| The rapid development of wireless communication technologies has put forward higher requirements on the antennas, that is integration (easiness of integration with RF circuits), miniaturization (with low profile and small volume), and high performance (broadband or multiband, high gain, low sidelobe, low cross-polarization, etc.). With its unique advantages, microstrip antennas have been widely used in many fields such as wireless communication, remote sensing, radio astronomy and so on. However, the bandwidths of microstrip antennas are usually very limited, which significantly astrict its scope of possible applications. The bandwidth enhancement technology of microstrip antennas is not only a hotspot in antenna research, but also an important research direction in the field of wireless communication technology. Hence, it is of great significance for the entire application field of microstrip antennas to develop wideband high-performance microstrip antennas.On the other hand, as one of the most rapidly developing areas of wireless communication, mobile communication systems have caught a lot of troubles after evolving over several generations. As to the traditional base station, the fan beam coverage and direction cannot be flexibly adjusted, the area resources are in a dramatic shortage due to the coexistence of various network standards, the construction and maintenance costs are continuing to rise up. In regarding to the Massive Multiple-Input and Multiple-Output (Massive MIMO) technology which is the development direction of 5G wireless communication system with the highest potential, its research and verification also have to face a dual challenge of cost and performance. In this context, the high performance active antenna system, which has a high degree of integration and ability to support multiple network standards, has come into being. As the well-known international equipment manufacturers launching experimental prototypes successively, the active antenna system has become a very important development tendency of the future mobile site pattern. Furthermore, the system’s ability of precisely beam scanning in a wide range makes it very suitable for the efficient realization of the Massive MIMO system architecture. Based on such background, it has extraordinary significance to do the study of low cost and high-performance active antenna system.The dissertation focuses on the research and development of wideband low profile microstrip antennas and low cost high-performance active antenna system. It is organized as follows:Chapter one mainly focuses on the wideband low profile linear polarized microstrip antennas. Through excitation of multiple operating modes, two kinds of novel wideband low profile microstrip patch antennas are proposed. Firstly, a single layer wideband low profile tooth-like-slot patch antenna fed by inset microstrip line is presented. Based on the current distributions on the patch antenna, the working mechanism is investigated and the design procedure is concluded. Then, a wideband low profile substrate integrated waveguide (SIW) cavity-backed E-shaped patch antenna is proposed. By analyzing the input impedance of the antenna, the design ideas are summarized. The comparison between the proposed antenna and conventional E-shaped patch antenna is also illustrated. Prototypes of both antennas are fabricated and measured. It is shown that both of them can exhibit more than 10% impedance bandwidth with a low profile of 0.033λ0. The designs of this part have been published in iWAT and IEEE Antennas Wireless Propag. Lett..Chapter two revolves around wideband low profile circular polarized (CP) microstrip antennas. By means of introducing parasitic resonances and building up antenna array, three kinds of wideband low profile CP microstrip antennas are proposed. Firstly, a single-fed low profile broadband CP stacked patch antenna is presented. A parasitic patch is introduced in the driven patch layer of the antenna to achieve much wider potential AR bandwidth.. Four stacked patches arranged in a loop style are employed to get loose coupling between the driven patch layer and stacked patch layer. Combing the two techniques, the antenna can achieve a wider AR bandwidth with a lower profile. Then, a wideband CP cavity-backed aperture antenna with a parasitic square patch is studied. A favorable CP performance can be achieved by suspending a patch above the low profile cavity-backed aperture antenna that has a deteriorative CP characteristic in itself. With cavity height decreasing, very good CP performance can be still maintained as long as the parasitic patch grows properly. At last, by applying the stacked coupling technique in the sequential phase feeding structure, a bandwidth- and gain-enhanced 2×2 CP patch antenna array using sequential phase feed is proposed. The working mechanism and main parameters which significantly affect the antenna performance are investigated in regarding to all the aforementioned antennas. Measured results of the prototypes validate the theoretical analysis. The designs of this part have been published in IEEE Trans. Antennas Propag. and IEEE Antennas Wireless Propag. Lett.Chapter three gives a in-depth study of the low cost high-performance 2.6GHz active antenna array for Massive MIMO system. Based on the analysis of some key technologies which should be considered seriously in the design procedure such as RF miniaturization, phase control techniques, etc., a frequency shiftable vector modulator (FSVM) module with low cost and high flexibility, which is very suitable for application in active antenna system, is proposed firstly. The FSVM module utilizes the vector modulator’s advantages of low cost and high precision, but overcomes its shortcoming of very limited operating band. In addition, the performance of the signal passing through the module will not be affected. By employing such module, a 1×8 active antenna array with a RF path beamforming structure is then developed. Measured results of the prototype validate the theoretical analysis and exhibit its excellent qualities of low cost, high flexibility, wide scanning angle (-50° to 50° with gain fluctuation less than 1.5dB) and high precision (beam position error better than 0.5°). The designs of this part have been submitted to Microw. Antennas Propag.. The developed active antenna array has been used by our customers to achieve Massive MIMO mobile communication system.Chapter four aims at the hotspot of the current research for Wireless Local Area Network (WLAN) applications, that is the smart antenna system with low cost, high-performance and capability of fully adaptive beamforming. Accordingly, a 1×8 active transmitting array based on the frequency shiftable vector modulator (FSVM) module is demonstrated. The antenna array can be combined with the pre-exsiting 8×8 MIMO system, which is developed by our lab for future WLAN, to provide Massive MIMO function. During the study, with the realization of the FSVM module in C band, the amplitude and phase modulation at high frequency, which originally need to pay a high price with a low accuracy, can be achieved by realizing the amplitude and phase modulation at low frequency, which now is conducted through low cost but high-precision analog ICs. By applying this module in each transmitting RF channel, the proposed active antenna system is sufficient to provide the fully adaptive beamforming function. Measured results of the prototype validate the theoretical analysis and exhibit its excellent qualities of low cost, high flexibility, wide scanning angle (-50° to 50° with gain fluctuation less than 2dB) and high precision (beam position error better than 1°). The designs of this part have been submitted to IEEE Antennas Wireless Propag. Lett..
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