tailieunhanh - Microwave Waveguides and Coaxial Cable

Microwave Waveguides and Coaxial Cable | MICROWAVE WAVEGUIDES and COAXIAL CABLE In general a waveguide consists of a hollow metallic tube of arbitrary cross section uniform in extent in the direction of propagation. Common waveguide shapes are rectangular circular and ridged. The rectangular waveguide has a width a and height b as shown in figure 1. Commonly used rectangular waveguides have an aspect ratio b a of approximately . Such an aspect ratio is used to preclude generation of field variations with height and their attendant unwanted modes. Waveguides are used principally at frequencies in the microwave range inconveniently large guides would be required to transmit radio-frequency power at longer wavelengths. In the X-Band frequency range of to GHz for example the . standard rectangular waveguide WR-90 has an inner width of cm in. and an inner height of cm in. . In waveguides the electric and magnetic fields are confined to the space within the guides. Thus no power is lost to radiation. Since the guides are normally filled with air dielectric losses are negligible. However there is some I2R power lost to heat in the walls of the guides but this loss is usually very small. It is possible to propagate several modes of electromagnetic waves within a waveguide. The physical dimensions of a waveguide determine the cutoff frequency for each mode. If the frequency of the impressed signal is above the cutoff frequency for a given mode the electromagnetic energy can be transmitted through the guide for that particular mode with minimal attenuation. Otherwise the electromagnetic energy with a frequency below cutoff for that particular mode will be attenuated to a negligible value in a relatively short distance. This grammatical use of cutoff frequency is opposite that used for coaxial cable where cutoff frequency is for the highest useable frequency. The dominant mode in a particular waveguide is the mode having the lowest cutoff frequency. For rectangular waveguide this .