___________________________________________________ Principles of Transmission Lines
from the outer conductor. 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. Rigid (Air) Coaxial Line
3-15. 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
3-16. 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.
3-17. 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.
3-18. 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.