Design and analysis of integrated millimeter wave antenna
Date of Issue2017-05-02
School of Electrical and Electronic Engineering
Centre for Integrated Circuits and Systems
The 24-GHz industrial, scientific and medical (ISM) band has been allocated by the European Commission and the Federal Communications Commission (FCC) for automotive short range radars applications, such as collision warning system, parking aid, adaptive cruise control, rear traffic crossing alert and blind spot detection. All of these systems require high-gain, low-profile and easily fabricated antennas. In the chip scale, comparing with antenna-on-chip (AOC), antenna-in-package (AIP) is a more promising candidate for 24-GHz single chip system, since AIP has a higher radiation efficiency and wider selection of antenna prototypes and substrate materials. Among various reported antennas that are used for 24-GHz application, microstrip grid array antenna is a potential candidate because it has all the usual benefits of patch array antenna plus high radiation efficiency, high aperture efficiency and low cross-polarization. Several problems need to be solved to further push microstrip grid array antenna for 24-GHz and millimeter-wave applications. The first is that the mutual coupling mechanism between grid array radiators is unclear. The second is that the polarizations and feeding methods of microstrip grid array antennas are worth exploring. Besides, the radiation performance of large microstrip grid array antenna is limited by non-uniformed current distribution which results in high sidelobes and low gain. Finally, a thorough comparison between microstrip grid array antenna and patch array antennas is needed to further investigate the potential advantages and disadvantages of microstrip grid array antenna. This thesis firstly presents an analysis of mutual coupling between sub grid arrays on electrically-thin substrate. It is found that the mutual coupling between sub grid arrays on electronically-thin substrate is mainly caused by dielectric polarization currents. High isolation between sub grid arrays can be achieved by proper selection of the antenna array factor and size. The high isolation between sub grid arrays enlarges design flexibility of the antenna, which makes it easy to implement simultaneous transmission and reception (STAR) application, monopulse antenna and generating orbital angular momentum (OAM) modes. To validate the theory, a particular example of microstrip grid array antenna with 5 sub arrays is developed in RT/duroid 5880 substrate for STAR application with high measured isolation of 65.07 dB between transmitting and receiving antennas. Measured results from this configuration agree well with the simulated results. Secondly, this thesis presents a multi-port microstrip grid array structure, which gives the flexibility to use it as a single antenna or multiple antennas. To realize such a structure, the key challenges are impedance matching and port isolation. It is shown that impedance matching can be achieved with stepped-impedance lines and port isolation can be enhanced from the defected ground plane. A particular example of 145 radiating elements with 4 ports is implemented for 24-GHz applications. Simulated and measured results are compared, which verifies the design, analysis, and versatility of the structure. Thirdly, the microstrip grid array antennas excited by slots are designed. A 24 GHz microstrip grid array antenna excited by capacitive coupling slot is designed. Furthermore, a microstrip grid array antenna excited by coaxial-fed slot is proposed. A novel coaxial line to slot transition is introduced. Fourthly, a dual-polarized microstrip mesh array antenna for 24-GHz applications is developed. The microstrip mesh array antenna enlarges the system polarization diversity with the capability of being utilized to support the simultaneous reception in two orthogonal polarizations, as well as transmission in either polarization. The proposed design is composed of two grid arrays in two orthogonal directions, where matching stubs are specially designed to compensate the alteration of input impedance due to the direct coupling of the complementary array of another polarization. Finally, a thorough and fair comparison between microstrip grid array antenna and patch array antenna is carried out. For basic elements, the performances of two-loop microstrip grid array antenna, conventional patch antenna and wideband patch antenna on the same substrate are compared. After that, a 145-element microstrip grid array antenna is compared with a bandwidth-enhanced patch array antenna. Both antennas are designed on single-layer RT/duroid 5880 substrate with size of 100 mm × 100 mm × 0.25 mm. The discrepancies between the performances of these two antennas are analyzed with emphasis on the side lobe level and cross-polarization level.