CHAPTER 1 INTRODUCTION TO AUTOMOTIVE ANTENNAS

Transcription

1 1 CHAPTER 1 INTRODUCTION TO AUTOMOTIVE ANTENNAS 1.1 INTRODUCTION Automotive communication has witnessed a tremendous growth in recent years due to modern communication technologies and the advent of new radio frequency (RF) receivers. In addition to the traditional AM and FM, the present day automobiles are equipped with multitude of information and entertainment systems such as Navigation, Global System for Mobile communication (GSM), Bluetooth, Satellite Radio, Remote Keyless Entry Service (RKES) and Tyre-Pressure Monitoring System (TPMS). Apart from these services, higher end cars are equipped with Digital Audio Broadcasting (DAB), Satellite In-vehicle TV, Satellite Digital Audio Radio Service (SDARS), Electronic Toll Collection (ETC), short range and long range radar system for adaptive cruise control, parking assistance, collision detection and avoidance. These value added services are included in automobiles with the view to increase passenger comfort and safety. The next generation vehicles will have Long Term Evolution (LTE) systems, WiFi and additional systems for automated driver assistance. Also, the Vehicle-to-Vehicle (V2V) and Vehicle-to-Infrastructure (V2I) communication networks will receive special attention for efficient vehicular information systems (Carmo & Riberio 2012). The new sophisticated services offered in automobiles rely on efficient wireless communication systems and the antennas form an integral part of such systems. In the present scenario, premium cars are integrated with

2 2 at least dedicated antennas (Koch 2012). For satisfactory performance of the antenna, researchers have identified only a few locations in the vehicle s body where the antennas can be integrated. Thus there is a constraint in space to load the antenna in vehicle resulting in deployment of antennas in close proximity leading to increased co-site interference. The increase in number of antennas also affects the aesthetics of the vehicle and increases the in-car SAR exposure (Martinez et al 2010). Hence, the design of antennas for automotive application derives a special interest among the antenna research community. A more detailed discussion on antenna design requirements for automotive applications, antenna placement in vehicle s body and commonly used automotive antennas are described in the following sections Automotive Antenna Design Requirements Antennas are the necessary component in any wireless communication system. Antennas transmit and receive radio signals in the form of electromagnetic waves. Modern cars are equipped with more number of information and entertainment systems involving satellite and terrestrial communications. Therefore, the antennas for automotive communication require a special attention in terms of application, aesthetics, engineering and manufacturing cost. The addition of more number of infotainment services in automobiles require more number of dedicated antennas to be loaded onto the vehicle s body. Figure 1.1 shows a typical scenario depicting more number of antennas projected from the vehicle s body. These projections affect the appearance of the vehicle making the car look more like a complex telecommunication system rather than a comfort vehicle. The antennas for modern automobiles are expected to have minimum projection from the

3 3 surface of the vehicle s body. The aesthetic value of the cars can be preserved by exploiting hidden antenna concepts which would increase the engineering cost. However, the engineering cost can be reduced through outdoor placement of integrated multiband/wideband antennas that has minimum projection and also complements the aesthetics of the vehicle (Koch 2012). Figure 1.1 Car fitted with multiple radiators The antennas used for automotive communication and its radiation performance depend on the type of application. Table 1.1 gives the list of services available in modern automobiles and the corresponding antenna radiation requirements. The radiators used in vehicles are grouped in three major categories, viz. omnidirectional, directional and isotropic (Pell et al 2011). A short description on these types of radiators is presented below. The first type of radiator is the omnidirectional antenna characterized by uniform radiation characteristics along the azimuth plane. This type of antenna is used to receive signals transmitted from earth bound base stations. Some of the services which require omnidirectional coverage are AM/FM, GSM, 3G and 4G. The second type of radiator is the directional antenna. The directional antenna has highly directive radiation pattern along a particular

4 4 direction. For example, the navigation antennas have directional patterns to communicate with the satellite. Another interesting example is the automotive radar antenna which has highly directive radiation pattern for efficient detection of obstacles to prevent collision. Table 1.1 Automotive applications and antenna requirements Sl. No. Application Frequency Type of radiation pattern 1 AM 1 MHz 2 FM MHz 3 In-vehicle TV MHz 4 DAB MHz 5 800/900 MHz GSM, 3G & 1800/1900 MHz LTE 2100 MHz 6 WiFi & WiMAX 2.4 GHz & 2.3, 2.5 & 2.7 GHz, 3.5 GHz 7 V2V 5.9 GHz 8 RKES 315 MHz / 413 MHz / 434 MHz GHz Navigation GHz System GHz 10 Satellite radio GHz 11 SDARS 2.3 GHz 12 ETC 5.8 GHz 14 Automotive Radar 15 TPMS 24 GHz & 77 GHz 315 MHz / 413 MHz / 434 MHz Omnidirectional Directional Isotropic 16 Bluetooth 2.45 GHz 17 WiFi (Internal) 2.45 GHz

5 5 The third category is the isotropic antenna which is a hypothetical, lossless antenna characterized by uniform distribution of field along the elevation and the azimuth plane. The isotropic radiator is used to communicate with receiver systems placed at different locations inside the vehicle. Bluetooth, WiFi are some of the applications which require isotropic radiators Automotive Antenna Technology The desired antenna radiation characteristics can be achieved by exploiting the most popular antenna technologies. The choice of antenna technology depends on the antenna s performance with respect to the intended application and the location where the antenna is to be placed in the vehicle. Some of the technologies are rod antennas, printed antennas, on-glass antennas and glued foil antennas (Koch 2012). A brief overview of these antenna technologies is presented in this section to understand the selection of technology based on services. Figure 1.2 Rod antenna in Toyota Prius Rod antennas are monopoles consisting of either a thick or flexible metallic conductor, having length equal to or greater than quarter wavelength, connected directly to a large ground plane. Figure 1.2 shows a car loaded with rod antenna in the front side of the roof. The antenna radiation pattern is

6 6 omnidirectional and these antennas are suitable for short and medium wave terrestrial communications. The rod antennas have large external projection and hence they are avoided by premium car manufacturers to preserve the aesthetic value of the vehicle. The rod antennas often suffer from aerodynamic drag and they are prone to damage and theft. The on-glass antenna technology is more popular since 1980 for VHF and UHF bands. This technology uses slot antenna principle where the window glass is realized as a slot surrounded by thin metallic conductor (Callaghan 1999). Resonant conditions are achieved by exciting the metallic structure surrounding the slot (glass) using external feeding network. In some designs, the conducting regions of the car are modeled as a part of the antenna. The on-glass antennas are limited by complex manufacturing processes and feeding techniques. However by careful selection of feed point, many orthogonal modes can be excited using this technology. Figure 1.3 shows a rear defogger whose conducting parts are used as antennas for medium wave communications. Figure 1.3 Rear defogger antenna Printed antennas are patch antennas printed on a flat dielectric material and have a large metallic ground. Owing to its simple profile, printed antennas have received more attention from manufacturers while designing antenna systems for cars. The printed antennas can be easily integrated with the vehicle s body and making it hidden can improve the aesthetics of the

7 7 vehicle. The choice of this antenna is made when the application frequency is beyond 1 GHz. Both omnidirectional and unidirectional radiation patterns are feasible using this technology. By carefully designing the feeding network, this antenna technology can also be used to realize isotropic radiation pattern. Another interesting automotive grade antenna used primarily for terrestrial communications is the glued foil antenna in which a conductive foil is glued to an insulator to produce low profile antennas (King & Forster 2007). Though the engineering and manufacturing costs are lesser in comparison with other antenna technologies, the antenna developed using this technique suffers from narrow bandwidth and performance degradation during harsh environmental conditions. In some cases, combination of these antenna technologies is used to improve the link quality and spatial coverage during mobility. One such technique is the diversity reception. Diversity techniques are exploited in modern automobiles to overcome the effects of multipath signal propagation. This technique employs distributed antenna system concepts where the signals received from multiple antennas are used to improve the overall Signal-to-Noise ratio (SNR). Whenever the signal received through one antenna degrades, the receiver switches or selects another antenna placed at a different location which receives better signal. For example, the rod antenna used in the car roof is supported by rear defogger antenna for FM diversity reception Antenna Placement in Automobiles The performance of automotive antennas severely depends on the location where the antennas are integrated. In this section, the ideal locations where the antennas can be installed are dealt in brief. In literature, several studies are made to identify the best practices to locate the antennas in

8 8 vehicle s body. According to Koch (2012), the following basic requirements have to be satisfied for efficient signal transmission and reception in mobile terminals. 1. The antennas should be located high above the ground to achieve good spatial coverage along the horizontal plane. 2. The antennas should be sufficiently decoupled from conducting parts and other radiators in the vehicle. 3. The distance between the antenna and the receiver system should be minimized to overcome additional signal attenuation due to cable induced losses. 4. Polarization losses at the antenna can be minimized by having prior information about the polarization of the transmitted radio signal. With these requirements, there are a few locations identified in the vehicle s body for ideal operation of the antenna. Figure 1.4 depicts the regions for placement of antennas in automobiles. Figure 1.4 Location for antenna placement (Koch 2012) It is the best practice to load antennas in the car s roof since it is high above the ground and remains unobstructed. Both omnidirectional and

9 9 directional antennas can be mounted in the car s roof. The rod antennas, shark fin multi- antenna system and satellite terminal antennas are usually located in this region. The spoilers, if present in the vehicle, can be used to install antennas for telemetry applications. The spoiler made of plastic provides abundant space and it is the second most preferred location for antenna placement. The side wing mirrors with plastic shells provide hollow space to locate antennas for terrestrial applications such as V2V and FM diversity reception. Certain manufacturers house a combination of several antennas in this region to cater to the needs of modern automotive communication. The FM diversity antennas and terrestrial TV antennas are commonly printed on the rear screen and windows. A thin metallic conductor printed along the edges of the mirror will give flexibility in integrating the antennas in the vehicle s chassis. In some cases, the antennas are printed directly on the glass using conductive inks and copper tapes. The sedan type cars have trunk on the rear side that is also sufficiently high above the ground and remains unobstructed. Therefore, antennas can also be placed in the trunk. For this purpose, either the trunk should be made of plastic or at least it should have a plastic top to support antenna transmission and reception. Satellite terminal antennas are located at this region. The plastic fenders and bumpers help to overcome the space limitations to load supporting antennas for diversity reception (Martinez et al 2010). The automotive radar antennas and the associated sensor circuitry are placed in the front and rear bumpers respectively for collision avoidance and parking assistance. This region is least preferred as it is close to ground and

10 10 the performance is affected due to ground reflections and engine noise. Further the antennas at this region are prone to damage during collision. Apart from these external locations, antennas can also be installed inside the vehicle. Figure 1.5 shows the interior view of an intelligent car. The dashboard (1) provides abundant space to load antennas along with its circuitry. Since the dashboard is just beneath the front screen, antennas can communicate with external infrastructure without any signal obstruction. The pillars (2) connecting the front screen and side mirrors can support antennas for vehicle to in-vehicle communications. Antennas for in-vehicle communication can also be placed within the side doors (3) made of fiber/plastic. Some antennas are also placed behind the rear mirrors, beneath the driver and passenger seats. Figure 1.5 Interior of Acura ILX (2014) 1.2 AUTOMOTIVE ANTENNA - TRENDS & DEVELOPMENTS This section analyzes the various types of antennas used in automobiles for information and entertainment systems; and their developments in recent times. This section is broadly classified into three parts. Initially, narrow band antenna systems using aforementioned antenna

11 11 technology are dealt. In the second part, multi-service antenna systems used in modern automobiles are discussed and the last part of this section gives an overview of antenna array technology used in constructing the automotive radar antennas Single Frequency Automotive Antennas The single frequency antennas are dedicated antennas intended for a specific application. Each single frequency antenna is connected to its own receiver system. Some of the commonly used single frequency antennas are presented in the following subsections Rod antennas The oldest form of entertainment in automobiles is the AM and FM. As discussed in section 1.1.2, rod antennas are predominantly used for this communication. The rod antenna is an omnidirectional radiator that is often limited by its large projection, narrow bandwidth, aerodynamic drag and vulnerability to wind noise. Antenna height reduction is achieved through helical antennas. However, the retractable antennas received much attention than the fixed height whip antennas and helical antennas. The retractable car antennas are invented to overcome the problem of large projection from the vehicle s body. The length of the antenna is adjustable and is hidden when not in use (Hillman 1993). The retraction mechanism can be either powered or manual. Figure 1.6 (a) shows a retractable mast antenna used for MHz automotive communication. The Bee Sting antennas replaced the standard mast antenna and it is in use for many years. The Bee Sting antenna has a small height in comparison with the mast antenna and is described in Figure 1.6 (b). These antennas theoretically aid in achieving a low drag coefficient and hence less

12 12 prone to wind noise. The performance degradation due to reduced length is compensated by an amplifier module following the aerial section. Apart from AM/FM communication, this type of antenna is suitable for DAB and cellular applications. Though this antenna is visually appealing, it is limited by its narrow band operation. Figure 1.6 (a) Retractable mast antenna (b) Bee sting antenna (BMW) On-glass antennas Low profile on-glass antennas have received wide attention in automotive industry as these antennas offer mounting flexibility. Installation of this antenna doesn t require drilling of holes. The antenna base is directly attached to the window with special glue and can be easily removed. Conductive inks are used to print antennas in the window. But this technique involves manufacturing complexity since conductive ink requires curing at high temperature to avoid peeling off of the antenna. Low et al (2007) proposed an on-glass antenna for FM reception. The antenna geometry consists of four elements and is fed from the top using an unscreened flying lead from a matched amplifier circuit. Conductive inks are used to print antenna on the inside face of the glass. The window aperture size is approximately quarter wavelength and the lengths of the vertical elements are much shorter than required for quarter wavelength monopole

13 13 antennas for FM. The presented design though provides broad bandwidth; the design is often limited by the complex feed network that affects both impedance matching and operating bandwidth. On-glass antennas for WiFi, GSM, UMTS/3G antennas are commercially available in the automotive market Patch antennas An edge truncated square patch radiator is the most popular automotive antenna for GPS receivers developed using patch antenna technology. Figure 1.7 shows the classical GPS antenna. High dielectric constant substrate materials are used for antenna construction to achieve size reduction. This antenna is protected with radome and has a magnetic base to fix the antenna to the vehicle. This antenna is usually located in the roof top or underside of the dashboard. Though the presented antenna is simple and compact, the manufacturing cost is high due to high dielectric constant substrate material. Figure 1.7 Edge truncated square patch GPS antenna Multi-service Automotive Antennas Multi-service automotive antenna systems derive surge of research interest due to limited amount of space to locate antennas in vehicle. This section presents some of the popular multi-service antennas used in automotive communication.

14 Integrated antennas Oh et al (2005) proposed a novel integrated automotive antenna for GPS, RKES and PCS. Figure 1.8 shows the integrated antenna system where the edge truncated square patch GPS antenna and a combined monopole and normal mode helix share a common substrate. This integrated system requires two independent feeds and occupies a large area in the car roof. Thus the aesthetic value of the car is compromised to achieve multiple services. Figure 1.8 Integrated multi-antenna system (Oh et al 2005) The patented work by Grant et al (2007) described a compact vehicle mounted antenna. Two independent planar inverted F antennas (PIFA) supported by dedicated feed and a common ground plane is proposed for multi-service application. The PIFA antennas are designed for cellular bands and the presented work suggests that a GPS antenna can be located between the PIFAs. Therefore a three element antenna system as shown in Figure 1.9 (a) is developed for vehicular communication and is suitable for positioning in the car roof and spoiler. Though the presented design achieves desired multi-service capability, the number of services offered is limited to

15 15 two and separate feed networks for each antenna element increases cable routing complexity. An integrated multi-service antenna system for cars is patented by Puente-Baliarda et al (2009). This multi-service antenna system has multiple antenna elements housed within a physical component of the car as depicted in Figure 1.9 (b) and (c). Each antenna is defined by an independent radiating arm and a feed point. As too many antennas are to be packed, the radiating arms are convoluted using space filling curves and are packed within a small volume. Therefore, the number of service offered by this antenna system depends on the number of antennas integrated into the physical component of the car. This technique also requires independent feeding and the field interactions are strong since too many radiators are placed in close proximity. Further the housing should be sufficiently large to house many such radiators. Figure 1.9 Technical diagrams of integrated antenna systems proposed by (a) Grant et al (2007) (b) & (c) Puente-Baliarda et al (2009) Mariottini et al (2010) used the GPS antenna in Figure 1.7 to construct an integrated antenna system for GPS and SDARS applications. The GPS antenna is developed on a ceramic substrate with high dielectric constant (ε r = 45) and the SDARS antenna is constructed using square loop printed on a standard FR4 material. Because of the choice of material, the SDARS antenna is larger in size in comparison with the GPS antenna. Therefore, GPS antenna is placed above the SDARS antenna to achieve integrated antenna

16 16 system with two separate feed. After the integration, a capacitive matching patch is added to the feed of SDARS antenna to compromise any loss due to impedance mismatch. The developed dual-band integrated antenna is shown in Figure The presented design occupies small space and does not require complex feeding and matching network. Figure 1.10 Integrated GPS and SDARS antenna (Mariottini et al 2010) Shark fin antenna The most popular and stylish antenna system used in modern automobiles is the shark fin antenna and is shown in Figure Shark fin antenna consists of a hollow painted outer cover (shell) and a plastic base screwed directly to the car s roof. This shell houses one or more antenna systems for automotive communications. A conventional shark fin antenna houses antennas for at least three services such as GSM, GPS, SDARS printed either on the same printed circuit board (PCB) or different boards assembled inside the housing. The limitation of this multi-antenna system is the use of dedicated antennas for each application. Thus only a few antennas can be accommodated inside the shark fin housing due to space limitations. Further additional filtering networks are used to increase the isolation between the

17 17 antenna elements to overcome performance degradation due to strong field interactions. Figure 1.11 Shark fin multi-antenna system Figure 1.12 Technical diagram of highly integrated shark fin antenna (Chakam et al 2012) Chakam et al (2012) patented a highly integrated multiband shark fin antenna for automotive communication. The technical diagram of the patented automotive antenna is shown in Figure The entire configuration consists of an antenna circuit board having a top and bottom layer. The

18 18 multiband antenna system consists of at least one transmitting and one receiving antenna arranged on the top layer. The presented design consists of a GPS antenna, SDARS antenna and an antenna for GSM. The module consisting of electronic circuits comprising matching or amplifier circuits that include at least one of the transceivers, tuners and receivers is located in the underside of the antenna circuit board. Shielding plates are used to shield the electronic circuits from the transmitting and the receiving antenna elements arranged on the top side of the circuit board. The shark fin housing is used to protect the antenna system from harsh environmental conditions and to preserve the aesthetics of the vehicle Other multi-service antennas Filipovic & Volakis (2004) presented a cavity-backed spiral antenna capable of integrating with the trunk lid of the car. The developed antenna is shown in Figure The proposed spiral antenna exhibits a wide impedance bandwidth. This specific antenna configuration can be operated in two modes. Under coplanar waveguide (CPW) mode, the antenna exhibits omnidirectional characteristics and in coupled slot-line (CSL) mode directional pattern is achieved. A metallic cavity is used to prevent back radiations into the vehicle s body. The presented antenna does not protrude from the vehicle s body and hence it is the most elegant solution. The only limitation with this design is the wideband noise coupling. Figure 1.13 Wideband cavity backed spiral automotive antenna (Filipovic &Volakis 2004)

19 19 Low et al (2006) presented a printed antenna system mounted in the aperture of car roof. The presented design is shown in Figure Radiating arms of unequal lengths are deployed to achieve broadband signal reception from MHz. The developed antenna is suitable for AM, FM, TV and DAB reception. The average gain of the developed antenna is better than -3 dbi for vertical polarization and -10 dbi for horizontal polarization. The return loss is approximately 2 db before the inclusion of matching sections. From the presented results, it is understood that the presented design suffers from low gain and poor impedance matching throughout the operational bandwidth. Figure 1.14 Multi-function automotive antenna (Low et al 2006) Arianos et al (2012) proposed two designs of multiband antennas for automotive communication. Both the designs are suitable for GSM, DCS and PCS application. These antennas can be easily integrated with the existing PCB and positioned inside the dashboard of the car. Therefore this antenna does not require a dedicated space for mounting the antenna. Though the

20 20 antenna reduces manufacturing cost and provides high mounting flexibility, the design provides low gain and poor impedance matching across the realized frequency bands. Thus from the study of the literature, several multi-service antenna systems are developed either as an integration of multiple antenna elements or single feed antenna with wide impedance bandwidth. In case of wide band antennas, the impedance matching realized throughout the frequency bands is not satisfactory and hence they suffer from low gain. Therefore it is evident that, a single feed antenna for multi-frequency application with good impedance matching across the operational bandwidth will overcome a number of problems such as minimizing the number of antennas required, co-site interference and flexibility in antenna placement. The problem of noise coupling from adjacent frequency bands can be eliminated by designing a multiband antenna with sufficient inter-band spacing. Further the antenna with such characteristics will also reduce the filter overhead that follows the antenna system Automotive Radar Antennas In recent years, the number of roadside accidents has increased drastically and in most situations, the accidents are due to human errors. To overcome this problem, modern automobiles deploy radar systems. The dedicated Short Range Radar (SRR) and Long Range Radar (LRR) systems provide safety functions such as Adaptive Cruise Control (ACC), collision detection and avoidance, rear traffic crossing alert and blind spot detection (Menzel & Moebius 2012). The Table 1.2 presents the application frequency and bandwidth requirement of automotive radar systems. The radar sensor system is expected to provide information about the vehicles and obstacles on the roadside based on the distance between the

21 21 source vehicle and the target, the relative speed and angle. The source vehicle calculates the range by measuring the time taken for the transmitted radar pulse to reach the source after reflecting from the target. Table 1.2 Automotive radar services Sl. No. Band Application GHz (NB) 24 GHz (UWB) Centre frequency Bandwidth ACC & Lane Change 24.2 GHz 0.2 GHz SRR 24.5 GHz < 5 GHz 3 26 GHz SRR 26.5 GHz < 4 GHz 4 77 GHz ACC & LRR 76.5 GHz 1 GHz 5 79 GHz Medium Range Radar & SRR 79 GHz 4 GHz Figure 1.15 shows the scanning regions of the automotive radar and ultrasonic sensors for driver assistance. The ultrasonic sensors are used for parking aids and it has a minimum range close to 2 m. The LRR can range up to m with velocity above 30 to 250 kmph and the SRR can range up to 30 m with speed of the vehicle ranging between kmph. On an average, the premium vehicles are fitted with at least 4 such radar systems and at most 10 radar systems are expected to be deployed in the near future. For efficient performance, the antenna and the associated sensor systems are usually loaded behind the front and rear bumper. Certain systems are also deployed on the fender region to avoid the impact along the sides of the vehicle. Care should be taken to provide sufficient transparency between the radar antenna and the target, since the bumper region can cause signal blockage resulting in erroneous target detection. The following subsections are attributed to the study of antenna technology used for the development of automotive radar antennas. The

22 22 antennas for radar applications attract additional attention in terms of its radiation properties such as high gain and narrow beam width. This property of the antenna is realized using low cost planar microstrip array antenna technology. In this section, microstrip based series fed array and grid arrays are presented for automotive radar sensor applications. Figure 1.15 Automotive radar and ultrasonic sensor systems for driver assistance (Courtesy: Planet Analog) Series fed microstrip array antenna Figure 1.16 Automotive radar sensor system with transmit and receive antenna arrays (Menzel & Moebius 2012)

23 23 The automotive radar antenna constructed using planar series-fed microstrip array technology is shown in Figure The proposed antenna array consists of three transmit and three receive sub-arrays. The antenna array consists of series fed half-wavelength dipole antennas. The radiating elements in the array are titled 45 o to eliminate interference due to vehicles equipped with similar radar sensors and moving in the opposite direction. The six sub-arrays are oriented parallel to one another. All sub-arrays provide relatively wide beam width along the azimuth plane Grid array antenna The Grid Array Antenna (GAA) has received considerable attention in recent times for automotive sensor antenna development. The GAAs are planar array antennas consisting of many grid cells formed by radiating elements and transmission lines. The transmission lines provide required phase delay for in-phase excitation of the radiating elements. The GAAs provide numerous advantages such as high gain, narrow beam, simple probe feed and easy construction. Since its inception, numerous designs of GAAs are reported in literature. The GAA for automotive radar sensors was first proposed by Zhang et al (2011). The design consists of multiple grid cells formed using long and short transmission lines. The long side of each grid acts as transmission lines and the short side functions as the transmission lines and radiating element. The amplitude tapering technique is exploited to achieve low side lobe levels. The number of cells along the horizontal axis is increased to achieve a narrow beam along the x direction. The number of grid rows is restricted to two to achieve a broad beam width along the vertical axis. The design achieves a high gain of dbi with 1.6% impedance bandwidth calculated at 24 GHz.

24 24 Another version of GAA is proposed by Zhang et al (2012) for automotive sensor. The developed antenna is shown in Figure This GAA consists of 32 meshes etched on the top surface of the substrate. In the proposed design, 41 radiating elements are arranged for operation in the 24 GHz band. This GAA offers 2% impedance bandwidth with gain 20.6 dbi at 24.2 GHz. Figure 1.17 A 24 GHz microstrip GAA (Zhang et al 2012) Both the presented designs are suitable for narrow band applications such as ACC and lane change. The only limitation with the narrow band radar system is the interference with passive sensors operating in the same GHz band. This interference cannot be neglected due to the increased number of cars and the number of radar systems deployed per car. Therefore UWB radar systems derive much interest and antennas for this UWB system gains importance Important Conclusions from Literature Based on the study of the literature, the following important conclusions are made using which the objectives of this thesis are framed. 1. There is a need for compact multi-service antennas

25 25 a. To increase the number of information and entertainment systems in automobiles without increasing the number of antennas required. b. To minimize the co-site interference c. To preserve the aesthetic value of the vehicle d. To reduce the engineering and manufacturing cost 2. UWB automotive antennas derives additional interest since, a. It performs well in the multipath environment b. It consumes minimum power c. It can handle large data with multimedia content d. Most suitable for vehicle to in-vehicle communication 3. Diversity techniques provide a. Good spatial coverage b. Enhanced link quality and data rate 4. UWB radar systems provide lesser interference than conventional narrow band radars. Hence, there is a challenge to design wideband antennas for radar application with minimum cost and easy fabrication techniques. 1.3 OBJECTIVES OF THE THESIS This thesis presents the antenna design techniques for use in vehicular environment. The outline of the thesis is presented in Figure The objectives of the research are: (i) To design a multi-frequency antenna with small frequency ratio for satellite services in the L-band. A slot loaded penta-band microstrip patch antenna is presented in this thesis. The presented design achieves

26 26 smallest frequency ratio with sufficient inter band spacing. The antenna can be located in the roof of the vehicle. Figure 1.18 Outline of the thesis (ii) To design compact multi-service antennas for Intelligent Transport System (ITS). The antennas used in automobiles should complement the aesthetics of the vehicle. Therefore, compact antenna structures are designed for ITS using PIFA and quad-band monopole technology. This antenna can be easily located inside the vehicle and merely making it hidden can improve the aesthetic value of the vehicle. (iii) To design an ultra-wideband (UWB) antenna for in-vehicle communications. The UWB technology provides large multipath immunity and it is capable of handling multimedia content with low power consumption. A simple compact UWB antenna is presented in this thesis for in-vehicle communications.

27 27 (iv) To evaluate the diversity performance of the compact antenna configurations The effects of multipath propagation are significant in automotive environment. Therefore diversity techniques are employed in modern automobiles to achieve good spatial coverage and link quality. The compact antennas developed are used for constructing spatial, polarization and pattern diverse antennas. (v) To construct bandwidth enhanced high gain antennas for automotive radar applications Dedicated short range UWB radar utilizing 24 GHz frequency spectrum is used for collision detection and avoidance. This technology requires a more directional antennas. GAA is found to be a promising technology which can be adopted to construct antennas having fan beam radiation pattern. This thesis demonstrates two GAAs that provide high gain and large bandwidth in comparison with the narrow band GAA constructed using line radiators. 1.4 SYNOPSIS OF THE THESIS The key aim of this thesis is the design and development of antennas for use in automotive environment. The antenna designs are carried out with a view to 1) Increase the number of services offered in the modern automobiles without increasing the number of antennas. 2) Develop compact antennas that gives high mounting flexibility 3) Achieve good spatial coverage 4) Reduce manufacturing cost and engineering cost

28 28 The thesis comprises of 6 Chapters as listed below: Chapter 1: Introduction to Automotive Antennas Chapter 2: Multi-Frequency Antenna for Satellite Services Chapter 3: Compact Multi-Service Antennas for Intelligent Transport Systems (ITS) Chapter 4: Diversity Analysis of Compact Antenna Configurations Chapter 5: Wideband Grid Array Antennas for Automotive Radar Systems Chapter 6: Conclusions and Future Scope Chapter 1 provides the broad overview of automotive antennas. The trends and developments of modern automotive communication are dealt in brief. The antenna requirements for these new services are discussed and explained. The objectives of the research work and the organization of thesis are described in this chapter. As an outcome of this chapter, the reader will understand the aim of the present research work and the contributions made in this thesis. Chapter 2 describes the conventional multi-frequency antenna design techniques. Based on the merits of slot-loaded antennas, a penta-band microstrip patch antenna (MSPA) is designed and demonstrated in this chapter for satellite based vehicular communications. The antenna reflection coefficient characteristics and radiation pattern are presented in this chapter. The salient features of this antenna have also been described. The proposed antenna can be loaded in the roof-top of the vehicle and hence the effect of car roof on the antenna s performance is evaluated and presented in this chapter.

29 29 Chapter 3 presents three compact antenna solutions for multiservice utility in automotive environment. The compact modified PIFA is designed and the development of shared aperture multiservice antenna for integration with PCB electronics is demonstrated. Further size reduced antenna is achieved using folded monopole antenna employing asymmetric coplanar strip-line (ACS) configuration. The proposed compact antennas are light weight, simple and cost effective. Both the antennas provide multifrequency operation within a small volume. Furthermore, a bandwidth enhanced antenna with hybrid geometry is presented in this chapter for automotive UWB communications. The performances of all the presented antennas are validated using free-space experimental measurements. The antenna housing effects such as antenna-to-antenna coupling and far-field radiation characteristics are estimated and the results are presented. Chapter 4 describes the diversity analysis of co-located compact antennas. The multipath signal propagation in automotive environment causes deep fading and depolarization of transmitted radio signal. Further the vehicle s body and other vehicles on-road create shadowing effects. Therefore, diversity techniques are adopted in automobiles to overcome these effects. Multiple identical antennas with same or different orientations are deployed in the vehicle s body to achieve spatial, polarization and pattern diversity. The construction and performance evaluation of diversity antennas is presented in this chapter. The diversity metrics such as envelope correlation coefficient (ECC), diversity gain (DG), mean effective gain (MEG) and Total Active Reflection Coefficient (TARC) are evaluated and presented. The conventional narrow-band radar systems suffer from severe interference effects due to passive sensors operating in the same frequency band and the co-existence of many vehicles fitted with similar radar systems. Chapter 5 demonstrates the design of two bandwidth enhanced GAAs for

30 30 application to wideband automotive radar systems. At first, bandwidth enhancement is demonstrated by reactance loading in the radiating lines of the GAA. The bandwidth of the GAA can also be increased by replacing the narrow band radiating lines with novel astroid radiating elements providing large impedance bandwidth. The design and discussion of this GAA constructed using astroid radiating elements is presented in the second part of the chapter. Chapter 6 presents the conclusions and future scope of the research work presented in this thesis.