With the improvement of the source and detection technology of terahertz wave, terahertz band (0.1THz-10THz) is gradually becoming a focus of present research. In the fields of material testing, biology, medicine, imaging, and explosive detection, etc., terahertz wave has wide application prospect. But up to now, the dielectric properties, especially the loss, of the material used by terahertz components are still unknown. And the design, fabrication, measurement and loss analysis of the passive components in terahertz band are quite different with those in microwave and millimeter wave band. Hence, it is of great significance for the study of dielectric properties and passive components in terahertz band.
Although the frequency of microwave band is lower than terahertz band, but its development is more mature. And its application range covers many fields such as communication, radar, medical treatment and so on. As the most commonly used device in microwave band, antenna can radiate linearly, circularly, or elliptically polarized wave, according to its structure. Compared with the linear polarization, circular polarization allows for the reduction in multipath reflections and other interference, simplification of the alignment between transmitter and receiver, and better weather penetration; Microstrip antennas have the advantages of low profile, low weight, easiness of fabrication and integration. Based on these advantages, the circularly polarized (CP) microstrip antennas are widely used in wireless and satellite communication, navigation and radar, etc. However, its inherent limitation of bandwidth restricts its scope of applications. Therefore, the study of high performance CP microstrip antennas and arrays has great significance in theory and practice.
The dissertation revolves around the terahertz material properties, passive componenets and broadband CP microwave antennas. It is organized as follows:
Chapter one focuses on the terahertz time-domain spectroscopy (THz-TDS) system and its application in the measurement of material properties. The working principles of THz-TDS system, and the beam path are introduced. The effect of water molecule in the air and the repeatability of multiple measurements are analyzed. Based on the transmission method, the formulas for the measurement are derived. Then the dielectric properties of some common substrates, like Rogers RT/Duroid 5880, FR4_epoxy, gallium arsenide (GaAs), and sillicon (Si) substrates with different resistivity are extracted and analyzed. The losses of these materials in terahertz band are presented in the form of loss tangent for the first time. The measurement results show that the high-resistivity Si and GaAs are suitable for the use in terahertz band due to their extremely low loss. The Rogers RT/Duroid5880 is only suitable for some simple structures in the low frequency band of terahertz. However, application range of the low-resistivity Si and the FR4_epoxy are limited by their high loss in terahertz band. Some works in this chapter have been published in Journal of Microwave and National Conference on Microwave and Millimeter Wave (NCMMW).
Chapter two studies on the designs, fabrication, measurement and analysis of terahertz components based on Micro-electro-mechanical System (MEMS) technology. Several layer structures are compared and detailed processing steps are presented. Taking MEMS straight waveguide as example, the sources of loss are found out and a simulation analysis method is proposed. Moreover, a low insertion loss bandpass filter using elliptic cavities and a high selectivity bandpass filter using rectangular cavities are fabricated and measured. And the measured results agree well with the fitting results which is based on the loss analysis method. In addition, a terahertz frequency scanning antenna is designed and simulated. The scanning range of the pattern is above ±45°. Some designs in this chapter have been published in IEEE Trans. on Terahertz Science and Technology.
Chapter three presents the designs, fabrication, measurement and analysis of terahertz components based on high precision milling process. A straight waveguide and a bandpass filter are fabricated to verify the process in terahertz band. On this basis, a planar offset-fed reflector is designed, and the effect of its size to the aperture efficiency and gain is discussed. Based on this planar offset-fed reflector, two methods for higher gain are proposed. One method is to connect the horn to the aperture of reflector. With this method, the gain can be increased form 20.8dBi to 31.1dBi@300GHz; another method is to make the side surface of the brick corrugated. With this method, the gain can be increased form 21.3dBi to 28.5dBi@310GHz. Both of these methods achieve good gain enhancement. Moreover, they all has the advantage of easy fabrication, and meet the application requirements in terahertz band. Some designs in this chapter have been published in the international conference of IEEE IWS’2015 and some have been submitted to IEEE Antennas Wireless Propag. Lett.
Chapter four proposes a broadband CP microstrip antenna with improved axial ratio (AR). According to the polarization error among the influencing factors of AR, a broadband CP microstrip antenna with lower polarization error and improved AR is designed. The feed network of this antenna includes a broadband balun. Through the metallic vias, the energy transmits from the feed network to L-shaped probes and then feeds into the patch. By inserting a metal sheet between two L-shaped probes, the cross-polarization could be reduced and the AR will be improved. The proposed antenna achieves the measured AR bandwidth of 61.7%, which is comparable to some other CP microstrip antennas with four feeding points. The proposed antenna exhibits good performance and easiness for design and optimization. Some designs in this chapter have been published in IEEE Antennas Wireless Propag. Lett and two patents have also been applied for the design.
Chapter five studies on the design of vehicle mounted antenna array for satellite communication. A double-layer antenna array is proposed, which can achieve the gain coverage more than 9dBi in the upper half-space, from 1.98GHz to 2.20GHz. The aperture coupled microstrip antenna is choosed as CP element. For the generation of CP wave, two coupling slots are placed 90° offset. The parasitic patch is introduced to improve the matching and enhance the gain. The CP element can achieve the gain of 8.95dBi within the required band. To meet the requirement of gain, subarray is formed with four CP elements. The measured gain of the subarray is above 13dBi and the AR is lower than 2.4dB. And the state of polarization can be changed easily between left-hand circular polarization and right-hand circular polarization. The whole array consists of 17 subarrays, which are mounted on the top and side surfaces of a double-layer octagonal frustum. The internal space of the frustum is used for radio frequency (RF) modules. The coverage of the upper half space can be achieved by using a combination of independent beams by single subarray and composite beams by multi-subarrays. Some designs in this chapter have been used to apply for patent.