Design of L-probe coupled patch antennas and arrays
Student thesis: Doctoral Thesis
Related Research Unit(s)
|Award date||3 Oct 2006|
With the increasing growth of mobile communication services, there is a need to build base stations with more sophisticated antennas to further enhance network coverage and to provide greater capacity. Due to their versatility in design, patch antennas are commonly used in modern wireless communications. They can be designed with linear or dual polarizations. Moreover, the patch antennas allow many different excitation techniques to be employed, such as the wideband L-probe feeding technique. In particular, they can be used as base station antennas for cellular communications. The coverage of a vertically-polarized patch antenna depends on the horizontal H-plane beam width. Base station antennas with different values of beamwidth in the horizontal plane (H-plane), ranging from 30 degrees to 120 degrees, are required to serve different environments. One way of increasing the beamwidth is to reduce the width of the patch, but resulted in reducing the usable impedance bandwidth and poor input matching. In this study, a two-element U-folded patch antenna array with an L-probe feed is first investigated. This antenna has wide impedance bandwidth of 20% with SWR<1.5 and a low cross polarization level of less than –20dB. Secondly, in order to further increase the beamwidth, the common technique is to reduce the width of ground plane. By this method, however, the backlobe level will be increased drastically. Therefore, a modified grounding structure for the first design, namely the U-folded patch antenna array with an L-probe feed, is proposed and tested. By cutting slots in the two vertical side walls of a box-shaped grounding structure , an impedance bandwidth of larger than 20% (SWR< 1.5), an H-plane beamwidth over 90 degrees and much reduction in back lobe radiation can be obtained. In order to serve a larger number of users, to provide higher transmission speeds and to operate in more complex environments, diversity techniques are commonly used to improve the system performance without additional interference. In many urban applications, the polarization diversity system has replaced the traditional space diversity system as the means to mitigate fast fading. The isolation of a single dual-polarized L-probe patch antenna is not high enough for real applications. So, thirdly, we design a two-element array of stacked patches surrounded by four sidewalls surrounding a planar ground plane to counteract the coupling between the vertical portions of the L-probes. As a result, a wider isolation bandwidth can be obtained in comparison to the case with a planar ground plane. However, in some cases, the half-power beamwidth in the horizontal plane should be 90 degrees in order to serve a wider sector of a cell. The beamwidth enlargement in dual polarization cannot be simply reducing the width of the patch. Fourthly, two smaller box-shaped grounding structures are added to surround the stacked patches used in the third design to increase the beamwidth to 90 degrees and to maintain good isolation. In addition, a 2 x 2 high gain antenna array is also developed to serve another environment. By locating each L-probe pair for each element appropriately, higher isolation and lower cross polarization in both ports can be obtained. Fifthly, for MIMO applications, an L-probe fed dual-polarized array is implemented on a finite conducting cylinder which yields wide bandwidth and high isolation. Furthermore, in order to maximize the capacity provided by a mobile phone network, the area of coverage of an individual base station is carefully controlled. Two genetic programming techniques which automatically synthesize the design of an eight- element L-shaped probe fed antenna array with electronic down tilt in the E-plane are developed. Different phase and magnitude are found for different elements. The upper sidelobe can be suppressed and the lower nulls can be filled to designate power levels.
- Microstrip antennas, Antenna arrays