3-6
Figure 3-7.—Flexible coaxial line.
Because of the high-frequency losses associated with rubber insulators, polyethylene plastic was
developed to replace rubber and eliminate these losses. Polyethylene plastic is a solid substance that
remains flexible over a wide range of temperatures. It is unaffected by seawater, gasoline, oil, and most
other liquids that may be found aboard ship. The use of polyethylene as an insulator results in greater
high-frequency losses than the use of air as an insulator. However, these losses are still lower than the
losses associated with most other solid dielectric materials.
Waveguides
The WAVEGUIDE is classified as a transmission line. However, the method by which it transmits
energy down its length differs from the conventional methods. Waveguides are cylindrical, elliptical, or
rectangular (cylindrical and rectangular shapes are shown in figure 3-8). The rectangular waveguide is
used more frequently than the cylindrical waveguide.
Figure 3-8.—Waveguides.
The term waveguide can be applied to all types of transmission lines in the sense that they are all
used to guide energy from one point to another. However, usage has generally limited the term to mean a
hollow metal tube or a dielectric transmission line. In this chapter, we use the term waveguide only to
mean "hollow metal tube." It is interesting to note that the transmission of electromagnetic energy along a
waveguide travels at a velocity somewhat slower than electromagnetic energy traveling through free
space.
A waveguide may be classified according to its cross section (rectangular, elliptical, or circular), or
according to the material used in its construction (metallic or dielectric). Dielectric waveguides are
