DESIGN AND DEVELOPMENT OF FRACTAL ANTENNAS FOR WIRELESS COMMUNICATION. Ph.D. Summary

Transcription

1 DESIGN AND DEVELOPMENT OF FRACTAL ANTENNAS FOR WIRELESS COMMUNICATION Ph.D. Summary SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY IN ELECTRONICS AND COMMUNI CATION ENGINEERING MAHARISHI MARKANDESHWAR UNIVERSITY MULLANA, AMBALA, INDIA KULDIP KUMAR Department of Electronics and Communication Engineering M.M Engineering College Maharishi Markandeshwar University Mullana (Ambala) October, 2012

2 ABSTRACT The rapid growth of wireless technologies has drawn new demands for integrated components including antennas. In particular, DQWHQQD VPLQLDWXUL]DWLRQLVQHFHVVDU\IRU achieving an optimal design of modern ha ndheld wireless communication devices. Antenna on chip is a new mantra the in area of antenna research. Numerous techniques have been proposed byresearchers for the miniaturization of microstrip patch antennas with multiband ch aracteristics. For many years, various antennas for multiband operation have been studied and designed for communication and radar systems. One of the solutions for th e multiband characteristics is the fractal antennas. The Fractal antennas are based on e thconcept of fractal geometries. Fractal structure is made in order to obtain a reduced size multiband patch antenna. The use of fractal geometries in antennas have proved to be a good strategy in order to obtain log periodic multiband behaviour. This is mainly due to the self-similarity property of fractals which means that some of its parts ve hathe same shape as a whole object but at different scales. In addition, due to their space filling prop erties, fractals are used in antenna miniaturization. The use of space filli QJFXUYHVLQFUHDVHVWKHDQWHQQD VHOHFWULFDO length. This allows low frequency operations. Therefore, the fractal geometry allows miniaturization of radiators with overall small dimensions and long electrical length. Thus, the miniaturization effect in fractal microstrip antenna is based on lengthening the surface current lines in the patch element. As result, a the electrical length of resonator is expanded and the entire structur e is miniaturized. One of the crucial aspect of fractal antenna is to maintain the radiation patternmainly shape and the characteristics. The etching of the metal portion does affect the diation ra properties of the antenna hence, is optimally done to ensure radiation properties Fractals antennas have peculiar properties that make them suitable for applications where wideband and multiband are important parameters of the overall performance. In recent years, several fractal geometries have beenintroduced for antenna applications with varying degrees of success in improving an tenna characteristics. Some of these geometries have been particularly useful in reducing the size of antenna, while other designs aim at incorporating multiband characteristics. 2

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4 antenna is a key component of the integr ated low profile wireless communication systems. In addition, antenna miniaturizati on is also another emerging demand to meet the requirements of modern wireless standa rds and devices. For this purpose, several antenna configurations (including designs) based on fractal geometries have been UHSRUWHGLQUHFHQW\HDUV>±@7KHVHDUHSUeferred, as these are low profile antennas with gain achievable in desired range and can also be made operative at multiple frequency bands. This is why, these are also referred as multifunctional antennas. Therefore, in research reported in this thesis, the multiband (in particularly, multifunctional) aspect of fractal antenna designs are investigated further, with the key IRFXVRQLGHQWLI\LQJWKHIUDFWDODQWHQQD VPXOWL band characteristics. Inparticularly, the reported research is primarily focused on microstrip patch type fractal antennas. Numerous techniques have alr eady been proposed for the mini aturization of microstrip patch antennas supporting multiband charac teristics [11-25]. A review of these technologies and design methodologies is presented in chapter 3. In modern wireless communication systems, e th key requirement is the need of multiband antennas. Because of this reason, for last many years, the performance of various antennas for multiband operation has been nvestigated i for various communication and radar systems. One of the solutions of intere st, for the multiband characteristics is fractal antennas [26-30]. Fractal structure is preferre d because of their small size and multiband characteristics [21-35]. Several engineers and researchers have already worked and/or are doing further developments in is tharea of fractal antennas. From these studies, the use of fractal geomet ries in antennas have shown to be a good strategy in order to obtain log periodic multiband behaviour [36-40]. This is primarily due to the similarities of fractals in shape, within the whole ante nna structure. More specifically, some parts of antennas i.e. fract als, have same shape as the whole object (antenna); however, at reduced scales [5]. This discussed further in detail in following chapters. In addition, due to their space-filling properties, fractals are preferably used in miniaturization of modern multiband antennas. It is because the use of space-filling geometries (e.g. curves) increas HVWKHDQWHQQD VHOHFWULFDO length [6]. Thus, the use of fractal geometry has a potential to minimize diators ra with smaller dimensions and longer electrical lengths. This is beneficial as it miniaturizes the fractal microstrip antenna by 4

5 lengthening the surface current lines in the patch elemen t [7-8]. Because of these benefits, this thesis is focused on the use of fractal geometry in designing antennas. Fractal is first defined by Benoit Mandelbrot [9] as a way of classifying structures whose dimensions are not whole numbers. These geomet ries have also been used previously to characterize unique occurrences in nature that are difficult to define with the Euclidean geometries. Typical example includes defining the length of coastlines, density of clouds, branching of trees, snowflakes and human lungs. When fractal geometry is used in context of designing antennas, a number of bene fits are reported such as improvement in multiband characteristics of antennas [10]. A review of these reported studies is mentioned in chapter 3. Fractals, in practice, can be defined as abstract objects that cannot be physically implemented. Nevertheless, some related geometries can be used to develop an ideal fractal, usef ul in constructing antennas. Th ese geometries are called prefractals or truncated fractals, and the techni ques involved in using the same in antennas are called fractal antenna technologies. More specifically, the te rm fractal antenna technology is used to describe those antenna engineering techniques that are based on mathematical concepts such as self-similarity and fractional dimensions. Self-similarity is useful in designing multi-frequency antennas in which the object to be designed is composed of sub-units and/or sub-sub-units on multiple levels resembling the structure of whole object. Whereas, fractional dimensionsmeans non-integer dimensions that differ its design from Euclidean geometry [4]. From the above discussion, it can be inferred that the use of fractal geometries based antennas ve haseveral benefits. Therefore, this thesis focuses on developing novel fractal geometry to design multiband microstrip patch fractal antenna. Fractal geometries are associ ated with complicated task and associated problems. An overview of these problems is presented below. 1.2 Key associated problems Antenna design is usually reported very complicated by engineers and researchers because of its many parameters especially related to directiona l characteristics. Commonly used designs such as Sierpinski sket Ga and Koch Curve, are usually reported sensitive to only a narrow range of frequencies, and are not efficient if the same have dimensions smaller than a quarter of the wavelength [41-43]. This problem is more critical for small, portable antennas, such as patch antennas used in cellular phones. In 5

6 addition, these devices are usually battery pow ered, so their design always demands for low power antennas with better gain and sma ller geometries. Moreover, because of the high demands of these devices, the vendors shifting are towards the higher frequencies in GHz, to meet the bandwidth criteria. This further leads to the demand of miniaturizing the size of antennas. These are contradictory requirements; therefor e, several engineers and researchers are working to find an op timum solution to address these problems. A potential solution is the use of fractal ante nnas; which has reported having a potential of offering low gain and low power handling. However, the fractal antenna suffers from spurious feed radiation and has narrow bandwidth [44-45]. 1.3 Potential solution Fractal antenna designs can provide a potential solution to the problems mentioned above. Research studies, such as Sierpinskifractal loop antenna, have shown that antennas built with only a small number of iterations of a fractal process can exhibit better usable sensitivity at several frequencies [46-57]. Also, with the increase in number of iterations, it is reported that the lowest frequency of the antenna gets further lower and additional higher frequencies are added [13]. Or in othe r words, the bandwidth of operation of antenna increases and can be rther fu enhanced by using, say, stacked fractal antenna. Also, fractal antennas can operate efficiently at one-quarter the size of more traditional designs. Because of all these reported advantages, the research reported in this thesis is primarily focused on developing a low powered fractal antenna. This approach is expected to provide a better compact and multi-frequency antennas, suitable for low powered devices such as cellular phones. The same design approach can also be used for designing antennas for other applications such as multi-frequency wireless LAN and maritime antennas. 1.4 Aim To provide solution to the antenna requirements mainly for low powered wireless communication systems ensuring size reduction and multiband characteristics. 1.5 Objectives 6

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