There is a variety of different
types of coax or coaxial cable that are in widespread use. Different
types of coax cable or feeder are needed for
different purposes and applications and accordingly it is necessary
have specifications and data to be able to determine the required type
or RF
cable easily.While it would be possible to manufacture an infinite
variety of RF cables, standard varieties are specified. There are two
basic systems that
are used for defining RF cables. One originated in the United Kingdom
and its type numbers all start with UR. The other system is American
with type
numbers commencing with the letters RG. The RG series was originally
used to specify the types of coax cables for military use, and the
specification took the form RG (RG from Radio Guide) plus two numbers.
In some instances these numbers were followed by the letter U which
indicated it was for multiple uses. These types of coax cable were all
listed in the MIL-HDBK-216 which is now obsolete. Although full MIL
specifications are now officially used for specifying most components
for military use, the RG series of RF cables continued to be used
because of its widespread acceptance. However it should be noted that
the RG specifications are no longer maintained so there is no complete
guarantee to the exact specification for the particular type of coaxial
cable.
Coaxial cable is an electrical cable
consisting of a round, insulated conducting wire, surrounded by an
insulating spacer, surrounded by a cylindrical conducting sheath,
usually surrounded by a final insulating layer.
The cable is designed to carry a
high-frequency or broadband signal, usually at radio frequencies.
Sometimes DC power (called bias) is added to the signal to supply the
equipment at the other end, such in direct broadcast satellite
receivers. Because the electromagnetic field carrying the signal
exists (ideally) only in the space between the inner and outer
conductors, it cannot interfere with or suffer interference from
external electromagnetic fields.
Coaxial cables may be rigid or flexible. Rigid types have a solid
sheath, while flexible types have a braided sheath, both usually of
copper. The inner insulator, also called the dielectric, has a
significant effect on the cable's properties, such as its
characteristic impedance and its attenuation. The dielectric may be
solid or perforated with air spaces. Coaxial cables are usually
terminated with RF connectors.
