Studies on Stacked Patch Antennas for WLAN applications

Abstract

The need for wireless computer communications is growing day by day; with WLAN, WiMAX and UWB technologies emerging as the major technologies in this field currently. As the communication is wireless in nature, an antenna is one of the main requirements of these systems. Typically for the WLAN systems, an antenna is required at the router as well as at the computer terminal that is preferably inexpensive, compact and rugged in nature. Microstrip antennas therefore act as one of the viable choices for such systems which can be preferably designed as multiband antennas with suitable bandwidths that are capable of covering multiple wireless applications for efficient utilization of the resources. A lot of complex techniques are available in the literature that can be used to design and develop Microstrip antennas for these wireless applications; a simpler technique in this context is the use of Stacked/Multilayer Microstrip patch antennas. As the effective height of the antenna increases when multiple substrate layers are used in the antenna structure, and the resulting antenna has loosely bound EM (electromagnetic) fields and that are radiated out better into free space. This leads to an increase in the antenna bandwidth if the two substrates are of same/ close by dimensions and multiple frequencies can be radiated out if the patch antennas are of varying dimensions. Therefore the purpose of this thesis is to investigate the behavior and the design and development of Stacked Patch antennas for wireless applications like WLAN, WiMAX and UWB etc and the methods to develop Stacked Microstrip patch antennas (MSAs) that exhibit multi-frequency and a wideband behavior at these bands. A defected ground structure (DGS) is another variation that can be incorporated into the antenna structure to enhance the existing bandwidth or get a multiband behavior from the stacked MSAs. On the basis of dimensions and shape of defect in ground plane, the protected distribution of current density in the ground plane is interrupted, leading to a restrained excitation and generation of the electromagnetic waves through the substrate layer that modify the characteristic properties of the transmission line. The shape of the defect may be changed from the simple shape to the complicated shape for getting the desired performance This concept has been used to design and develop four main types of stacked patch antennas in the current research work mainly; a simple rectangular stacked patch antenna (two layers of stacking) with improved bandwidth, a stacked Microstrip patch antenna with an ‘E’ shaped defected ground structure for dual broadband operation, a fractal antenna with a cross shaped DGS for dual broadband operation and a complementary rectangular stacked patch antenna with a pie shaped DGS have been designed for the application in wireless bands of IEEE 8.2.11a/b WLAN bands, U-NII-1 ,U-NII-2/2e band, Radio Astronomy, STM band applications, UWB band from (5.28-5.8) GHz, and WiMAX wireless application bands. All the proposed designs have been developed with a VSWR of less than 2 at their respective resonant bands of operation so that a good impedance matching allows maximum power to be coupled to the antenna from the transmission line. The designed antennas aim at getting an appreciable reflection coefficient of less than 0.33, wide impedance bandwidth, increased gain for long range applications, perfect impedance matching, increased frequency ratio and reasonable antenna size with Dual-wideband operation as the main goals of present research work. This thesis describes original work done on stacked microstrip antennas by using multiple substrate layers (up to three) in the antenna structure with an aperture coupled feeding mechanism with different types of geometries for getting multi-frequency and wideband behavior from the proposed stacked Microstrip patch antennas. The research work starts with a thorough study of literature available in context to broad banding and multi banding of Microstrip antennas as discussed in chapter two. Chapter 3 presents the design, analysis and verification of results of a simple rectangular stacked patch antenna. A Stacked MSA with aperture coupled feeding has many parameters that can be optimized to obtain a desired behavior from this antenna. A specific design example of a stacked patch antenna at 5.4GHz with a bandwidth of 630 MHz has been considered for analysis purpose. By modifying the shape of the slot cut in the ground plane (DGS) of the aperture coupled stacked patch antenna, a dual band behavior can be achieved , which is explained in chapter 4 where the antenna is able to achieve 200MHz bandwidth at 2.53GHz and 530MHz at the 3.66GHz frequency. By incorporating fractal shaped defects in MSAs; multiband behavior can be obtained and the use of DGS with these antennas can lead to an improved bandwidth at the two bands, this is illustrated in chapter five of the thesis work. Use of complementary slotted stacked patch antennas with the help of a DGS can again lead to an improved bandwidth resulting in an ultra wideband antenna; this is explained in chapter six of the dissertation. Chapter seven concludes the research work presented in this thesis and the future scope of the work is also presented. This entire research work is verified through analytical simulations using three dimensional Full Wave electromagnetic simulator, Computer Simulation Technology Microwave Studio Version 10.0 (CST MWS V10.0). Experimental studies and measurements have been carried out using Vector Network Analyzer (VNA) E 5071C and an anechoic Chamber operating in the range of 2 KHz to 20 GHz having a standard horn antenna with a calibrated gain of 12dBi.