TAILIEUCHUNG - designing a wireless network phần 3

chúng tôi họ muốn một dịch vụ dựa trên Internet mà có thể mở rộng và nâng cao giá trị của các cuốn sách. Căn cứ vào đọc những phản hồi và kế hoạch chiến lược của chúng tôi, chúng tôi đã tạo ra một trang web mà chúng tôi hy vọng sẽ vượt quá mong đợi của bạn | Radio Elements and Frequency Spectrums Chapter 2 53 The driven element is the only active element on a Yagi antenna and is the only element that connects to the transceiver via a remaining elements are known as parasitic elements because they feed off of the radiated power from the driven element. As we described earlier if a piece of metal receives a signal and it is not drained from the metal it will be re-radiated from the metal. This is how the parasitic elements of a Yagi work. Broadcast television antennas are examples ofYagi type antennas. Planar Array Antennas Planar array antennas are similar in concept to Yagi antennas except all elements both active and parasitic lie in the same plane. This results in a flat antenna that can be mounted flat on a wall yet still have the properties and gain of a directional antenna. Figure shows a planar array antenna. Figure Planar Array Antenna and Associated Directional Beam Pattern Multiple Antenna Elements Shown with Dipole Antennas 54 Chapter 2 Radio Elements and Frequency Spectrums Planar arrays can have more than one active element. By changing the phase and power of the signal to hit specific active elements the beam can be steered without the antenna physically moving. A useful application of this is on military tracking radars. A computer can adjust the input signals to the various elements of a planar array and steer the beam faster than the whole array could be moved physically thus allowing for tracking of multiple fast-moving targets. Sectorized Array Antennas Sectorized array antennas are a type of phased array antenna designed to split up a circular coverage area into sectors to help in channel allocation and reuse. Most sectorized antennas will have a beam width of about 120 that allow them to divide a circle into three sectors. Sectorized antennas are commonly used in wireless phone applications and can be seen on wireless phone towers all over America. Figure .

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