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Coaxial cables carry a
nominal impedance of between 35 and 185 ohms. The three most common
coax cables are 50 ohms (most widely used in thin-net Ethernet), 75
ohms (the cable your most likely using at home for your TV or cable
modem) and 93 ohms, which is rarely used.
Coax cable can
support much higher bandwidths than unshielded twisted pair (UTP)
cable. The most efficient transfer of energy, over coax, occurs when
all parts of the system have the same impedance. For example, a
transmitter, interconnecting cable, and receiver should all have
matching impedance. This need for impedance matching is especially
critical at higher frequencies, where the consequences of mismatches
are more severe.
Knowing how to
install and terminate coax cable properly is critical to achieving
impedance matching. Knowledge and selection of the best available
materials greatly increases efficiency.
The most common
type of coax, called Flexible Coax, is a flexible cable, which uses
a braided shield of extremely fine wires. This braid helps to make
the coax flexible, but at a cost: energy or RF (radio frequency)
signals leak through the small gaps in the braid. To combat this
attenuation (energy loss), manufacturers have added several layers
of braid and placed thin foil between the layers. This provides
better coverage for greater shielding effectiveness. We normally use
a quad shield (two layers of braid, two of foil) for 75-ohm
applications.
Even though coax
makes up a small percentage of our total installations, it is still
a critical piece of the infrastructure puzzle for our customers.
Coax has been the medium of choice for high fidelity audio,
television, satellite and broadband communications. |
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