Page:Electronics Technician - Volume 7 - Antennas and Wave Propagation - NAVEDTRA 14092.pdf/69

 Figure 3-19.—Two-wire transmission line.

Figure 3-20.—Quarter-wave section of transmission line shorted at one end.

Note that quarter-wave sections are insulators at only one frequency. This severely limits the bandwidth, efficiency, and application of this type of two-wire line.

Figure 3-21 shows several metallic insulators on each side of a two-wire transmission line. As more insulators are added, each section makes contact with the next, and a rectangular waveguide is formed. The lines become part of the walls of the waveguide, as illustrated in figure 3-22. The energy is then conducted within the hollow waveguide instead of along the two-wire transmission line.

The comparison of the way electromagnetic fields work on a transmission line and in a waveguide is not exact. During the change from a two-wire line to a waveguide, the electromagnetic field configurations also undergo many changes. As a result of these changes, the waveguide does not actually operate

Figure 3-21.—Metallic insulator on each side of a two-wire line.

Figure 3-22.—Forming a waveguide by adding quarter-wave sections.

like a two-wire line that is completely shunted by quarter-wave sections. If it did, the use of a wave-guide would be limited to a single-frequency wave length that was four times the length of the quarter-wave sections. In fact, waves of this length cannot pass efficiently through waveguides. Only a small range of frequencies of somewhat shorter wavelength (higher frequency) can pass efficiently.

3-11