Aperture Coupled Microstrip Patch Antenna for WIFI, WiMAX, and Radar Applications by Christopher Keith Clayton A thesis submitted to the Graduate Faculty of Auburn University In partial fulfillment of the Requirements for the Degree of Masters of Science Auburn, Alabama May 4, 2014 Keywords: microstrip, patch, WIFI, WiMax, radar Copyright 2014 by Christopher Keith Clayton Approved by Lloyd Riggs, Chair, Professor of Electrical and Computer Engineering Robert Dean, Associate Professor of Electrical and Computer Engineering Stuart Wentworth, Associate Professor of Electrical and Computer Engineering ii Abstract The aperture-coupled microstrip patch antenna is presented. The operation frequency range of antenna is 4.8 - 5.2 GHz. This frequency range is possible due to the use of low dielectric materials for the patch and microstrip substrate, g2013g3045 = 1.03 g1853g1866g1856 g2013g3045 = 2.6. The antenna is fed by a microstrip feed line designed for 50? impedance. The g1845g2869g2869 for the antenna at 5 GHz is -38.3 dB. The -10dB bandwidth of the antenna exceeds 400 MHz which equates to 8% bandwidth overall. The realized gain for the antenna at 5 GHz is 8.33 dB when g2016 = 0?and g2038 = 90?. The maximum sidelobes are -3.02 dB and -2.29 dB when g2016 = ?55?and g2016 = 55?, respectively. The sidelobe level or the difference between the main beam maximum and the sidelobe, equals to around 10.62 dB. The co-polarization radiation is above -4 dB while the cross-polarization is below -32 dB. The overall performance is sufficient to support WIFI, WiMAX, and radar applications in mobile devices, routers, electronic devices, and for military purposes. iii Acknowledgments First, I am grateful for God for providing me with the knowledge and tools in order to complete my master?s degree. Second, I would like to thank my advisor Dr. Lloyd Riggs; mentors Dr. Robert Nelms, Dr. Stuart Wentworth, Dr. Robert Dean, and Jim Killian; family Rufus Clayton, Brenda Clayton and Antonio Washington; and friends especially Udarius Blair, Alana McClain, Marque Wright, Aisha Wright, and Whitney Davis for encouraging and motivating me throughout my entire educational experience at Auburn University. iv Table of Contents Abstract ........................................................................................................................................... ii Acknowledgements ........................................................................................................................ iii Chapter 1: Introduction ....................................................................................................................1 History of Microstrip Patch Antennas .................................................................................1 History of Aperture Coupled Microstrip Patch Antennas ....................................................2 Chapter 2: Microstrip Patch Antenna ..............................................................................................4 Design Specifications...........................................................................................................4 Advantages ...........................................................................................................................7 Disadvantages ......................................................................................................................9 Design Adaptations ............................................................................................................10 Chapter 3: Aperture Coupled Microstrip Patch Antenna ...............................................................13 Design Specifications.........................................................................................................13 Advantages .........................................................................................................................16 Disadvantages ....................................................................................................................17 Design Adaptations ............................................................................................................18 Chapter 4: Feed Techniques ...........................................................................................................21 Proximity Coupled Feed ....................................................................................................21 v Stripline Feed .....................................................................................................................22 Coplanar Waveguide Feed .................................................................................................24 Chapter 5: Aperture Coupled Microstrip Patch Antenna Applications .........................................27 WiMAX .............................................................................................................................27 WIFI ...................................................................................................................................30 Radar ..................................................................................................................................30 Chapter 6: High Frequency Structure Simulator (HFSS) ..............................................................31 Absorbing Boundary Conditions (ABC) ...........................................................................31 Perfectly Matched Layer (PML) ........................................................................................31 Chapter 7: Aperture Coupled Microstrip Patch Antenna ...............................................................33 Design Specifications.........................................................................................................33 Simulation Results .............................................................................................................38 Chapter 8: Conclusion and Future Work .......................................................................................42 References ......................................................................................................................................44 Appendix 1: MATLAB Patch Antenna Design Calculator ...........................................................46 vi List of Tables Table 1. Design parameters for aperture coupled microstrip patch antenna ..................................34 Table 2. Calculated and simulated patch dimensions ....................................................................35 Table 3. Nominal and simulated feed, stub, and total length at 5 GHz .........................................36 vii List of Figures Figure 1. Schematic diagram of a microstrip patch antenna ............................................................4 Figure 2. Schematic diagram of a microstrip transmission line.......................................................5 Figure 3. Efficiency versus substrate thickness for microstrip patch antenna .................................9 Figure 4. Bandwidth versus substrate thickness for microstrip patch antenna .............................10 Figure 5. Geometry of the wideband multislot patch antenna .......................................................11 Figure 6. Geometry of the compact printed patch antenna ............................................................12 Figure 7. Schematic diagram of an aperture coupled microstrip patch antenna ............................13 Figure 8. Circuit model for an aperture coupled microstrip patch antenna ...................................14 Figure 9. Schematic diagram of the aperture coupled stacked patch microstrip antenna ..............18 Figure 10. Schematic diagram of the cavity-backed aperture coupled microstrip patch antenna .19 Figure 11. Schematic diagram of the (a) backed substrate integrated cavity aperture coupled microstrip antenna and (b) geometry of the cross shape metallic via configuration ....................20 Figure 12. Proximity coupled fed patch antenna ...........................................................................22 Figure 13. Schematic diagram of stripline-fed aperture coupled patch antenna ............................23 Figure 14. Schematic diagram of stripline-fed aperture coupled double patch antenna ................24 Figure 15. Schematic diagram of a CPW-fed aperture coupled patch antenna .............................25 viii Figure 16. Schematic diagrams of broadband coplanar waveguide fed aperture coupled patch antenna ...........................................................................................................................................26 Figure 17. 3-D schematic diagram ................................................................................................33 Figure 18. Nominal HFSS feed and stub length dimensions .........................................................36 Figure 19. Side view of aperture coupled patch antenna with ABC ..............................................37 Figure 20. Simulated g1845g2869g2869 of the aperture coupled microstrip antenna ...........................................38 Figure 21. Smith chart for aperture coupled microstrip patch antenna ..........................................39 Figure 22. 3-D plot of realized gain for aperture coupled microstrip patch ..................................40 Figure 23. Rectangular plot of realized gain for aperture coupled microstrip patch .....................40 Figure 24. Co-polarization and cross polarization for E-plane and H-plane .................................41 1 Chapter 1 Introduction History of Microstrip Patch Antennas The idea of microstrip radiators was first mentioned by Deschamps in 1953 and later patented by Gutton and Baissinot in 1955 [1]. The concept and development of microstrip patch antennas was later introduced to the antenna engineering community in the early 1970s. Howell and Munson were the first documented researchers to fabricate a microstrip patch antenna. During the early stages of its development, the microstrip patch antenna received skepticism from researchers due to its extremely narrow bandwidth (around 1-2%) which is unsuitable for wireless communication systems [2]. During this time, two methods were developed to calculate the input impedance response and model the microstrip patch antenna: the transmission line model and the cavity model. Even though these analytical techniques were revolutionary, both techniques were limited in their ability to precisely calculate the performance of the microstrip patch. As research and development continued into the 1980s, design and analytical techniques for microstrip patch antennas also progressed. Phased array microstrip patch antennas were analyzed in order to explore their overall size limitations [1]. Circular polarization and other polarization methods for microstrip patch antennas were also examined during this period. Many of the major concerns regarding the practicality of microstrip patch were either resolved or under investigation by the end of the 1980s. By the start of the 1990s, microstrip patch antennas were being implemented into commercial applications such as mobile communication technology [2]. More advanced computation tools such as finite difference time domain (FDTD) and finite element 2 method (FEM) were capable of evaluating external environments and microstrip patch issues faster than previous computational tools. Due to these more sophisticated computational methods, impedance bandwidth enhancements up to an octave (67%) were produced. Other advancements include multiband microstrip patches, efficient printed antennas, high-gain printed patch antennas, and size reduction techniques for patch conductors. History of Aperture-Coupled Microstrip Patch Antenna Aperture coupled microstrip patch antennas have become a viable option for wireless and telecommunication systems over traditional microstrip patch antennas. Research and development in the 1980s contributed to the discovery of the aperture-coupled microstrip patch antenna [2]. David Pozar is credited as being one the main pioneers in the development of the aperture coupled microstrip patch antenna. The fundamental structural difference between the microstrip patch and aperture coupled microstrip patch antenna is the feed method used to excite the patch. While the microstrip patch antenna uses direct feed in order to excite the patch, the aperture coupled microstrip patch antenna uses non-contact feed lines which is accurately described as the magnetic equivalent of the edge-fed procedure in [2]. The non-contact feed is also capacitive in nature which counteracts with the natural high inductance of the excitation. Having indirect feed contact allowed for the aperture coupled microstrip patch antenna to achieve low current discontinuity which is fundamental for achieving good impedance and radiation performance. 3 Since its introduction, aperture coupled microstrip antennas have continually been used in numerous applications requiring a wideband frequency range. This includes, but is not limited to, global positioning satellites, cellular phones, personal communications systems, GSM, wireless local area networks, cellular video, direct broadcast satellites, automatic toll collections, collision avoidance radar, and wide area computer networks [3]. 4 Chapter 2 Microstrip Patch Antennas Design Specifications A traditional microstrip patch antenna consists of a microstrip line fed directly to a patch resting on a single layer dielectric substrate. A ground substrate is also added to the lower dielectric substrate which serves as a path for return currents. The microstrip patch is considered a resonant type radiator meaning that its feed length is approximately half of the guided wavelength [4]. Figure 1 shows the basic schematic diagram of an edge-fed microstrip antenna. Figure 1. Schematic diagram of a microstrip patch antenna. The microstrip transmission line is designed to transmit electromagnetic energy to the radiating patch. Figure 2 shows the schematic diagram of a traditional microstrip transmission line. 5 Figure 2. Schematic diagram of a microstrip transmission line. When designing the microstrip transmission line for an antenna, three design rules must be considered. First, the length of the microstrip transmission line L must be half the guided wavelength of the desired resonant frequency. (Eq. 1) A half guided wavelength is recommended for the length because the current will be excited on the patch and vertical electric fields will be generated between the patch and the ground plane [2]. This effect will cause the radiated fields to add constructively creating an efficient, resonant radiator [2]. Second, the dielectric constant g2035g3045 should be as low as possible. The reason is because the permittivity of the dielectric substrate affects the intensity of fringe fields which directly relates to the radiation of the antenna [1]. Typical permittivity values (g2035g3045 ? 10) for microstrip patch antennas vary depending on the substrate?s availability, application, and costs. However, the permittivity should be as low as possible (g2035g3045 ? 2.5) in order to increase the strength of fringe fields. Third, a thicker substrate should be used. Even though a thicker substrate h is recommended, the designer should make sure to keep it below a fraction of a wavelength. The substrate thickness h along with the transmission line width w determines the characteristic 6 impedance g1852g3042. Equations 1-3 show the mathematical calculations used for microstrip transmission line design [5]. g1839g1861g1855g1870g1867g1871g1872g1870g1861g1868 g1864g1857g1866g1859g1872? = g2971g3282g2870 g1875?g1857g1870g1857 ?g3034g1861g1871 g1872?g1857 g1859g1873g1861g1856g1857g1856 g1875g1853g1874g1857g1864g1857g1866g1859g1872? (?g3034 = g3030g3493g3084 g3280g3281g3281g3033 ) (1) g1831g1858g1858g1857g1855g1872g1861g1874g1857 g1868g1857g1870g1865g1861g1872g1872g1861g1874g1861g1872g1877 = g2013g3032g3033g3033 = g3084g3293g2878g2869g2870 + g3084g3293g2879 g2869 g2870g3495g2869g2878g3117g3118g3283g3298 (2) g1829?g1853g1870g1853g1855g1872g1857g1870g1861g1871g1872g1861g1855 g1861g1865g1868g1857g1856g1853g1866g1855g1857 = g3422 g1852g3042 = g2874g2868g3493g2013 g1857g1858g1858 lng4672g2876g3035g3050 + g3050g2872g3035g4673g2007 g1858g1867g1870 g3050g3035 ? 1 g1852g3042 = g2869g3493g2013 g1857g1858g1858 g2869g2870g2868g3095 g3298 g3283g2878g2869.g2871g2877g2871g2878g2868.g2874g2874g2875g3039g3041g4672 g3298 g3283g2878g2869.g2872g2872g2872g4673 g2007 g1858g1867g1870 g3050g3035 > 1 g1 (3) The patch element is crucial in the overall performance of the microstrip antenna. The patch?s length controls the resonant frequency and the width of the patch affects the resonance impedance level and bandwidth [2]. These conditions are only plausible as long as the dielectric substrate thickness remains below 0.03?g3042. Equations 4-8 show the mathematical equations used to calculate the patch length L and width W [6]. g1852g3042 = g2869g2870g2868g3095g3035g3024g3493g3084 g3280g3281g3281 (4) ?g1864 = 0.412?g3436 g3084g3280g3281g3281g2878g2868.g2871g3084 g3280g3281g3281g2879g2868.g2870g2873g2876 g3440g3435 g3024 g3035g3415 g3439g2878g2868.g2870g2874g2872 g3435g3024 g3035g3415 g3439g2878g2868.g2876 (5) g1858g3045 = g3030g2870g3493g3084 g3280g3281g3281(g3013g2878g2870g2940g3039) (6) g1849 = g3030g2870g3033 g3293 g4672g3084g3293g2878g2869g2870 g4673g2879 g2869 g2870g3415 (7) (g1838 +2?g1864) = g3090g3282g2870 = g3090g3290g2870 g3493g3084g3280g3281g3281 (8) 7 Advantages Since its discovery, microstrip patch antennas have been continually researched and developed in order to improve its reliability and efficiency. Major advantages of the microstrip patch antenna are its low profile, conformal nature, low production cost, compatibility with microwave integrated circuits, capability to be easily formed into arrays, and efficiency [1-2, 4]. The remainder of this section will further discuss each advantage previously mentioned. Since most wireless and telecommunications applications require compact components, low profile designs are becoming the standard in the antenna community. Typical substrate layer thickness for a single layer microstrip patch antenna is no more than five-hundredths of a wavelength while the substrate thickness for a multilayer microstrip patch antennas is no more than a tenth of a wavelength [2]. Having minimal substrate thickness allows for easy integration into various applications such as airplanes, consumer electronics, military weaponry, etc. Being conformal, or flexible, is an advantage of the microstrip patch antenna. Several military applications such as missile guidance systems and aircraft communications require rugged antennas. Soft laminates are used for the dielectric substrate of the microstrip patch allowing it to be easily molded to the body of the missiles or aircrafts [2]. Since standard printed circuit etching techniques are used to create microstrip patch antennas, production costs are relatively low [2]. Using printed circuit methods allows for the entire antenna including the patch and feed to be built in one process which also 8 significantly reduces manufacturing time and costs. By utilizing printed circuit techniques, microwave integrated circuits are easily integrated with microstrip patch antennas. Standard printed circuit board materials such as FR4 can be used for applications requiring an operation frequency less than 1 GHz [2]. Since microstrip patch antennas are low-gain antennas (less than 8dBi), antenna arrays are necessary in applications requiring a high amount of gain [2]. The manufacturing process for microstrip patch arrays is relatively simple since it is possible to fabricate the entire array network on a single layer. Also, the production cost of multiple antenna elements does not increase significantly, allowing for use in numerous commercial applications. Essentially, a microstrip patch antenna is an efficient radiator [2]. Due to its resonant nature, the efficiency of a microstrip patch antenna is greater than its wideband variants. Three loss mechanisms contribute to decreased efficiency: conductor loss, dielectric loss, and surface wave loss [2]. In order to minimize loss and maximize efficiency, it is recommended to use direct contact feed techniques rather than non-contact methods. Using a thin, low permittivity substrate will also improve the overall efficiency of the antenna. Figure 3 shows efficiency versus the substrate thickness based on the substrate permittivity [4]. 9 Figure 3. Efficiency versus substrate thickness for microstrip patch antenna. Disadvantages Even though the microstrip patch has a wide array of advantages, many disadvantages hinder its use in numerous applications. Some disadvantages of microstrip patch antennas include narrow impedance bandwidth, spurious feed radiation, and poor polarization purity [4, 7]. The remainder of this section will further discuss each disadvantage previously mentioned. The narrow impedance bandwidth issues are due to the highly inductive nature of the method used to excite the microstrip patch antenna [2]. As the feed line is excited, the frequency response is dominated by a high level of inductance at frequencies below resonance. The microstrip patch antenna is a resonant-styled antenna meaning the reactance of the impedance must equal zero. Therefore, the capacitance and inductance must cancel for optimum performance. The excess inductance reduces the substrate 10 thickness which reduces the overall impedance bandwidth. Figure 4 shows a plot the bandwidth versus substrate thickness based on the substrate permittivity [4]. Figure 4. Bandwidth versus substrate thickness for microstrip patch antenna. Poor polarization is the product of high levels of cross-polarization and mutual coupling within the microstrip patch antenna [1]. Cross-polarization radiation is the radiation field that is produced from the antenna in the opposite direction of the desired radiation field. For example, if the desired field is meant to radiate horizontally, then the cross-polarization field will radiate vertically. The increased cross-polarization levels can be attributed to surface waves diffracting off the antenna?s finite ground plane [2]. Design Adaptations Numerous adaptations of the traditional microstrip antenna have occurred over the past several decades. Several design innovations have successfully addressed and resolved some key issues mentioned in the previous section. In [8], a wideband multislot 11 antenna for broad band wireless applications is presented. Since a microstrip patch antenna is naturally narrowband, the proposed antenna uses three different narrow slots along with a patch-like tuning stub to enhance bandwidth and maintain modest polarization. The antenna was capable of achieving an impedance bandwidth of around 88% from 5.1 to 13.GHz. Figure 5 shows the geometry of the wideband multislot patch antenna. Figure 5. Geometry of the wideband multislot patch antenna. In [9], a novel, compact printed patch antenna for WiMAX applications was presented. The unique innovation about this design is the designer?s ability to reduce the size of the antenna by adjusting the complimentary split ring resonators (CSRR) etched into the ground plane. The reported frequency band and gain are 5.405-5.595GHz and 6.4dBi, respectively. Figure 6 shows the geometry of the compact printed patch antenna. 12 (a) (b) Figure 6. Geometry of the compact printed patch antenna (a) top view and (b) bottom view. 13 Chapter 3 Aperture Coupled Microstrip Patch Antenna Design Specifications Figure 7 shows the schematic diagram of an aperture coupled microstrip patch antenna. The dielectric substrates are separated by a ground plane. The aperture in the ground plane allows for electromagnetic coupling to occur between the feed structure and the patch. Equation 9 shows the mathematical expression for the coupling amplitude between the feed structure and patch, where g1876g3042 is the offset of the aperture from the patch edge, M is the magnetic current density, H is the magnetic field, and L is the length of the microstrip feed [1]. Figure 7. Schematic diagram of an aperture coupled microstrip patch antenna 14 g1829g1867g1873g1868g1864g1861g1866g1859 = ? g1839 ?g1834g1856g1874 = sin (g3095g3051g3290g3013 )g3023 (9) Based on circuit theory, the microstrip is seen as an open circuited stub which makes the feed naturally capacitive. The circuit equivalent for an aperture coupled patch antenna is shown in Figure 8. Figure 8. Circuit model for an aperture coupled microstrip patch antenna. Unlike microstrip patch antennas, aperture coupled microstrip patch antennas have numerous design parameters which allows more degrees of freedom for antenna designers [2]. Design parameters include the antenna substrate, patch dimensions, open circuit stub termination, and slot length. Each of these parameters assists in controlling the outcome of the antenna?s input impedance and the front-to-back ratio, which is the ratio of the radiated far-fields in the forward and backwards direction. The remainder of this section will further discuss each design parameter previously mentioned. The patch substrate is responsible for controlling the bandwidth, radiation efficiency, and coupling level of the antenna [3]. A thick, low permittivity substrate is ideal because it will increase the impedance bandwidth while reducing the surface wave excitation. 15 The main disadvantage of using a thick, low permittivity substrate is the decrease in coupling for a given aperture size. The mathematical calculations for the patch substrate are shown in Equations 4-8. The patch dimensions are responsible for dictating the resonant frequency, resonant resistance, and polarization levels [3]. Increasing the patch length will decrease the resonant frequency [1]. The patch width controls the resonant resistance. If a lower resonant resistance is desired, a wider patch is required. If the antenna is critically coupled, minimizing the patch width will result in over-coupling which decreases the front-to-back ratio. This reduction in the front-to-back ratio occurs because the slot becomes larger as the patch size decreases resulting in the slot radiating equally on both sides of the ground plane [1]. The open circuit stub termination is used to match the input impedance response [2]. Adjusting the stub helps tune the excess reactance produced from the slot [3]. Typically, the stub length g1838g3046 is less than g3090g3282g2872 in length. When the stub is shortened, the impedance locus on the Smith Chart will move in the capacitive direction; when the stub is lengthened, the impedance locus will move in the inductive direction. The slot length controls the size of the impedance locus on the Smith Chart [3]. Increasing the slot length will cause the diameter of the locus to increase consequently making the impedance bandwidth increase. For optimum matching, the locus needs to be large enough to pass through the center of the Smith Chart. To prevent over coupling, the designer must only make the slot length no larger than required. 16 Advantages The aperture coupled microstrip patch antenna has several advantages comparable to the edge-fed microstrip patch antenna. Advantages of the aperture couple microstrip patch antenna include wide impedance bandwidth, low cost, compact profile, and very pure radiation [2,3]. The remainder of this section will further discuss each advantage previously mentioned. Wide impedance bandwidth is the primary benefit of the aperture coupled microstrip patch antenna design. The wideband characteristics are a product of coupled resonances referred to as mutual resonance [2]. Whenever overcoupling occurs to the microstrip feed structure with a low-Q resonance of the patch, an impedance loci is produced on the Smith Chart [2]. In order to optimize the wide bandwidth capabilities, a large impedance locus is desired meaning multiple resonances must be produced. Pure radiation allows for minimal cross-polarization levels [2]. Since the aperture coupled patch is excited using a thin slot, cross-polarized current are reduced. The major issue that occurs with a thin slot is the increased occurrence of diffracted fields interacting with the finite sized ground plane [2]. Compact profile and low cost advantages of the aperture coupled microstrip patch antenna are similar to the microstrip patch antenna. The only major cost and profile difference is a result of the additional substrate layer between the aperture ground plane and the patch. Current printed circuit technologies are capable of fabricating the feed line on one layer and the radiating patch on a layer within close proximity to it without increasing manufacturing cost [2]. 17 Disadvantages One disadvantage of aperture coupled microstrip antennas is the multilayer fabrication process. Issues such as patch alignment, small gaps between dielectric layers, and material bonding can significantly alter the performance of the antenna [4]. Small gaps between dielectric layers would change the antenna?s input impedance and bandwidth. Another disadvantage is the possibility of a poor front to back ratio. This issue is a consequence of using a large aperture in order to increase impedance bandwidth. Since the fields from the ground aperture radiates towards the upper (patch) and lower (feed) half, a chance of high backward radiation can occur [4]. High backward radiation can lead to lower power availability in the power budget which is important for communication systems. In order to resolve the issue, the addition of a cavity-backed substrate and/or reflector has been used to decrease the amount of backward radiation from the large aperture. Scalping of the radiation pattern is another concern when using a large aperture design. Whenever a large aperture is combined with a small ground plane, the radiated power, specifically the electric field, can easily be diffracted off of the ground plane resulting in an altered radiation pattern [4]. In order to avoid this issue, the ground plane should be at least a couple wavelengths large in respect to the center of the patch and slot. The use of absorbing material around the edges of the ground plane also will reduce radiation scalping. 18 Design Adaptations Since its introduction into the antenna community, several adaptations of the aperture coupled microstrip antenna have been implemented in order to increase gain, directivity, and bandwidth. The remainder of this section will discuss various design adaptations with their results. In [1], the aperture coupled stacked patch microstrip antenna is presented. Figure 9 shows the schematic diagram of the aperture coupled stacked patch microstrip antenna. In this configuration, impedance bandwidths up to 70% have been achieved which is relatively high for a microstrip antenna [2]. The increased impedance bandwidth is due to the additional patch which excites additional resonances by increasing electromagnetic coupling. Figure 9. Schematic diagram of the aperture coupled stacked patch microstrip antenna. 19 In [10], the author improved the bandwidth of the antenna by adding a cavity-backed substrate and a lower ground plane as shown in Figure 10. These design alterations allowed for an increase in coupling between the aperture and patch resulting in a 40% 10dB bandwidth and lower back radiation. Figure 10. Schematic diagram of the cavity-backed aperture coupled microstrip patch antenna. In [11], the backed substrate integrated cavity aperture coupled microstrip antenna is introduced. This antenna is an innovative design because it uses metallic vias along with the shape of the backed substrate integrated cavity in order to improve coupling between the patch and aperture. A reported 48% bandwidth was produced along with size reductions. Figure 11 shows the schematic design backed substrate integrated cavity patch with the cross shaped metallic via geometry. 20 (a) (b) Figure 11. Schematic diagram of the (a) backed substrate integrated cavity aperture coupled microstrip antenna and (b) geometry of the cross shape metallic via configuration. 21 Chapter 4 Feed Techniques Microstrip feed lines are not the only type of feed lines used with aperture coupled patch antennas. Proximity coupled, stripline, and coplanar waveguide feed substrates have been used in numerous designs in order to achieve improved impedance bandwidth, directivity, and gain. The remainder of this section will discuss in detail the feed techniques mentioned above. Proximity Coupled Feed Proximity coupled feed is another non-contacting feed technique similar to the aperture coupled microstrip method. It consists of two substrate layers where the microstrip feed is on the bottom layer and the patch is on the upper layer [1]. The microstrip feed is an open end transmission line making it naturally capacitive. In order to increase bandwidth and reduce the spurious radiation effect from the open end microstrip feed, the substrate characteristics of the two layers must be chosen carefully. Typically, the lower substrate layer is relatively thin in order to manage coupling. Since the patch is placed between two substrate layers, the bandwidth increases dramatically. The downside of this antenna configuration is the fabrication process since accurate alignment of the patch and feed line is crucial. Figure 12 shows the schematic diagram and circuit equivalent model for the proximity coupled fed patch antenna [1, 2]. 22 (a) (b) Figure 12. Proximity coupled fed patch antenna (a) schematic diagram and (b) circuit equivalent model. Stripline Feed A typical stripline-fed aperture coupled patch antenna consists of a stripline feed placed in between two dielectric substrates with a ground plane placed on the lower dielectric substrate. Figure 13 shows an example schematic diagram of a stripline-fed aperture coupled patch antenna [12]. 23 Figure 13. Schematic diagram of stripline-fed aperture coupled patch antenna. Parallel plate modes are excited primarily by the coupling slot making the stripline feed negligible when considering a symmetric structure [13]. In order to obtain optimum efficiency, the spacing between the coupling slot and ground plane must be half a wavelength. This specific spacing is below the high order modes cut-off frequency. Since size constraints are a factor for practical applications, a substrate thickness of around a quarter wavelengths is expected and produces good results. In regards to design importance, the patch dimensions have a more dominant effect on the overall performance of the antenna than the slot dimensions [13]. Since the distance between the slot and patch control the coupling, this spacing determines the maximum efficiency. The patch length primarily controls the center frequency resonance. Along with dictating the center frequency, the patch element heavily influences the impedance bandwidth, input impedance and radiation properties of the antenna. In [13], a stripline-fed aperture coupled double patch antenna was presented. The bandwidth and efficiency of double patch antenna is greater than a single patch design. Similar to the aperture coupled stacked patch microstrip antenna, the upper patch acts as 24 the radiating element while the lower patch is used to increase coupling. Figure 14 shows the schematic diagram of the stripline-fed aperture coupled double patch antenna. Figure 14. Schematic diagram of stripline-fed aperture coupled double patch antenna. Coplanar Waveguide Feed Coplanar waveguide (CPW) feed substrates are an efficient method used to integrate the antenna with monolithic microwave integrated circuits (MMIC). [2] Two methods of excitations have been investigated using a CPW feed. The first method involves the CPW?s center conductor splitting the coupling slot into two. The second method changes the CPW into a slot which makes the coupling between the patch and CPW inductive. The primary advantage of a CPW feed is the feed?s negligible radiation due to the odd mode excitation of the coupled slot line [2]. Figure 15 shows a schematic diagram of a CPW-fed aperture coupled patch antenna [2]. 25 Figure 15. Schematic diagram of a CPW-fed aperture coupled patch antenna. In [14], a unique broadband coplanar waveguide fed aperture coupled patch antenna using a metamaterial based superstrate is presented. In this design, high directivity and bandwidth is achieved using an array of frequency selective superstrate (FSS) structures to construct the patch. The CPW feed is used to increase wideband performance of the antenna. Figure 16 shows the cross sectional view of the proposed antenna along with the FSS patch array and CPW feed. In this configuration, the antenna achieved an impedance bandwidth in excess of 50% and directivity of about 13.6 dBi. 26 Figure 16. Schematic diagrams of broadband coplanar waveguide fed aperture coupled patch antenna (a) cross-sectional view (b) FSS patch array and (c) CPW feed. 27 Chapter 5 Aperture Coupled Microstrip Patch Antenna Applications WiMAX WiMAX was created in order to increase broadband communication over a wide network area. In recent years, WiMAX has developed as the standard for present digital networks with a greater range of services [15]. Since its introduction by the industry group WiMAX forum in June 2001, WiMAX has grown from a communication theory to a reliable source for wireless broadband access networks [16]. Formally, WiMAX is a wireless Ethernet alternative to wire technologies (i.e. cable modems, DSL, etc.) which provides broadband access to desired customers. WiMAX was originally developed in order to provide a low-cost alternative for fixed and mobile services (i.e. Voice Over Internet Protocol [VoIP], Information Technology and Video). Unlike WiFi which only offers wireless connectivity for a small coverage area, WiMAX is capable of providing coverage to a large area (around 50km) while supplying bandwidth speeds up to 72 Mbps to users. Two different versions of WiMAX were established in order to optimize the overall performance of WiMAX network. The first version is IEEE 802.16-2004 which provides fixed and nomadic access to WiMAX networks. The second version is IEEE 802.16e which provides network support for portable and mobile communication electronics. The initial frequency profiles that have been developed for mobile applications for WiMAX are 2.3, 2.5, 3.5, and 5.1-5.8 GHz [17]. Each WiMAX version will be discussed in further detail in the remainder of this section. 28 IEEE 802.16-2004 and IEEE 802.16e Standards The IEEE802.16 standard is used to supply coverage over large wireless metropolitan area networks (WMAN) [16]. It includes the Physical (PHY) and Medium Access Control (MAC) layers. It uses both Point-to-Multi-Point (PMP) and Mesh modes in order to communicate between subscriber stations and the base station. The difference between PMP and Mesh modes is that in PMP mode, two subscriber stations can only correspond with each other through the base station; while in Mesh mode, two subscriber stations are allowed to communicate directly with each other. Both PHY and MAC layers implement the PMP and mesh mode when transmitting information to one another. The main modulation techniques used in transmitting information to end-users are orthogonal frequency division multiplexing (OFDM) and orthogonal frequency division multiplexing access (OFDMA). Based on the amount of noise and the transmission distance, both of these methods provide adaptive modulation to available subscribers. By using these methods effectively, WiMAX is capable of supporting fast-speed mobile communications at a higher spectral efficiency while reducing the amount of external interference. The IEEE 802.16e-2005 standard was completed in February 2005 in order to supply high-speed mobile support for subscriber stations [16]. Two major issues occurred when attempting to add mobile access that forced many changes to PHY and MAC layer in order to establish mobility support. The first issue was the handover, which manages base station switching and handoff methods for inter-cell and inter-sector handover between multiple sized cells. The second issue was power management which consisted of taking advantage of two modes of operation in order to reduce mobile power 29 consumption: idle and sleep mode. Once these problems were resolved, IEEE 802.16e- 2205 was ready to be employed in current generation technology. Impact of WiMAX Technology The overall impact of WiMAX into the consumer market has been substantial. Based on [18], WiMAX has become a more advantageous alternative to current 3G systems due to the fact that its frequency spectrum is cheaper, it is 100 percent IP based, has a higher quality of service (QoS) and data rate, is an open system, and it is acceptable to the large developing country markets. The need for WiMAX broadband service has increased from around 430 million customers in 2009 to nearly 800 million consumers in 2010 worldwide, which is nearly double the market coverage for mobile users. This exponential growth for reliable broadband services has contributed to a rise in portable internet-accessible devices such as laptops, mobile phones, and tablet computers. Even though laptops are still the major medium for access to the internet, mobile phones and tablet computers have gained popularity among end-users. Most present-day technology giants have begun including WiMAX into their mobile chipsets. For example, Intel introduced the Intel WiMAX Connection 2300 chipset in December 2006 which includes mobile WiMAX and Wi-Fi [18]. Another example is the Samsung SWD-M1000 Mondi, which was the tech giant?s first mobile WiMAX enabled handheld device [19]. These two devices are prime examples of the ever-expanding market for WiMAX-enabled devices and WiMAX?s relevance in future mobile innovations. 30 WIFI WIFI (or IEEE 802.11) was first released in 1997 with the capability to transmit raw data at speeds of 1 and 2 megabits per second (Mbit/s) using the Industrial Scientific Medical (ISM) frequency band at 2.4GHz [20]. Since its original release, the IEEE 802.11 standard has been amended several times in order to increase availability and overall performance. The first amendment was introduced in 1999 as the IEEE 802.11b. IEEE 802.11b increased the raw data rate to 11 Mbit/s using the 2.4 GHz frequency band [20]. Depending on user?s desired signal quality, the 11 Mb/s data rate was scaled down to 5.5 Mb/s, 2 Mb/s, and/or 1 Mb/s. The second amendment was the 802.11a standard which was the first standard to operate in the 5 GHz frequency band. Since the 2.4 GHz frequency band has a limited number of non-overlapping channels and is heavily used, the 5 GHz frequency range allows for less interference from other wireless devices such as Bluetooth, computers, and other electronics [21]. The major tradeoff for using the 5 GHz is the reduced network range available meaning propagation though solid obstacles (such as walls, trees, etc.) are decreased significantly. Radar Interests in radar applications at the 5 GHz range has increased over the past decade. In 2003, the International Telecommunication Union Radiocommunication Sector (ITU-R) created the sharing criteria for the 5GHz range [22]. One notable radar application in the 5 GHz range is weather radars. Due to their short pulse type and scanning patterns, weather radars are hard to detect in this frequency range. 31 Chapter 6 High Frequency Structure Simulator Methods High Frequency Structure Simulator or HFSS is a software program with the ability to calculate the electromagnetic behavior of a structure [23]. It can solve basic open boundary problems, generalized S-parameters, and eigenmodes (resonances) of a structure by using a finite element method (FEM) solver. HFSS designs include, but are not limited to, antennas such as helical, pyramidal horn, and coax fed patch; signal integrity structures such as vias, coax bend transient, and connectors; RF/microwave structures such as bandpass filter, combiner, and tee; and integrated circuits such as package models and spiral inductors. Absorbing Boundary Conditions Radiation boundaries, also known as absorbing boundary conditions (ABC), are used to simulate radiating electromagnetic waves in an infinite space [23]. In order to replicate infinite space, HFSS absorbs any radiating waves and infinitely expands the boundary area. This simulation method is useful in antenna design where the near- field and/or far-field patterns assist in determining the performance of an antenna design. Typically, an ABC effectively absorbs incident energy that is normal to the surface of the radiating element. ABCs are usually placed at least quarter wavelength away from strongly radiating structures for precise simulation results [23]. The distance between the radiating structure is crucial to accurate simulation results. Perfectly Matched Layer A perfectly matched layer (PML) boundary is described as a fictitious lossy anisotropic material that fully absorbs electromagnetic fields from radiating elements 32 [24]. Compared to absorbing boundary conditions, PMLs absorb a wider range of frequency and directional waves at the cost of more computing RAM. This method allows for boundary conditions to be closer to radiating elements resulting in smaller 3-D models [23]. Unlike ABC where the distance between the boundary and the radiating structure is crucial, the boundary distance for PMLs is more flexible. 33 Chapter 7 Aperture Coupled Microstrip Patch Antenna Design Specifications An aperture coupled microstrip patch antenna was created and simulated in HFSS. Figure 17 shows the 3-D schematic of the proposed aperture coupled microstrip patch antenna. Table 1 shows the design parameters used for the aperture coupled microstrip patch antenna. Figure 17. 3-D schematic diagram of aperture coupled microstrip antenna. 34 Design Parameters Value (in mm) Microstrip Substrate Thickness (Arlon AD260A) ?g2869 1.016 Patch Substrate Thickness (Eccostock PP-2) ?g2870 6.35 Patch Length g1838g3052 25.5087 Patch Width g1849 g3051 29.7775 Slot Length g1838g1845g3052 23.7 Slot Width g1849g1845g3051 2.1 Stub Length g1864g3046g3047g3048g3029 1.5 Microstrip Feed Length g1838g3033g3032g3032g3031 37.2725 Microstrip Feed Width g1849g3033g3032g3032g3031 3 Table 1: Design parameters for aperture coupled microstrip patch antenna. The patch element rests on Emerson & Cuming Eccostock PP-2 polyethylene foam (g2013g3045g2870 = 1.03 and loss tangent = 0.0001). As mentioned in previous sections, an ideal substrate for the patch element is one that has a low permittivity and thick substrate for increased bandwidth which Eccostock PP-2 meets both specifications. Additional benefits of this material include lightweight, durability (weather resistant), and ability to return to its normal thickness after being compressed. The microstrip feed line is placed on Arlon AD260A (g2013g3045g2869 = 2.6 and loss tangent = 0.0017). As mentioned in previous sections, the microstrip substrate should have a low permittivity with a relatively thin substrate in order to increase fringing fields which Arlon AD60A meets both specifications. Additional benefits of this material include low insertion loss and high antenna efficiency. The patch length and width dimensions were determined using Equations 4-8. In order to expedite the calculation process, a MATLAB patch dimension calculator 35 created by Anurag Ghosh was used. [Appendix 1] The patch dimensions were determined using the known patch substrate thickness, permittivity, and desired resonant frequency. Table 2 shows the calculated and simulated patch dimensions for the antenna. As shown in Table 2, the patch length was lengthened in order to achieve an optimum resonance at 5 GHz. Calculated (mm) Simulated (mm) Patch Length 20.6622 25.5087 Patch Width 29.7775 29.7775 Table 2. Calculated and simulated patch dimensions. The slot length and width dimensions were determined using the parametric sweep function in HFSS. According to [3], the ratio of slot width to length is around g2869 g2869g2868 = 0.1. Based on my design, the ratio of slot length to width is g2870.g2869 g2870g2871.g2875 = 0.09 which is close to the initial ratio. The microstrip feed length was determined using information found in [25]. The total feed length for an open-circuit microstrip line is 0.739?. [25] The stub length is included with the total feed length which is a length of 0.211? from the center of the slot. Therefore, the feed length from the edge to the center of the slot is 0.528?. Figure 18 shows a schematic diagram of the nominal HFSS feed and stub length dimensions. 36 Figure 18. Nominal HFSS feed and stub length dimensions. Using these nominal HFSS values, the feed and stub lengths were calculated and implemented in the initial design. After conducting a parametric sweep in HFSS, the optimum feed and stub length were determined. Table 3 shows the nominal and simulated feed and stub length at a frequency of 5 GHz. Based on Table 2, the nominal and simulated total feed lengths have a percent difference of 14.35%. Nominal (mm) Simulated (mm) Feed Length 31.68 37.2725 Stub Length 12.66 1.5 Total Feed Length 44.34 38.7725 Table 3. Nominal and simulated feed, stub, and total length at 5 GHz. The microstrip feed width was determined using Equation 2. In order to expedite the calculation process, the online microstrip calculator on Microwaves101.com was used 37 in order to obtain accurate, consistent values. Based on the known substrate thickness and permittivity, the microstrip feed width was calculated to be 3 mm for a substrate thickness of 1.016mm where g2013g3045g2869 = 2.6. As mentioned in Chapter 6, the amount of separation between the antenna structure and the absorbing boundary is crucial for accurate simulation results. Figure 19 shows the side view of the aperture coupled patch antenna with the ABC. When using an absorbing boundary condition (ABC), the distances between both the microstrip feed and radiating element from the ABC is approximately a quarter wavelength as shown in Equation 10 where c is the speed of light, g1858 is the desired frequency, and . The calculated distance is equal to 14.78mm. g3090g3282 g2872 = g2869 g2872 g3030 ?g3084g3293g3033 = g2869 g2872 g2871g3051g2869g2868g3124 g3040 g3046? ?g2869.g2868g2871?(g2873g3008g3009g3053) = 14.78g1865g1865 (10) Figure 19. Side view of aperture coupled patch antenna with ABC. 38 Simulation Results Figure 20 shows the simulated g1845g2869g2869 of the aperture coupled microstrip antenna. The -10 dB bandwidth is approximately 400 MHz from 4.8-5.2 GHz. The percent bandwidth achieved from this antenna is around 8% based on Equation 11 where g1858g3009 is the highest frequency at -10 dB and g1858g3013 is the lowest frequency at -10 dB. One major resonance occurs at the frequency of 5 GHz. %g1828g1849 = 2g3033g3257g2879g3033g3261g3033 g3257g2878g3033g3261 g1876 100 (11) Figure 20. Simulated g2175g2778g2778 of the aperture coupled microstrip antenna. Figure 21 shows the Smith Chart impedance locus for the aperture coupled microstrip patch antenna. When the locus passes through the center of the Smith chart, the optimum matching condition has been achieved. Based on the figure, optimum 39 matching occurs when the frequency is 5 GHz which validates the resonance in Figure 20. Figure 21. Smith chart for aperture coupled microstrip patch antenna. The realized gain represents the gain that includes the reflection losses at the input of the antenna. The realized gain is calculated by taking the ratio of the power radiated to the power into the antenna. Figure 22 shows the 3-D plots of the realized gain at 5 GHz. Figure 23 shows the rectangle plot of the realized gain at 5 GHz. As shown in the figures, the maximum realized gain is 8.33 dB at 5 GHz when g2016 = 0 g1853g1866g1856 g2038 = 90, respectively. The maximum sidelobes are -3.02 dB and -2.29 dB when g2016 = g339855g953and g2016 = 55g953, respectively. Therefore, the sidelobe level or the difference between the main beam maximum and the sidelobe, equals to around 10.62 dB. Having low sidelobes is 40 often imperative in antenna design since excessive sidelobe radiation wastes energy and may cause interference to other equipment. Figure 22. 3-D plots of realized gain for aperture coupled microstrip patch at 5 GHz Figure 23. Rectangular plot of realized gain for aperture coupled microstrip patch at 5 GHz. 41 Figure 24 shows the co-polarization and cross-polarization E-field and H-field patterns at 5 GHz. As described in previous sections, the cross-polarization pattern is desired to be as low as possible in order for the co-polarization pattern to be dominant. As shown in Figure 24, the cross-polarization levels remain below -32 dB while the co- polarization levels remain above -4 dB. Figure 24. Co and cross polarization for (a) E-plane at 5 GHz (b) H-plane at 5 GHz. 42 Chapter 8 Conclusion and Future Work An aperture coupled microstrip patch antenna has been presented. The antenna is capable of operating in the 4.8 ? 5.2 GHz range. The g1845g2869g2869 for the antenna at 5 GHz is - 38.3 dB. The -10dB bandwidth exceeds 400 MHz which equates to 8% bandwidth overall. The total realized gain at 5 GHz is 8.33 dB when g2016 = 0g953and g2038 = 90g953. The maximum sidelobes are -3.02 dB and -2.29 dB when g2016 = g339855g953and g2016 = 55g953, respectively. The sidelobe level equals to around 10.62 dB below the main beam maximum. The co- polarization radiation is above -4 dB while the cross-polarization is below -32 dB which is desired. The designed antenna is capable of supporting WIFI, WiMAX, and radar applications at 5 GHz. Several modifications could be made in order to maximize the overall performance of the antenna. The bandwidth, realized gain, and size could be greatly improved if various enhancements were applied. The remainder of this section will cover methods that could be implemented in future work. First, the antenna?s bandwidth could be increased if the slot aperture was altered. The H-shaped slot has shown to increase the coupling level between the slot and patch which could excite more resonances. Another method that could be used is to increasing the number of slots in the aperture to produce multiple resonant frequencies. Adding a substrate integrated cavity with metallic vias has also proven to improve the coupling level. The major tradeoff of each of these methods is that the antenna design becomes more intricate and complex causing an increase in both fabrication time and costs. 43 Second, an array could be used to increase the overall gain of the antenna. Even though the patch antenna has a realized gain of around 8.33 dB, a higher gain may be required for most long-range wireless applications. The main tradeoff of using an array is increased design complexity especially for impedance matching and electromagnetic interference issues. Another method that would increase the gain would be to add a reflector element to the antenna. The reflector element would reduce the amount of back radiation caused by the large aperture slot and therefore would decrease the front to back ratio. Third, decreasing the size would allow for the antenna to be implemented in more wireless mobile applications where board space is limited. These applications include, but are not limited to, cellular phones, WiFi/WiMAX routers, gaming consoles, weather radars, WiFi-enabled televisions, etc. 44 REFERENCES [1] R. Garg, Microstrip Antenna Design Handbook, Artech House Inc., 2001. [2] R.B. Waterhouse, Microstrip Patch Antennas: A Designer?s Guide, Kluwer Academic Publishers, 2003. [3] D. M. Pozar. ?A Review of Aperture Coupled Microstrip Antennas: History, Operation, Development, and Applicatons,? University of Massachusetts at Amherst, 1996. [4] Lal Chand Godara, Handbook of Antennas in Wireless Communications, Chapter 6 Microstrip Patch Antennas, CRC Press LLC, 2002. [5] S. M. 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