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chapter 1 thesis proposal about radar antenna optimizations system
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  1 CHAPTER I INTRODUCTION 1.1   Background: Radio frequency (RF) technology has been used in many fields such as telecommunication, medical, and military. One of RF applications in military field is radar. Radar is a technology which is widely used for range detection, weather sensing, navigation process, and detection of specified object. The most important part in radar technology is antenna. Radar antenna is a hardware device which radiates the electromagnetic waves to the targets, then receives the echo signals which is reflected by the targets. Most of the radiation  pattern which is needed for radar application (scanning and tracking) cannot be achieved by only using single element antenna, because the typical of single antenna element has wide radiation pattern and low directivity. Therefore, the antenna with more than one element or namely array comes to overcome these  problems. Array antenna does not only have narrow radiation pattern but also has higher directivity than single element antenna. Traditional radar antennas use mechanical method to conduct the beam steering while scanning. Mechanical method uses a rotator to mechanically change the direction of the radiation pattern. However, mechanical method has influential errors because of rotator error. In several decades, beam steering by electronically changing the array antenna elements phase variable in radar is widely used. These methods is known as phased array and widely discussed in (Hansen, 1998). Radiation pattern of phased array antenna can set to desired direction with setting the phase shifter on each antenna elements. Therefore, the use of phased array antenna is more efficient than mechanical method because it quickly and accurately steers the radiation (Stutzman and Thiele, 2012). Moreover, target with weak signal can be detected easily and can handle the interference from undesired target (Haupt, 1985). Transmit/receive (TR) module is an active element in phased array antenna which is used to give antenna performance improvement. TR module  2 which is embedded on each antenna elements has functions as a transmitter and receiver which are integrated into a module. Obviously it makes the design to be more complex and the cost getting enlarged (Agrawal, , 1998). Accordingly, this proposed research uses passive elements in phased array antenna because  passive elements have lower cost than active element. The use of many elements in array antenna makes the energy consumption and consequently the production cost get higher. The most effective method to reduce the production cost is reducing the number of antenna element and it makes the element spacing become larger (sparse). However, the grating lobes will occur when the element spacing is greater than a wavelength. The presence of grating lobes in radiation pattern can make the undesired signals enter the antenna and will reduce the antenna performance. In order to suppress grating lobes, sparse array usually reconfigured to unequally-spaced arrays, or aperiodic array. Some of the researcher studied about these issued are (Ishimaru, 1962), (Leahy and Jeffs, 1991) and (Hohn, , 1997). The aperiodic arrays configuration have several advantages, which are it have higher spatial resolution and lower side lobes than equally-spaced arrays configuration while using a fewer number of antenna elements. Many researchers in the last few years investigate about how to reduce the negative effects of the sparseness in the array antenna elements, such as (Haupt, 1985). Design comparison base on elements spacing parameter in sparse array antenna is investigate by (Athley, , 2007), and cross-entropy (CE) method is  proposed by (Minvile, , 2011) to optimize the sparse array antenna design, which only needs a few parameter to accelerate the processing time. (Leahy and Jeffs, 2007) implement sparse array in radar application to detection and direction finding. However, the most widely used method to implement in array antenna is genetic algorithm (GA), which investigate by (Haupt, 1994) which use GA method to thinning the array antenna, (Cen, , 2012), they proposed improved genetic algorithm to optimize the weight coefficient and sensor positions of the array, (Yan and Lu, 1997) discussed about genetic algorithm to control the grating lobes. The advantages of GA method are can optimize with continuous or discrete  parameters, can work with large number of variables, and easy to solved  3 electromagnetic problems. Another method to synthesize the arrays pattern is use tapering in antenna design, which employs a distribution function to arrange the arrays antenna both amplitude and density or even size. One of the well-known distributions commonly used to tapering the antenna radiation pattern is Taylor line-source distribution, this method was introduced by (Taylor, 1955). The implementation of sparse arrays in phased array antenna design is one of the major concerns to reduce the cost. In this proposal, we must realize that the sparse phased array problem not only from implementation point of view, but also look into more fundamental problems, that are grating lobes, gain, and system complexity. Nowadays, many methods have been proposed to investigate the just-mentioned fundamental problems, as mentioned in above paragraphs. Several topics related to sparse phased array antennas are: 1.   Strategy to destroy the periodicity to avoid grating lobes and to allow wide angular scanning A traditional linear array has a periodicity in spacing between each antenna element which causing the appearance of grating lobes. So, our aim is to produce a linear array with non-periodicity in element spacing without  producing grating lobes. By increasing the average spacing we can design a sparser linear array. By combining spatial/density tapering via non-periodic  placement with amplitude tapering we aim to develop a strategy for side lobe level control in a linear sparse array. 2. Analyze the effects of gain. Gain is a most important antenna evaluation parameter. We have to look at gain effects from fundamental viewpoint. We know that in a sparse array with reduced number of elements        (1.1) where G  is gain and  N   is the number of antenna elements. In a dense array (with more antenna elements) Gmax  can be larger at broadside but reduces with scan angle. The maximum scan angle while scanning, which can be achieved for the Field of View (FOV) of a phased array antenna is 120° (-  4 60° until 60° from broadside). This phenomenon is much less in sparse arrays. In sparse array, we reduce the number of antenna elements. From (1) we can see that the maximum gain is lower at broadside then for the dense array due to the reduced number of antenna elements. At maximum scan angle the difference in Gmax  between a dense and a sparse array  becomes less. Therefore, the strategy at maximum scan angle for reducing the number of antenna elements without reducing the antenna gain have to  be find. In other words, we should develop a strategy for scanning arrays by considering maximum scan angle, so that we have the same gain for sparse array (with less elements) as the fully populated array (with more elements) at maximum scan angle. In this research we want to develop this strategy leading to the same gain for sparse array and for dense array in case we analyze the array performance at maximum scan angle 3. Optimizing antenna aperture and radiation of an individual antenna element. The antenna effective aperture is given by            (1.2) where   is the wavelength and    is the gain of each antenna element. In (2) an effective aperture is related to the gain of the antenna element. In this topic attention should be paid to optimizing the element in order to get the smallest reduction of the array gain at maximum scan angle with minimum beam broadening of the array pattern over all scan angles. According to problems describes above, which are about the presence of grating lobes and gain reduction, in this proposal we implement joint amplitude-density-size tapering using GA method to optimize the phased array antenna design.
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