The TWISTED PAIR transmission line is illustrated in figure 3-4. As the name implies, the line
consists of two insulated wires twisted together to form a flexible line without the use of spacers. It is not
used for transmitting high frequency because of the high dielectric losses that occur in the rubber
insulation. When the line is wet, the losses increase greatly.
Figure 3-4.Twisted pair.
The SHIELDED PAIR, shown in figure 3-5, consists of parallel conductors separated from each
other and surrounded by a solid dielectric. The conductors are contained within a braided copper tubing
that acts as an electrical shield. The assembly is covered with a rubber or flexible composition coating
that protects the line from moisture and mechanical damage. Outwardly, it looks much like the power
cord of a washing machine or refrigerator.
Figure 3-5.Shielded pair.
The principal advantage of the shielded pair is that the conductors are balanced to ground; that is, the
capacitance between the wires is uniform throughout the length of the line. This balance is due to the
uniform spacing of the grounded shield that surrounds the wires along their entire length. The braided
copper shield isolates the conductors from stray magnetic fields.
There are two types of COAXIAL LINES, RIGID (AIR) COAXIAL LINE and FLEXIBLE (SOLID)
COAXIAL LINE. The physical construction of both types is basically the same; that is, each contains two