Chapter 1 Introduction
1.1 Background
With advances in technology, wireless communication has become an integral part of daily life. For example, wireless local area networks (WLAN), wireless personal area networks (WPAN), and RFID systems, all sorts of wireless applications makes people’s lives more convenient than the past. Antennas play an important role in wireless communications.
The performance of the antenna will affect the communication quality.
With the increase of wireless applications and rapid development of technology, wireless communications desire antennas with frequency diversity and polarization diversity to adapt in different application platforms and to improve polarization mismatch loss. Antennas with frequency diversity, such as multi-band or broadband antennas, make antennas operate in different frequency bands to transmit and receive messages in various telecommunication services. Polarization diversity can improve the communication quality when receiving signal, so the signal will not suffer from multipath diffraction or scattering, resulting in polarization mismatch loss, such as using different polarization antenna to achieve polarization diversity.
Pattern diversity, controlling null position in the radiation patterns, can avoid receiving noise from other devices and transmitting signals to unrelated devices, such as using antenna arrays
Chapter 1 Introduction
to achieve pattern diversity.
Although multiband or broadband antennas can operate in different frequencies, they receive unwanted signals from other frequency bands, which makes the system require stringent filters and results in increasing costs and space requirement. Different polarization antennas can receive signals with various polarizations which minimize the polarization mismatch loss. However, using multiple antennas will greatly increase the space requirement.
Array antennas, which consist of a number of antennas, due to space constraints and multiple antenna coupling effect, are not suitable for application in handheld devices.
1.2 Related Works
In recent years, software defined radio (SDR) and cognitive radio (CR) have received significant attention in the field of wireless communications. The SDR/CR device changes its transmission or reception parameters to communicate efficiently between licensed and unlicensed users. These parameters include operating frequency, spectrum, signal format modulation, etc. Hence, some researchers proposed reconfigurable antennas to be a good front-end solution to accompany the development of SDR/CR networks [1]. These reconfigurable antennas are capable of achieving diversity in the operating frequencies, polarizations, radiation patterns as well as gain; therefore, these antennas can fulfill the requirements of SDR/CR networks. Moreover, signals in the real world suffer multipath, fading, and diffraction as well as interference. The wireless system uses the reconfigurable antenna to increase the signal-to-noise ratio of the whole system by achieving diversity capabilities.
Generally, reconfigurable antennas are categorized in terms of frequency, radiation
Chapter 1 Introduction
while maintaining radiation characteristics. Compared to multiband antennas, the frequency reconfigurable antenna could be used in the multiband communication system for better out-of-band rejection. The frequency reconfigurable antenna only has one operating frequency in each operation. In other words, it has inherent bandpass characteristics that eliminates the demand of an isolated filter and hence reduces the front-end circuit complexity [2]-[5].
Consequently, the pattern reconfigurable antenna which changes radiation pattern while maintaining operating frequency and bandwidth can greatly enhance system performance by steering its radiation beams or nulls in several predefined reception directions. Therefore, it also achieves angle/space diversity at a given operating frequency. Pattern reconfigurable antennas are applied in handheld devices to decrease the interference with other devices and to improve communication quality [6]-[8]. A pattern reconfigurable bow-tie antenna using a CPW–to-slot line transition and a switchable polarization patch antenna have been investigated to increase the signal-to-noise (SNR) ratio [9]-[11]. Finally, the polarization reconfigurable antenna has been reported in [12]-[21]. Such antennas are commonly designed based on the microstrip patch antenna with truncated corners or slots so as to switch the antenna polarization states between RHCP, LHCP, LP, vertical polarization, or horizontal polarization.
1.3 Organization of Thesis
The thesis is organized as follows.
Chapter 2: The essential backgrounds are described. It starts from antenna parameters in antenna design. And then the introduction of microstrip antennas is given.
Chapter 3: A novel multiple-ear patch antenna for polarization and frequency diversities is
Chapter 1 Introduction
proposed. The antenna configuration, design concept, and bias network are described firstly. The parameter study and the antenna operating mechanism is also described and explored. Next, the simulated and measured results such as axial ratio and radiation patterns are presented.
Chapter 4: Conclusion
The previous work (a frequency reconfigurable slot antenna using PIN diodes) of multiple-ear patch antenna for polarization and frequency diversities is given in appendixes.
Appendix A: Introduction of slot antenna.
Appendix B: A frequency reconfigurable slot antenna using PIN diodes is proposed. The proposed antenna structure and design concept are presented firstly. And then the measured and simulated results of the proposed antenna such as return loss and radiation patterns are given. Finally, it is a brief summary.
Chapter 2 Introduction of Microstrip patch Antennas
Chapter 2 Introduction of Microstrip patch Antennas
2.1 Introduction
In this chapter, we introduce the essential background for the antenna design, such as antennas parameters and theory of microstrip antennas.
2.2 Antenna Parameters
z Directivity D ─ a ratio of the maximum power density in the main beam direction to the average radiated power density.
z Radiation Efficiency η ─ a ratio of radiated power to the input power.
z Gain G ─ a ratio of the maximum radiation intensity to the radiation intensity obtained if power received by the antenna were radiated isotropically.
z VSWR ─ Voltage Standing Wave Ratio, is a measure of how efficiently radio-frequency power is transmitted from a power source, through a transmission