Practical_Antenna_Handbook_0071639586

112 P a r t I I : F u n d a m e n t a l s
Outer
conductor
Coaxial Lines
The second form of transmission line commonly used at sub-microwave frequencies is
coaxial cable (Figs. 4.1F through 4.1L), often abbreviated as coax (pronounced “co-ax”).
This form of line consists of two cylindrical conductors sharing a common axis (hence
“coaxial”), and separated by a dielectric (Fig. 4.1F). Of necessity, the outer conducting
cylinder is hollow, but the inner one is usually solid (although that is not a requirement).
For low frequencies (in flexible cables) the dielectric may be polyethylene or
polyethylene foam, but at higher frequencies Teflon and other specialized materials are
common. In most of the inexpensive cables on the market, the dielectric completely fills
the space between the two conductors over the entire length of the cable. But for extremely
low loss requirements, circular discs are spaced along the inside of the line to
keep the inner conductor centered with respect to the outer conductor, and the line is
often filled with dry air or dry nitrogen and kept slightly pressurized.
Several additional variations in coaxial line construction and characteristics are
available. Flexible coaxial cable, characterized by the RG-8, RG-58, RG-59, RG-213, et
al., families, is perhaps the most common form. The outer conductor in such cable is
made of either braid or foil (Fig. 4.1G). Cable and satellite TV system home installations
are wired with coaxial cables, which should never be confused with audio cables. Another
form of flexible or semiflexible coaxial line is helical line (Fig. 4.1H), in which the
outer conductor is spiral wound. Hardline (Fig. 4.1I) is coaxial cable that uses a thin-wall
aluminum tube as the outer conductor; it is ubiquitous in the cable television industry’s
outdoor distribution systems, often seen midway up the utility poles in our neighborhoods
and as drop cables to our homes. Some hardline used at microwave frequencies
has a rigid outer conductor and a solid dielectric.
Gas-filled line is a special case of hardline that is
hollow (Fig. 4.1J); the center conductor is supported
by a series of thin ceramic or Teflon insulators. The
dielectric is usually anhydrous (i.e., dry) nitrogen or
some other inert gas.
Some flexible microwave coaxial cable uses a
solid “air-articulated” dielectric (Fig. 4.1K), in which
the inner insulator is not continuous around the center
conductor but, rather, is ridged. Reduced dielec-
Dielectric
Inner
conductor
Figure 4.1F Coaxial cable (end view).
Outer
insulator
Inner
conductor
Insulating
sheath
Dielectric
Braid
Inner
insulator
Shield
Inner
conductor
Figure 4.1g Coaxial cable (side view).
Figure 4.1H Helical coaxial cable.