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Electromagnetic Waves and Antennas combined - Chapter 9

Ống dẫn sóng hình chữ nhật được sử dụng thường xuyên để chuyển lượng lớn của quyền lực vi sóng ở tần số lớn hơn 3 GHz. Ví dụ tại 5 GHz, sức mạnh truyền có thể là một megawatt và sự suy giảm chỉ có 4 dB/100 m. Sợi quang hoạt động ở tần số quang học và hồng ngoại, cho phép một băng thông rất rộng. Thiệt hại của họ là rất thấp, điển hình là 0,2 dB / km. Sức mạnh truyền là của thứ tự của milliwatt. . | 9 Waveguides Waveguides are used to transfer electromagnetic power efficiently from one point in space to another. Some common guiding structures are shown in the figure below. These include the typical coaxial cable the two-wire and mictrostrip transmission lines hollow conducting waveguides and optical fibers. In practice the choice of structure is dictated by a the desired operating frequency band b the amount of power to be transferred and c the amount of transmission losses that can be tolerated. Fig. 9.0.1 Typical waveguiding structures. rectangular waveguide waveguide Coaxial cables are widely used to connect RF components. Their operation is practical for frequencies below 3 GHz. Above that the losses are too excessive. For example the attenuation might be 3 dB per 100 m at 100 MHz but 10 dB 100 m at 1 GHz and 50 dB 100 m at 10 GHz. Their power rating is typically of the order of one kilowatt at 100 MHz but only 200 w at 2 GHz being limited primarily because of the heating of the coaxial conductors and of the dielectric between the conductors dielectric voltage breakdown is usually a secondary factor. However special short-length coaxial cables do exist that operate in the 40 GHz range. Another issue is the single-mode operation of the line. At higher frequencies in order to prevent higher modes from being launched the diameters of the coaxial conductors must be reduced diminishing the amount of power that can be transmitted. Two-wire lines are not used at microwave frequencies because they are not shielded and can radiate. One typical use is for connecting indoor antennas to TV sets. Microstrip lines are used widely in microwave integrated circuits. 362 9. Waveguides Rectangular waveguides are used routinely to transfer large amounts of microwave power at frequencies greater than 3 GHz. For example at 5 GHz the transmitted power might be one megawatt and the attenuation only 4 dB 100 m. Optical fibers operate at optical and infrared frequencies allowing a