3-5
The rigid coaxial line consists of a central, insulated wire (inner conductor) mounted inside a tubular
outer conductor. This line is shown in figure 3-6. In some applications, the inner conductor is also tubular.
The inner conductor is insulated from the outer conductor by insulating spacers or beads at regular
intervals. The spacers are made of Pyrex, polystyrene, or some other material that has good insulating
characteristics and low dielectric losses at high frequencies.
Figure 3-6.—Air coaxial line.
The chief advantage of the rigid line is its ability to minimize radiation losses. The electric and
magnetic fields in a two-wire parallel line extend into space for relatively great distances and radiation
losses occur. However, in a coaxial line no electric or magnetic fields extend outside of the outer
conductor. The fields are confined to the space between the two conductors, resulting in a perfectly
shielded coaxial line. Another advantage is that interference from other lines is reduced.
The rigid line has the following disadvantages: (1) it is expensive to construct; (2) it must be kept dry
to prevent excessive leakage between the two conductors; and (3) although high-frequency losses are
somewhat less than in previously mentioned lines, they are still excessive enough to limit the practical
length of the line.
Leakage caused by the condensation of moisture is prevented in some rigid line applications by the
use of an inert gas, such as nitrogen, helium, or argon. It is pumped into the dielectric space of the line at
a pressure that can vary from 3 to 35 pounds per square inch. The inert gas is used to dry the line when it
is first installed and pressure is maintained to ensure that no moisture enters the line.
Flexible coaxial lines (figure 3-7) are made with an inner conductor that consists of flexible wire
insulated from the outer conductor by a solid, continuous insulating material. The outer conductor is made
of metal braid, which gives the line flexibility. Early attempts at gaining flexibility involved using rubber
insulators between the two conductors. However, the rubber insulators caused excessive losses at high
frequencies.
