Quiz 17: Microwave Devices and Circuits

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In electromagnetic spectrum, microwave and radio waves are adjacent to each other and there is no exact or specific distinct difference between them in terms of band width. The term micro in microwave refers very small or tiny but not it is as img . Micro waves are used for, in general, high frequency applications such as satellite communication, military applications, radio wave propagation and so on. The range of microwave can be spanned from 1 GHz to 300 GHz and the most typical operating frequency is 3 GHz.

The microwaves can be launched down the wave guide in different ways such as Transverse electric (TE) mode or Transverse magnetic (TM) mode. When the wave is launched into the wave guide, it strikes the wall with an angle. Depending on the type of mode (lower order or higher order modes of TE or TM), the angle of contact with the inner surface of the guide is varied. If the angle of contact with the inner surface of the guide increases then the wave travels more distance in the guide and takes more time to reach end point. The lowest frequency at which the wave inside the guide bounces back and forth is known as cutoff frequency. The frequency below the cutoff frequency, wave cannot propagate in the guide. From guide to guide the cutoff frequency is varied. The mode at which the cutoff frequency is the lowest one is known as dominant mode and img is the dominant for rectangular waveguide having lowest cutoff frequency than any other mode. If other than the dominant mode is preferred for a particular guide, the cutoff frequency must be almost near to multiple of lowest cutoff frequency of dominant mode. As a result below of this cutoff frequency the wave cannot be propagated through the guide. More over the energy of the signal is distributed throughout the guide that results in difficulty of extraction of power at the output end if other than the dominant mode is preferred. Therefore, for proper wave propagation with in the guide the dominant mode is the only mode that is most suitable to extract the signal energy at the output end.

The lowest frequency at which the wave inside the guide bounces back and forth is known as cutoff frequency. The frequency below the cutoff frequency, the wave cannot propagate in the guide. Therefore, for proper wave propagation with in the guide the dominant mode is the only mode that is most suitable to extract the signal energy at the output end. For rectangular wave guide img is the dominant mode. Refer to Figure 17.3 in the textbook for wave pattern of different modes. In the notation img , suffix '0' indicates number of half-cycles of the wave along the long dimension of the rectangular guide and second number '1' indicates the number of variations along the short dimension of the guide.

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