Broadband microstrip antennas with switchable polarizations

Abstract

Microstrip antenna consists of a patch of metallization on grounded substrate. These are low profile, lightweight antennas, most suitable for aerospace and mobile application. They are replacing many conventional antennas used in defense and commercial applications. They have got certain drawbacks such as narrow bandwidth, low gain, and low power handling capability and polarization impurity. Many researchers are trying to overcome these drawbacks. In this thesis also some efforts have been done to overcome certain drawbacks of microstrip antenna. One of the serious limitations of the microstrip antennas is its narrow bandwidth. The impedance bandwidth of MSA is around 1% only for thin substrates. The bandwidth of the MSA can be increased by increasing thickness of the substrate. If the thickness is increased it creates problems for impedance matching, produces radiations from the feed and distortions in the radiation patterns due to higher order modes. Therefore to avoid these problems thicker substrate is not used. The common techniques to improve bandwidth are Planner multiresonator configurations, Electromagnetically coupled MSA; Aperture coupled MSAs, Impedance matching networks for broad band MSAs & Log periodic MSA configurations. It is advantageous to use Electromagnetically coupled MSA because of its small size and no back radiations. Therefore Electromagnetically coupled MSA is used to design the antenna. The MSA gives linear polarization. Many times we need circular polarization with low cross polar level (generally below 10-12 dB). This circular polarization is obtained in this antenna using a shorting pin. This is the major achievement. Many papers discuss about bandwidth only, a few papers discuss just about polarizations using shorting pins. Many times we need large bandwidth, desired polarization and high gain. In order to obtain high gain we have to form an array of antennas. It is convenient to feed the array elements using microstrip feed. Therefore the Electromagnetically coupled MSA is fed using a microstrip line as described in chapter 4. Some times we need right circular or left circular or linear polarization depending on situation. These polarizations can be obtained using two shorting pins instead of one as described in chapter 4. In satellite TV transmission vertical and horizontal polarizations (Eθ and EФ) are used. In order to minimize adjacent channel interference they are placed alternately on vertical and horizontal polarizations. These additional Eθ, EФ polarizations as well as RHCP and LHCP are obtained using additional shorting pins as described in chapter 4. for satellite TV transmission we need high gain around 30 dB. This can be obtained using an array of above elements. The design is given in chapter 4. Various feeding techniques, transmission lines, bends, power dividers and quarter wave transformers are discussed. Spacefed microstrip antennas are also designed, simulated and studied in chapter 5. Recently fractal antennas are becoming very popular because of their small size, multiband response and high efficiency. The basic types of fractal antennas are designed and simulated in chapter 6. Bandwidth is the major problem in microstrip antennas. In chapter 7, it is discussed how do we get large bandwidth, and the concept is used to obtain very large or ultra-wide bandwidth using rectangular microstrip antenna. The same concept is used to obtain very large bandwidth using sierpinski fractal antennas.