desired receiver sensitivity for achieving reliable biotelemetry communication, Low Power Consumption, to extend the lifetime of the implantable medical device.

2.2 Antenna Miniaturization

In modern wireless communication systems, the microstrip patch antennas are commonly used in the wireless devices. Therefore, the miniaturization of the patch antenna has become an important issue in reducing the volume of entire communication system. Miniaturization usually comes at a price of reduced bandwidth, usually bandwidth is proportional to the volume of the patch cavity [22].

2.3 Miniaturization Techniques

The common methods for reducing the microstrip patch antenna are:

a) High Permittivity Material

b) Quarter Wavelength Patch

c) Planner Inverted F Antenna (PIFA)

d) Capacitive Loading of the Antenna

e) Slots in Antenna

f) Meandering in Antenna.

CHAPTER 2: LITERATURE REVIEW

2.3.1 High Permittivity Material

One of the most direct means of reducing the size of a microstrip antenna is to increase the relative permittivity (εr) of the dielectric used for the substrate material. The lowering of resonant frequency results from the relationship between the speed of light and the dielectric permittivity [23],as shown in equation 2.1 𝑐=1√𝜀𝜇=𝑐0√𝜀𝑟𝜇𝑟

Thus, as the relative permittivity is increased, the speed of light decreases. For a resonant structure, this slower speed means an object loaded with dielectric materials of εr > 1 will have a lower resonant frequency than an unloaded identical size structure. Therefore, these loaded structures are said to be “electrically larger” than their unloaded counterparts of the same physical size. Figure 2.1 shows the higher dielectric permittivity material ( = 1 = 4) gives a reduced antenna size where the smaller patch has about one-fourth the bandwidth of the original patch because bandwidth is inversely proportional to the permittivity.

Figure 2.1: Miniaturization using higher dielectric permittivity material [22].

2.3.2 Quarter Wavelength Patch

Another technique is Quarter wave patch where new patch has about one half the bandwidth of the original patch.Figure 2.2 shows the technique for reducing the size of antenna where the width remains the same but the length is divided by two. So we get the half of the radiating magnetic current.A smaller quarter wave patch have the same aspect ratio W/L as the original patch [22].

Figure 2.2: Quarter wavelength patch technique [22].

The new patch has about one half the bandwidth of the original quarter wave patch, hence the one fourth the bandwidth of the regular patch. The figure 2.3 shows, in case of smaller quarter wave patch technique the width and length both are reduced to half.

Figure 2.3: Smaller quarter wavelength patch technique [22]

2.3.3 Planner Inverted F Antenna (PIFA)

There has been always eternal demand for cheaper, small and dense wireless systems. The demand of reduced size of the wireless systems can be accomplished through small scale and compressed antennas. In order to have such antennas, Planar Inverted F Antenna is the best choice [24].The Planar Inverted F Antenna is can be easily mounted on portable equipment’s because of its small size, low weight and height. The main elements of PIFA are rectangular planar element, [25] ground plane and the short circuit strip. In PIFA, a short circuit plate is placed between the radiator plate and the ground plane in order to reduce the length of the rectangular element.

In figure 2.4, a short circuit plate is placed between the radiator plate and the ground plane in order to reduce the length of the rectangular element. The variation in the size of the ground plane certainly affects the impedance bandwidth of the antenna. The appropriate positioning of feed pin and ground aids to better impedance matching of PIFA. The use of high dielectric materials reduces the size of PIFA on the cost of degraded performance of the antenna. The height of the antenna is very crucial, widening the air gap between the radiating element and ground provides better gain and broad bandwidth [25].

Figure 2.4: Top view and side view of PIFA antenna [22].

2.2.4 Capacitive Loading of the Antenna

Capacitive loading offers some distinct advantages when designing a microstrip antenna. A cavity backing provides a metallic boundary around the antenna that can be used to isolate the antenna from its surroundings. This allows close integration of an antenna onto circuit boards, or surfaces where the antenna must be placed near surfaces that might absorb or scatter energy. Also, a microstrip antenna is planar and can conform to the surface it is mounted on, providing compact and unobtrusive antenna placement on the exterior of automobiles, airplanes and other surfaces. When a cavity is placed around a microstrip antenna, this allows the antenna to be recessed into the mounting surface (the ground plane), flush with the surface [26].

The cavity also provides some electrical benefits. The walls of the cavity, if close to the patch antenna, load the edges of the patch similar to that of a lumped parallel plate capacitor, which lowers the overall resonant frequency of the patch. Often miniaturized patch antenna designs use thick, high permittivity substrates to reduce the resonant frequency while maximizing bandwidth for a given area. The side-effect of this method is the excitation of surface waves, which results in a loss of power out along the grounded substrate – lowering the efficiency of the antenna and/or distorting the radiation pattern. By placing a cavity behind the patch, surface waves are suppressed by the metallic walls, which essentially “short out” the TE/TM surface wave modes [27].

2.3.5 Slots in Antenna

The TM100 mode that develops on the patch has a resonant frequency dependent on the length of the patch. While a high permittivity substrate will make the metal patch look electrically larger by changing the wave propagation speed, another method used in tuning a microstrip antenna is loading the patch with slots.

Figure 2.5: Current distributions on the patch layer (a) without slots (b) with slots

In the figure 2.5 the slots can be viewed as obstructions to the path of the current, forcing a longer physical distance for the current to travel. The patch without slots allows a straight path across the patch, whereas the slots force currents to take a longer path [23].

Figure 2.6 shows the slots are placed at the midpoint of the patch, but they can be located anywhere along the patch if they change the current paths. One important consideration in placement of the slots is the polarization desired, as asymmetric slot placement can potentially cause cross-polarization levels to rise. For asymmetric slots, resonant current paths can develop off the main axes of the patch, such as along a diagonal axis, producing radiation components along both of the main axes instead of only one axis. Increased cross-polarization will result in poor axial ratio for circular polarization, and coupling between the two orthogonal feeds will increase. Another representation of the slots is that of a lumped circuit inductor, placed in series with the transmission line model for the patch antenna [28]-[30].

Figure 2.6: Top view of different slot antennas in circular shape [22].

2.3.6 Meandering in Antenna

Meandering is also another technique which forces the current flow through a longer path, increasing the effective dimensions of the patch. Meandering also increases the capacitance of the PIFA line. The design of meander line antenna is a set of horizontal and vertical lines which forms turns. As number of turn’s increases, efficiency increases. In case of meander line if meander spacing increases with respect to that the resonant frequency decreases [31]. Meander line antenna shown in figure 2.7, has significant advantages, it is electrically small, low profile antenna and has simple structure [32]. But these antennas have some disadvantages, low radiation efficiency. When the size of antenna is reduced, the radiation resistance is reduced. This results in the problem of decreased radiation efficiency because the ratio of the Ohmic loss of the antenna conductor to the radiated power is increased [33].

Figure 2.7: Meandering for (a) quarter wave patch and (b) PIFA antenna [22].

2.4 Selected Miniaturization Technique

Among all described techniques, PIFA and meandering are very much suitable for biomedical applications, in biomedical most of antennas are implantable and for implantable situations the perfect match is PIFA antenna [24], on the other side meandering helps create better electrical path that is why these two techniques are the often applied in this sector.In this work, two techniques including PIFA and meandering are considered and combined to develop miniaturized antennas.

2.5 Antenna Performance Parameters

Different antenna performance parameters are described as follows:

Antenna directivity: The directivity of an antenna is given by the ratio of the maximum radiation intensity (power per unit solid angle) to the average radiation intensity (averaged over a sphere). [4] The directivity of any source; other than isotropic, is always greater than unity.The equation of directivity D is given by, 𝐷=𝑈𝑈0=4𝜋𝑈𝑃𝑟𝑎𝑑 (2.2)

where, U = radiation intensity (W/ unit solid angle)

= Total radiated power (W)

Antenna efficiency: The total antenna efficiency accounts for the two following losses: Reflection because of mismatch between the feeding transmission line and the antenna and secondly the conductor and dielectric losses [4].Antenna efficiency, e is given by, 𝑒=𝑃𝑟𝑎𝑑𝑃𝑎𝑐𝑝 (2.3 )

where, = Power accepted by the antenna

Return loss: Return loss is an important parameter when testing the antenna. It is related to impedance matching and the maximum transfer of power theory. It is also a measure of the effectiveness of an antenna to deliver of power from source to antenna. The return loss (RL) is defined by the ratio of the incident power of the antenna Pin to the power reflected back from the antenna of the source Pref, hence the mathematical expression is [34], 𝑅𝐿=10log10𝑃𝑖𝑛𝑃𝑟𝑒𝑓(𝑑𝐵) (2.4)

Antenna Pattern: The antenna pattern is a graphical representation in three dimensions of the radiation pattern of the antenna as a function of angular direction. Antenna radiation performance is usually measured and recorded in two orthogonal principal planes (such as Eplane and H-plane or vertical and horizontal planes). The pattern is usually plotted either in polar or rectangular coordinates. The pattern of most base stations contains a main lobe and several minor lobes, termed side lobes. A side occurring in space in the direction opposite to the main lobe is called back lobe [3].

E-plane Pattern: “For a linearly polarized antenna, the plane containing the magnetic field vector and the direction of maximum radiation”. For base station antennas, the E-plane usually coincides with vertical plane [4].

H-plane Pattern: “For linearly polarized antenna, the plane containing the magnetic field vector and the direction of maximum radiation”. For base station antennas, the H-plane usually coincides with the horizontal plane [4].

Far-field region: That region if the field of the antenna where the angular field distribution is essentially independent of the distance from specified point in the antenna region is known as far field region. The radiation pattern is measured in the far field [4].

Frequency Bandwidth: “The range of the frequencies within which the performance of the antenna, with respect to some characteristics, conforms to a specified standard”. VSWR of an antenna is the main bandwidth limiting factor [3].

Input Impedance: “The impedance presented by an antenna at its terminals”. The input impedance is a complex function of frequency with real and imaginary parts. The input impedance is graphically displayed using a Smith chart [3]. Input impedance is a coplex quantity that varies with frequency and expressed as, 𝑍𝑖𝑛(𝑓)=𝑅𝑖𝑛(𝑓)+𝑗 𝑋𝑖𝑛(𝑓) (2.5)

where, f is the frequency

Radiation efficiency: “The ratio of the total power radiated by an antenna to the net power accepted by the antenna from the connected transmitter” [4].

Scattering parameters: The reflection and transmission coefficients between the incident and reflection waves. They describe completely the behavior of a device under linear conditions at microwave frequency range. Each parameter is typically characterized by magnitude, decibel and phase. The expression in decibel is 20log (Sij) because s-parameters are voltage ratios of the waves [3]-[4].

2.6 Gain Considerations

The effective aperture of an antenna relates how large of an area over which an antenna efficiently accepts an incoming signal, and is related to the size of an antenna. It is related to directivity (and therefore gain), and is defined as [35], 𝐷=4𝜋𝜆2 𝐴𝑒𝑓𝑓 (2.6)

While for small antennas the effective aperture size is, in general, larger than the physical aperture size, as operating frequency decreases for a fixed antenna size, the effective aperture size will also decrease. For miniaturized antennas, the directivity will be lower than that of a regular antenna, and will have a directivity pattern that broadens, and looks more like an omnidirectional antenna as size is further reduced. However, this is not the only factor working against the gain of small antennas. The currents of the antenna are confined to a smaller area on the antenna surface, contributing to conductive losses, and stronger fields near the antenna contribute to the stored energy [36]. A helpful figure of merit is the concept of the “quality factor”, also referred to as simply “Q”, of a circuit – in this case an antenna. Fundamentally, in antenna design Q is defined as the ratio of the total time averaged energy stored in a given volume to the power radiated (i.e. power “loss”) [37].

2.7 Feeding Techniques

Microstrip patch antennas can be feed by a variety of methods. These methods can be classified into two categories- contacting and non-contacting. In the contacting method, the RF power is fed directly to the radiating patch using a connecting element such as a microstrip line. In the non-contacting scheme, electromagnetic field coupling is done to transfer power between the microstrip line and the radiating patch. The four most popular feed techniques used are the microstrip line, coaxial probe (both contacting schemes), aperture coupling and proximity coupling (both non-contacting schemes) [18].

2.7.1 Microstrip Line Feed

Figure 2.8 shows a conducting strip is connected directly to the edge of the microstrip patch. The conducting strip is smaller in width as compared to the patch and this kind of feed arrangement has the advantage that the feed can be etched on the same substrate to provide a planar structure. The purpose of the inset cut in the patch is to match the impedance of the feed line to the patch without the need for any additional matching element. This is achieved by properly controlling the inset position. However as the thickness of the dielectric substrate being used, increases, surface waves and spurious feed radiation also increases, which hampers the bandwidth of the antenna. The feed radiation also leads to undesired cross polarized radiation [38].

Figure 2.8: Microstrip Line Feed [18].

2.7.2 Coaxial Feed

The Coaxial feed or probe feed is a very common technique used for feeding Microstrip patch antennas. Figure 2.9 shows the inner conductor of the coaxial connector extends through the dielectric and is soldered to the radiating patch, while the outer conductor is connected to the ground plane. The main advantage of this type of feeding scheme is that the feed can be placed at any desired location inside the patch in order to match with its input impedance [39]. This feed method is easy to fabricate and has low spurious radiation. However, its major disadvantage is that it provides narrow bandwidth and is difficult to model since a hole has to be drilled in the substrate and the connector protrudes outside the ground plane, thus not making it completely planar for thick substrates (h > 0.02λo). Also, for thicker substrates, the increased probe length makes the input impedance more inductive, leading to matching problems. It is seen above that for a thick dielectric substrate, which provides broad bandwidth, the microstrip line feed and the coaxial feed suffer from numerous disadvantages. The non-contacting feed techniques discussed below, solve these problems [39].

Figure 2.9: Coaxial Feed [39].

2.7.3 Aperture Coupled Feed

Figure 2.10 shows the radiating patch and the microstrip feed line are separated by the ground plane. Coupling between the patch and the feed line is made through a slot or an aperture in the ground plane. The coupling aperture is usually centered under the patch, leading to lower cross polarization due to symmetry of the configuration. The amount of coupling from the feed line to the patch is determined by the shape, size and location of the aperture. Since the ground plane separates the patch and the feed line, spurious radiation is minimized. Generally, a high dielectric material is used for the bottom substrate and a thick, low dielectric constant material is used for the top substrate to optimize radiation from the patch. The major disadvantage of this feed technique is that it is difficult to fabricate due to multiple layers, which also increases the antenna thickness. This feeding scheme also provides narrow bandwidth [40].

Figure 2.10: Aperture Coupled Feed [40].

2.7.4 Proximity Coupled Feed

This type of feed technique is also called as the electromagnetic coupling scheme. As shown in Figure 2.11, two dielectric substrates are used such that the feed line is between the two substrates and the radiating patch is on top of the upper substrate. The main advantage of this feed technique is that it eliminates spurious feed radiation and provides very high bandwidth (as high as 13%), due to overall increase in the thickness of the microstrip patch antenna. This scheme also provides choices between two different dielectric media, one for the patch and one for the feed line to optimize the individual performances. Matching can be achieved by controlling the length of the feed line and the width-to-line ratio of the patch. The major disadvantage of this feed scheme is that it is difficult to fabricate because of the two dielectric layers which need proper alignment. Also, there is an increase in the overall thickness of

Figure 2.11: Proximity Coupled Feed [40]

Table 2.1summarizes the characteristics of the different feed techniques.

Table 2.1Comparison of Different Feeding Methods

2.8 Considerations for Biomedical Application

The design and construction of an implantable antenna must make provision for avoiding different side effects after the implantation in the human body which comes into contact with a foreign object.

2.8.1 Biomaterial

A biomaterial is any substance that has been engineered to interact with biological systems for a medical purpose - either a therapeutic (treat, augment, repair or replace a tissue function of the body) or a diagnostic one. As a science, biomaterials is about fifty years old. The study of biomaterials is called biomaterials science or biomaterials engineering. It has experienced steady and strong growth over its history, with many companies investing large amounts of

Characteristics

Microstrip

Coaxial

Aperture

Proximity

line Feed

Feed

coupled Feed

coupled Feed

Spurious feed

More

More

Less

Minimum

Radiation

Reliability

Better

Poor due

Good

Good

to soldering

Ease of

Easy

Soldering

Alignment

Alignment

fabrication

and drilling

required

Required

needed

Impedance

Easy

Easy

Easy

Easy

Matching

Bandwidth

2-5%

2-5%

2-5%

13%

(achieved with

impedance

matching)

18

money into the development of new products [41]. The materials used today as coatings or as base materials for the construction of other provisions are metals, polymers or ceramics [42].

2.8.2 Biocompatibility

Biocompatibility is defined as the property of some materials do not cause toxic reactions or effects or injuries in the human body. This means that the host, the human body and its immune system, is not directed "against" this mater

CHAPTER 3: METHODOLOGY

3.1 Frequency Selection

The antenna resonant frequency was selected from ISM band frequency. The range of frequency is 2.40 GHz to 2.50 GHz, so the resonant frequency becomes 2.45 GHz having 100 MHz bandwidth, This frequency can be used all over the world, and it is also very suitable for biomedical application such as implantable medical devices IMD’s. That is the reason this frequency was selected.

3.2 Antenna Design Equations

Two types of antenna were designed, Circular and Rectangular, they have different design equations as they are different in size and shape therefore different equations

3.2.1 Circular Antenna

There are available equations for circular patch antenna [3]-[4] Radius of the patch is given by the equation 3.1 𝑎=𝐹√1+2ℎ𝜋𝜀𝑟𝐹[ln𝜋𝐹2ℎ+1.7726] (3.1)

where, 𝐹=8.791×109𝑓𝑟√𝜀𝑟

ℎ= Height of substrate

𝜀𝑟= Dielectric constant for substrate

𝑓𝑟= Operating frequency of antenna

And the effective radius of the patch is expressed as[4], 𝑎𝑒=𝑎√1+2ℎ𝜋𝜀𝑟𝐹[ln𝜋𝐹2ℎ+1.7726] (3.2)

where, 𝑎= actual radius of the circular patch.

3.2.2 Rectangular Antenna

Figure 3.1 (a) shows a design in 2D (b) shows in 3D antenna structure. This antenna resonates at 2.45 GHz. So the frequency of operation f0 = 2.45 GHz and FR-4 lossy material was used for substrate whose dielectricProperty (Ԑr) was 4.3 and the height (hs) of the substrate was selected 1.5 mm. using these parameters the height and width of the antenna was calculated using the following equations [3]-[4]:

𝑊=𝑐2𝑓0√𝜀𝑟+12

where c is the velocity of light

Figure 3.1 (a) 2D view of the conventional antenna (b) 3D view of the conventional antenna

To find the effective length the value of effective dielectric constant was calculated usingthe following equation [3]-[4], 𝜀𝑒𝑓𝑓=𝜀𝑟+12+𝜀𝑟−12(1+12ℎ𝑊)−0.5 (3.4)

Hence the effective length, is

𝐿𝑒𝑓𝑓=𝑐2𝑓0√𝜀𝑒𝑓𝑓 . (3.5)

3.3 Design Process and Simulation Setting

To design a rectangular microstrip patch patch antenna, the width and length of the patch was calculated using the equation 3.1 and 3.3 respectively. The calculated value of width and length was, 37.6 mm and 29.195 mm respectively. To design the feed line (Inset feed line technique used) the input impedance is usually 50 Ω for this input impedance the width of feed line (Wf) was calculated 3.137 mm. For the patch copper was used of thickness (ht) 0.035 mm and the dimension of ground plane and substrate was twice as the normal patch width and length. The substrate was FR-4 (lossy) with 1.5mm thickness. All the parameters are shown in the Table 3.1 and figure 3.2 shows the structures of these parameters distribution.To simulate the design antenna, frequency range was set and field monitors (electric field, magnetic field, and far field) were applied.

3.3 Conventional Antenna Simulation:

Fig.3.3 (a) shows the S11 parameter of conventional microstrip patch antenna with calculated parameter as described in previous chapter 3.2 article for antenna design in selected frequency. The designed antenna resonates at 2.39 GHz with return loss of -25.22 dB. The bandwidth of 70 MHz is found at -10 dB. The simulated input impedance of designed antenna is obtained as Zi=51.645-5.37i Ω at 2.39 GHz. The gain and directivity were found 2.653 dB and 6.993 dBi respectively. 36.8% radiation efficiency was obtained.

Using engineering simulation, big compute and 3-D printing, Optisys achieves orders-of-magnitude reduction in antenna size and weight while reducing development time. By

leveraging Ansys electromagnetic and structural simulation tools running on Rescale's big compute platform, this startup's engineers take full advantage of the design freedom offered by 3-D printing to meet radio frequency (RF) performance requirements for an integrated array antenna High-frequency antennas are traditionally built by fabricating and assembling dozens to a hundred or more individual components plus hardware to provide the required RF performance and structural integrity. The RF energy propagates from component to component

through interfaces, seams and discontinuities, so the RF path length must be increased to compensate for these obstructions.

Figure: “E” shape Antenna

3.4 Antenna Feeding

There are different feeding technique can be used for designing an antenna, previously in 3.2 inset feeding technique is used, the problem of this technique is that it creates a bulky antenna. Among many techniques coaxial feed line technique is used for miniaturization process in this paper. The reason of choosing this technique is that impedance matching is easy and good bandwidth can be obtained.The Coaxial feed or probe feed is a very common technique used for feeding Microstrip Patch antennas.

3.5 Miniaturized Antennas

As described in previous chapter 2.3 article for miniaturization PIFA and meandering techniques are used for both circular and rectangular shape. The design for circular and rectangular are given below

3.5.1 Rectangular Antenna

A Rectangular antenna was designed using PIFA and meander line techniques which operates at 2.45 GHz.

3.9.1.1 Antenna Configuration

Figure 3.9 shows miniaturized antenna, circular planner inverted F antenna technique is used along with meandered lines. Coaxial feeding technique is used to obtain resonance at the selected frequency.FR-4 (lossy) material which permittivity and height is (Ԑr) 4.3 and 1.5 respectively is used as substrate and the area of this antenna is 12x12 mm2. Copper (annealed) is used as meandering material with 0.035mm thickness. The electrical path of the designed antenna is 80.6 mm.

Figure 3.3 Rectangular miniaturized antenna 2D and 3D view

3.5.1.2 Antenna Simulation in Free Space

Fig.3.11 (a) shows the S11 parameter in free space where the antenna resonates at 2.45GHz with return loss of -39.7 dB. The bandwidth of 41.7 MHz (2.4259 GHz-2.4677 GHz) is found at -10 dB return loss which includes ISM band of 2.45 GHz. The simulated input impedance of designed antenna is obtained as Zi=50.41+0.9443i Ω at 2.45 GHz. The gain and directivity were found -15.77 dB and 1.891 dBi. The radiation efficiency was - 17.66 dB. The radiation pattern are shown in fig. 3.10(b) and 3.10 (c)

3.5.1.3 Antenna Simulation in Biological Tissue

As shown earlier biocompatibility analysis has done in two steps, firstly a biocompatible material is used to cover the whole antenna i.e. biocompatible material coating and secondly a muscle tissue is used for cover the entire configuration to make a human body phantom formed as a human body phantom.

3.5.1.4 Simulation with Biocompatible Material Coating

After applying the biocompatible material coating, the antenna resonates at 2.40 GHz with return loss of -41.873 dB without any changes in design parameter. After some parameter changes the antenna resonates at 2.45GHz with return loss of -37.15 dB. Now the new electrical path becomes 79.06 mm and the bandwidth remains same. The simulated input impedance of designed antenna is found as Zi=49.9-1.38i Ω. The gain and directivity were found -15.65 dB and 1.944 dBi without changing any design parameter and -15.16 dB and 1.91 dBi after changes respectively. The radiation efficiency was -17.07 dB. Fig 3.11(a) shows the antenna design in biocompatible material coating and 3.11(b) shows the s11 parameter before and after the changes were made.

3.5.1.5 Simulation with Biological Tissue

After inserting the antenna in biological phantom it resonates at 2.41GHz with return loss of -43.39 dB without change any design parameter the bandwidth increases to 43.8 MHz. After few changes in the configuration it resonates at 2.45GHz with return loss of - 32.14 dB having newelectrical path of 77.66 mm and the bandwidth increased to 45.3 MHz. The simulated impedance is obtained as Zi=50.41-2.35i Ω at 2.45GHz. The gain and directivity were found -18.33 dB and 1.763 dBi without change any design parameter and -17.46 dB and 1.821 dBi after changes respectively. The radiation efficiency was -19.28 dB. Fig 3.12(a) shows the

structure of the antenna after inserting it into biological phantom, 3.12(c) shows the comparison of s11 parameter between before and after the inserting the antenna in the phantom.

3.5.1.6 Performance Analysis of the Rectangular Antenna

Table3.2 shows the comparison among free space, biocompatible material coating and biological phantom calculations. The performance parameter in open space gives bandwidth of about 41.7 MHz with return loss of -39.7 dB and the radiation efficiency was -17.6 dB, gain and directivity were found -15.77 dB and 1.891 dBi respectively.

After biocompatible coating is applied the bandwidth remained same with -41.873 dB in return loss, and slight improvement in directivity was again obtained.

When the antenna was inserted in biological phantom the performance parameter such as return loss of -32.14 dB, bandwidth of 45.3 MHz, gain of -14.68 dB, directivity of 3.228, and radiation efficiency of -17.46 dB were obtained which are comparatively better.

The maximum gain value of -17.46 dB and radiation efficiency of -19.28 dB of designed antenna is much higher than the maximum gain of -41.03 dB and radiation efficiency of - 40.69dB for miniaturized meandered patch antenna designed by Nayla Ferdous [17]. The bandwidth of the designed antenna of value of 54 MHZ is greater than 40 MHz for novel miniature antenna designed by A. Kiourti [18]

3.5.2 Circular antenna:

A circular antenna was designed using PIFA and meander line techniques which operates at 2.45 GHz.

3.4.2.1 Antenna Configuration

Figure 3.4 shows miniaturized antenna, circular planner inverted F antenna technique is used along with meandered lines. Coaxial feeding technique is used to obtain resonance at the selected frequency.FR-4 (lossy) material which permittivity and height is (Ԑr) 4.3 and 1.5 respectively is used as substrate and the radius of the substrate is 8.5mm. Copper (annealed) is used as meandering material with 0.035mm thickness. The electrical path of the designed antenna is 72.7mm.

Figure 3.4 circularly miniaturized antenna 2D and 3D view

3.5.2.2 Antenna Simulation in Free Space

Fig.3.6 (a) shows the S11 parameter in free space where the antenna resonates at 2.45GHz with return loss of -20.9 dB. The bandwidth of 47.4 MHz (2.4285 GHz-2.4759 GHz) is found at -10 dB return loss which includes ISM band of 2.45 MHz. The simulated input impedance of designed antenna is obtained as Zi=57.71-6.1586i Ω at 2.45 GHz. The gain and directivity were

found -14.84 dB and 2.964 dBi. The radiation efficiency was - 17.80 dB. The radiation pattern are shown in fig. 3.6(b) and 3.6 (c)

3.5.2.3 Antenna Simulation in Biological Tissue

The biocompatibility analysis has done in two steps, firstly a biocompatible material is used to cover the whole antenna i.e. biocompatible material coating and secondly a muscle tissue is used for cover the entire configuration to make a human body phantom. In the first step, a biocompatible material, Silica is used to cover the antenna so that the antenna do not cause toxic effects in the immune system. Thickness of this silica layer is 0.5 mm all over. Distance of this layer from the main antenna is 1 mm. In the second step the antenna was formed as a human body phantom. Distance of this muscle tissue layer from main antenna is very close like 1-2 mm

3.5.2.4 Simulation with Biocompatible Material Coating

After applying the biocompatible material coating, the antenna resonates at 2.43GHz with return loss of -21.2 dB without any changes in design parameter. After some parameter changes the antenna resonates at 2.45GHz with return loss of -20.21 dB. Now the new electrical path becomes 71.4 mm and the bandwidth remains same. The simulated input impedance of designed antenna is found as Zi=59.40-7.7i Ω. The gain and directivity were found -15.14 dB and 3.155 dBi without changing any design parameter and -14.98 dB and 2.975 dBi after changes respectively. The radiation efficiency was -17.96 dB. Fig 3.7(a)

shows the antenna design in biocompatible material coating and 3.7(b) shows the s11 parameter before and after the changes were made.

3.5.2.5 Simulation with Biological Tissue

After inserting the antenna in biological phantom it resonates at 2.44GHz with return loss of -31.6 dB without change any design parameter the bandwidth increases to 53 MHz. After few changes in the configuration it resonates at 2.45GHz with return loss of -31.6 dB having new electrical path of 71 mm and the bandwidth increased by 1. The simulated impedance is

obtained as Zi=48.86-2.5i Ω at 2.45GHz. The gain and directivity were found -14.68 dB and 3.228 dBi without change any design parameter and -14.50 dB and 3.137 dBi after changes respectively. The radiation efficiency was -17.90 dB. Fig 3.8(a) shows the structure of the antenna after inserting it into biological phantom, 3.8(c) shows the comparison of s11 parameter between before and after the inserting the antenna in the phantom.

3.5.1.6 Performance Analysis for Circular Antenna

Table3.3 shows the comparison among free space, biocompatible material coating and biological phantom calculations. The performance parameter in open space gives bandwidth of about 47.4 MHz with return loss of -20.9 dB and the radiation efficiency was -17.8 dB, gain and directivity were found -14.84 dB and 2.964 dBi respectively.

After biocompatible coating is applied the bandwidth remained same with slight change in return loss, and slight improvement in directivity was obtained.

When the antenna was inserted in biological phantom the performance parameter such as return loss of -31.6dB, bandwidth of 54 MHz, gain of -14.5 dB, directivity of 3.228, and radiation efficiency of -17.67dB were obtained which are comparatively better.

The maximum gain value of -14.5 dB and radiation efficiency of -17.67 dB of designed antenna is much higher than the maximum gain of -41.03 dB and radiation efficiency of - 40.69dB for miniaturized meandered patch antenna designed by Nayla Ferdous [17]. The bandwidth of the designed antenna of value of 54 MHZ is greater than 40 MHz for novel miniature antenna designed by A. Kiourti [18]

3.5.3 Discussion

Both Circular and Rectangular antenna simulated performance were compared. Rectangular antenna performed better if return loss which is -32.14 dB considered. But in terms of 54 MHz bandwidth found in circular antenna is better. Gain and directivity of circular shape antenna is also better -14.5 dB and 3.125 respectively compared to rectangular shape antenna. Radiation

efficiency of circular shape antenna is also found better. But the calculated area of rectangular shape antenna is less than circular shape antenna. The circular shape antenna performed better in biological tissue on the other hand rectangular shape antenna perform better with biocompatible material coating but bandwidth increases when it is inserted into biological tissue.

Both the circular and rectangular antenna have better bandwidth compared to 5 MHz [14], and better gain achieved than -41.24 dB [20] and good radiation efficiency than [21], and also well miniaturized than circular antenna having 11.2 mm radius[14] compared to 8.50 mm radius and rectangular antenna having 22x18 mm [14] compared to 13x13 mm size.

CHAPTER 4: RESULT AND DISCUSSION

4.1 Radiation pattern directivity

The directivity, D, of an antenna is the maximal value of its directive gain. Directive gain is represented as D(θ,Ф)and compares the radiant intensity (power per unit solid angle) U(θ,Ф) that an antenna creates in a particular direction against the average value over all directions:

Here θ and Ф are the zenith angle and azimuth angle respectively in the standard spherical coordinate angles; U(θ,Ф) is the radiation intensity, which is the power per unit solid angle; and P(tot)is the total radiated power. The quantities U(θ,Ф) and P(tot) satisfy the relation

Figure 4.1: radiation pattern directivity (3D)

4.2. Directivity (theta):

Directivity is a fundamental antenna parameter. It is a measure of how 'directional' an antenna's radiation pattern is. An antenna that radiates equally in all directions would have effectively zero directionality, and the directivity of this type of antenna would be 1 (or 0 dB).

Figure 4.2: directivity (Theta)

4.3 Far Field Gain(Phi):

Sound transmission in underwater acoustic communication channels and signal-processing systems has been described. A basic purpose in this research is to develop digital underwater acoustic communication equipment suitable for practical applications. Transducers play an important role in underwater acoustic communication equipment, thus their structures and operating characteristics will be the first topic introduced in this chapter, which will help us to design suitable and feasible underwater acoustic communication equipment. Then, three types of civil digital underwater acoustic communication equipment being developed by authors will be discussed: (1) Underwater acoustic telecontrol communications equipment in which a digital time correlation accumulation decision system has been employed, (2) underwater acoustic multimedia communication equipment in which an improved FH-SS system has been employed, and (3) a digital underwater acoustic communication prototype using the innovative APNFM

system. Using these systems or prototypes, some expected experimental test results have been obtained in shallow-water acoustic channels.

Figure 4.3: Farfield directivity (theta)

4.4 Far field directivity (phi)

The directivity of an antenna relates to its radiation pattern. An antenna which radiates uniformly in all directions in three-dimensional space is called an isotropic antenna. Such an antenna doesn’t exist, but it is convenient to refer to it when discussing the directional properties of an antenna. All real antennas radiate stronger in some directions than in others. The directivity of an antenna is defined as the power density of the antenna in its direction of maximum radiation in three-dimensional space divided by its average power density. The directivity of the hypothetical isotropic radiator is 1 or 0 dB. The directivity of a half-wave dipole antenna is 1.64 or 2.15 dB

Figure 4.4: Farfield directivity (phi)

The radiation pattern of a wire antenna of short length compared to a half wavelength is shown in Fig. 3.1A. The antenna is high enough so as not to be affected by the ground. If the antenna wire direction is parallel to the earth, then the pattern represents the intersection of a horizontal plane with the solid pattern of the antenna shown in Fig. 3.1B [1]. A vertical wire antenna is omnidirectional; that is, it has a circular horizontal radiation pattern and directivity in the vertical plane.

4.5 Gain

Antenna obtain is commonly described as the ratio of the power produced by means of the antenna from a far-field source on the antenna's beam axis to the energy produced by way of a hypothetical lossless isotropic antenna, which is equally touchy to signals from all directions.

Gain is calculated by means of comparing the measured power transmitted or received by the antenna in a specific route to the electricity transmitted or received with the aid of a hypothetical perfect antenna in the identical situation. If no route is specified, attain refers to top value in the course of the antenna's primary lobe.

4.6 S parameter:

Scattering parameters or S-parameters (the elements of a scattering matrix or S-matrix) describe the electrical behavior of linear electrical networks when present process a number steady state stimuli by way of electrical signals.

Figure 4.6: S-parameters

The parameters are useful for various branches of electrical engineering, which include electronics, conversation systems design, and in particular for microwave engineering. The S-parameters are participants of a family of similar parameters, different examples being: Y-parameters,[1] Z-parameters,[2] H-parameters, T-parameters or ABCD-parameters.[3][4] They differ from these, in the sense that S-parameters do no longer use open or quick circuit prerequisites to represent a linear electrical network; instead, matched masses are used. These terminations are an awful lot less complicated to use at high signal frequencies than open-circuit and short-circuit terminations. Contrary to popular belief, the quantities are no longer measured in phrases of strength (except in now-obsolete six-port network analyzers).Modern vector community analyzers measure amplitude and phase of voltage touring wave phasors using in fact the equal circuit as that used for the demodulation of digitally Mmodulated wireless signals.

4.7 Voltage standing wave ratio (VSWR):

For a radio (transmitter or receiver) to supply electricity to an antenna, the impedance of the radio and transmission line need to be properly matched to the antenna's impedance. The parameter VSWR is a measure that numerically describes how properly the antenna is impedance matched to the radio or transmission line it is connected to.

VSWR stands for Voltage Standing Wave Ratio, and is additionally referred to as Standing Wave Ratio (SWR). VSWR is a function of the reflection coefficient, which describes the electricity mirrored from the antenna. If the reflection coefficient is given by means of s11 or reflection coefficient or return loss, then the VSWR is described by using the following

formula:

Figure 4.6: S-parameters

5.1 Conclusion

In this thesis, small size circular and rectangular PIFA antenna is presented operating in ISM band. The both miniaturized antenna have resonant frequency at 2.45 GHz and a bandwidth 47.4 MHz (for circular) and 45.3 MHz (for rectangular). In order to understand the performance of those two antennas, when implanted in a lossy material (phantom) and to achieve a thorough miniaturization of their size based in these designs, a complete study of the effect of each parameter was undertaken. Simulations using biocompatible material coating and human phantom model were done and it is observed that the performance of the antenna is quite same even better in the phantom model in the both cases. As two different antenna were designed and miniaturized, the performance of the circular antenna is better than the rectangular because of greater bandwidth, gain and directivity. The literature review also indicates that in this work better gain is achieved in higher frequency with proper radiation efficiency. The circular antenna also have versatile design which makes it a uniqueantenna. Both these antenna can be used for implantable medical devices i.e. biomedical applications. Good performance of these proposed antenna is appropriate to be applied in biomedical application.

5.2 Limitations of the Work

In this thesis FR-4 (lossy) material is used as substrate, it has low permittivity compared to other materials, using other high permittivity material also increases the cost of the antenna. Only two feeding techniques have used in this thesis.

5.3 Future Work

In future these miniaturized antennas can be implemented as hardware which can be used in implantable medical devices, more feeding techniques can be used for increasing the performance of the antenna and by using high permittivity material, thus more miniaturization can be possible in future.

CHAPTER 5: CONCLUSION

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