Dual-polarized patch antennas with L-shaped probe feeds
Student thesis: Master's Thesis
Related Research Unit(s)
|Award date||2 Oct 2002|
This thesis presents novel techniques for increasing the isolation of dual-polarized patch antennas and arrays fed by L-shaped probes. The L-probe feeding technique was proposed in the literature in 1998 for enhancing the bandwidth of patch antennas. With the utilization of "T"-configuration of elements allocation, a two-elements array can be operated with dual polarization and good input-port isolation. In addition, dual-polarized patch antennas, employing "dual feeds", can perform high input-port isolation. Two techniques, one by cutting slots in the ground plane and the other by adding vertical metallic walls surrounding elements, for enhancing the isolation between array elements, are introduced. Finally, a back-lobe suppression technique is preformed through adding an external box-shaped ground plane under the designed array which can be used as base station antennas operated at the GSM1800 band. Firstly, the array with T-configuration of elements placing is studied. This array consists of two orthogonal individual sub-arrays, and the arrays are separated such that the coupling between them is minimized. A comparison between arrays with dual-feed and without dual-feed is studied too. There is 7dB improvement in the isolation after the array uses the dual L-shaped probe feeds. Then, a patch excited by two L-shaped probes for two orthogonal polarizations, k45', is investigated. The antenna exhibits poor input-port isolation due to strong coupling between vertical portions of the two feeding probes. In order to solve the problem of strong coupling, a dual-feed design for each polarization is introduced. This approach can effectively suppress the coupling and furnish good input-port isolation. The antenna achieves a bandwidth (SWR≤2) of 23% and an isolation of 30dB across the operating bandwidth. The far field radiation patterns are stable across the passband. In the study of dual-polarized antenna array, with high input-port isolation, two alternatives proposed arrays are considered. On one hand, an idea of cutting slots in the ground plane is presented for isolation enhancement for array designs. Specially selected patterns of slots are investigated and tested experimentally and theoretically. The proposed slots are able to reduce the coupling between elements of an array, with an isolation of 30dB across the operating bandwidth. On the other hand, a method of using metallic vertical walls to surround elements is described. This method is also applied for reducing the coupling between elements and provides good input-port isolation for the dual-polarized antenna array. The vertical walls act as obstacles to prevent strong coupling between elements. As a result, the array has 30dB isolation across the bandwidth. Finally, a back-lobe suppression technique is proposed. An additional box-shaped ground plane is added under the designed antenna array. The function of this box-shaped ground plane is to suppress and reflect the back radiation from the original ground plane. With the use of this additional ground plane, the array yields more than 25dB front-to-back ratio for the co-polarization across the operating bandwidth effectively.
- Antenna arrays, Microstrip antennas, Antenna feeds