Antenna Arrays and Diversity Techniques Một ăng-ten trong một hệ thống viễn thông là thiết bị thông qua đó, ở chế độ truyền dẫn, tần số vô tuyến năng lượng (RF) được kết từ các máy phát không gian miễn phí, và từ không gian miễn phí cho người nhận ở chế độ nhận [57-59]. | 21 CHAPTER 3 Antenna Arrays and Diversity Techniques An antenna in a telecommunications system is the device through which in the transmission mode radio frequency RF energy is coupled from the transmitter to the free space and from free space to the receiver in the receiving mode 57-59 . ANTENNAARRAYS In many applications it is necessary to design antennas with very directive characteristics very high gains to meet demands for long distance communication. In general this can only be accomplished by increasing the electrical size of the antenna. Another effective way is to form an assembly of radiating elements in a geometrical and electrical configuration without necessarily increasing the size of the individual elements 9 . Such a multielement radiation device is defined as an antenna array 59 . The total electromagnetic field of an array is determined by vector addition of the fields radiated by the individual elements combined properly in both amplitude and phase 58 59 . Antenna arrays can be one- two- and three-dimensional. By using basic array geometries the analysis and synthesis of their radiation characteristics can be simplified. In an array of identical elements there are at least five individual controls degrees of freedom that can be used to shape the overall pattern of the antenna. These are the 59 i. geometrical configuration of the overall array linear circular rectangular spherical etc. ii. relative displacement between the elements iii. amplitude excitation of the individual elements iv. phase excitation of the individual elements v. relative pattern of the individual elements 22 INTRODUCTION TO SMART ANTENNAS ANTENNA CLASSIFICATION In general antennas of individual elements may be classified as isotropic omnidirectional and directional according to their radiation characteristics. Antenna arrays may be referred to as phased arrays and adaptive arrays according to their functionality and operation . Isotropic Radiators An isotropic .
