Information Technology Reference
In-Depth Information
8/U, RG-11/U, RG-58A/U, RG-59/U, and RG-62/U. The outer jacket of RG-8/U
and RG-11/U is roughly 1/2 inch (12.7 mm), while RG-58A/U, 59/U, and 62/U are
on the order of 1/4 inch (6.4 mm). Plenum versions of these cables are usually a lit-
tle smaller. The wire sizes of the center conductor and the diameter of the outer
jacket (including dielectric and shield) are defined by the RG number and the insu-
lation type. You must specify whether you want connectors for PVC or plenum insu-
lation types, since the cable dimensions are not the same.
Coax Electrical Characteristics
Coax cables are available in a variety of standard characteristic impedances.
Because the primary use for coax is the transmission of RF signals, the coax imped-
ances often reflect the needs of RF equipment.
These standard impedances are 50 ohms, 75 ohms, and 92 ohms. The dimen-
sions and properties of the center conductor, dielectric, and shield combine to
define the coax cable's characteristic impedance. The dielectric constant in flame-
resistant cable is very different from that of conventional polyethylene or foam
dielectric cable. It is for this reason that plenum cable typically has a smaller cable
diameter.
The attenuation of coax cable is less than 1.5 dB per 100 ft at 10 MHz. At 100
MHz, the typical attenuation is under 5 dB per 100 ft. Obviously, the useful coax
cable length is reduced as the frequency goes up. Some types of networks base their
maximum coax cable runs on the total allowable loss from end to end (the loss
budget), while other types of networks have timing constraints that limit the usable
distance.
As with other wiring types, the capacitance of the cable is the primary contrib-
utor to attenuation at higher frequencies. Even so, coax has a greater maximum
usable length than twisted pair. For example, a 10 Mbps Ethernet segment can be
500 m (1640.5 ft) using thicknet and 185 m (607 ft) using thinnet. (Note: Segment
lengths in Ethernet are limited by attenuation and frequency-based signal distortion,
while network span is limited by timing constraints.) ARCnet, which runs at 2.5
Mbps, allows 2000 ft (610 m) for each coax leg with an active hub. For compari-
son, twisted-pair wiring is limited to 100 m (328 ft).
The standard coax dielectrics are polyethylene and PTFE or FEP, although sev-
eral other materials are also used. The loss characteristics of the dielectric material
contribute to the loss at any given frequency.
Low-loss dielectric materials and constructions, such as foam or helical
dielectrics, are available. However, those low-loss cables are cost-prohibitive for
normal LAN cable installations.
