Developing the Waveguide from Parallel Lines

1-8 NOTE This method of labeling waveguides is not standard in all texts. Different methods may be used in other texts on microwave principles, but this method is in accordance with Navy Military Standards (MIL-STDS). The ability of a waveguide of a given dimension to transport more than one frequency may be better understood by analyzing the actions illustrated in figure 1-10A, B, and C. A waveguide may be considered as having upper and lower quarter-wave sections and a central section which is a solid conductor called a BUS BAR. In figure 1-10A, distance mn is equal to distance pq, and both are equal to one quarter-wavelength (!/4). Figure 1-10A.—Frequency effects on a waveguide. NORMAL OPERATING FREQUENCY. NOTE Throughout NEETS, 1/4! and !/4 are both used to represent one quarter-wavelength and are used interchangeably. Also, !/2 and 3/2! will be used to represent one half-wavelength and 1 1/2 wavelengths, respectively. Distance np is the width of the bus bar. If the overall dimensions of the waveguide are held constant, the required length of the quarter-wave sections DECREASES as the frequency increases. As illustrated in figure 1-10B, this causes the width of the bus bar to INCREASE. In theory the waveguide could function at an infinite number of frequencies higher than the designed frequency; as the length of each quarter-wave section approaches zero, the bus bar continues to widen to fill the available space. However, in practice, an upper frequency limit is caused by modes of operation, which will be discussed later.