Coaxial cables conducts
electrical signal using an inner conductor mostly copper or copper
plated wire surrounded by an insulating wire and enclosed by a shield
mostly one to four layers of woven braided aluminum or copper wire, and
aluminum foil. Common applications of coaxial cables include video and CATV distribution, RF and microwave transmission and computer instrumentation and connectors.
Coaxial Cables Construction
Coaxial cable consists of a center wire
which is mostly copper wires that is surrounded by insulation, grounded
shield and a braided wire. Some of the cables consist of both braided
wire and aluminum foil. Coaxial cabling is primarily used by the cable
television industry and is also used for computer networks such as
Ethernet. Many coaxial cables have outer insulating sheath which
protects them from the external environment. The term coaxial (co-axial)
comes from the inner conductor and shield sharing a geometric axis.
Coaxial cables differ than those that carry high frequency signal and
those that carry low frequency signals such as audio signals.
Operation of Coaxial Cables
During conduction of electricity through
a conductor, say copper wire, not all the electrical energy gets to its
destination as some of the energy is lost to the surrounding in form of
heat. The energy lost is the energy used up when overcoming the
internal resistance of the copper wire. The energy conveyed through the
copper wire is done through its electrons. Electrical energy which is in
form of heat is not the only thing the copper wire emits during
conduction.
Electrons do get excited and when they
receive or absorb minimum amount of energy and when they do, they jump
from one energy level to the other. During this process, electromagnetic
radiation with a varying range of frequency, wave length is emitted.
Application of Coaxial Cables
Coaxial Cables are used as a
transmission line for radio frequency signals. It can be used as a
deadline connecting radio transmitter and receivers with their antennas,
computer network and is also used for distributing television signals.
An advantage which a coaxial cable has over other types of radio
transmission line is that in an ideal coaxial cable, the electromagnetic
field carrying electrical signals exists only in the spaces between the
inner and outer conductors. This makes it possible for the cables to be
installed close to a metallic objects such as gutter without having power losses
occurring during transmission or propagation of the signals unlike
other transmission lines that leaks the signals to the surrounding.
Signal Leakage in Cables
Signal leakage is the passage of electromagnetic fields
and or electromagnetic radiations through the shield of cables that
occurs in all directions. Signals can also pass from the surrounding of
the cable into the cable which might cause interference. This process is
called ingress. Interference causes noise. The signals propagated by
the cables can likewise leak from the cable to the surrounding. When
this happens, it leads to interference too. It is called egress. This
process reduces efficiency of the propagated signals.
Why Coaxial Cables are highly Insulated?
The electrical cables found in our homes
are insulated. Electrical cables are insulated for safety purpose and
also to minimize energy loss. The insulation
offers great resistance to leakage of energy to the surrounding. Since
the loss of energy is minimal in such cables, when there is excessive
current, it damages the cables and that is why we have fuse in
our home devices. Just as electrical cables emits electromagnetic
radiation and electrical energy in form of heat, coaxial cables can do
the same but are hindered from doing so by insulating them. For a single core cable, the resistance offered by the insulation to electrical energy is given as
Rins = ρdr / 2πrl
When integrated;
Rins [ ρ / 2πrl (loge (r1/r2))]
where ρ = resistivity of the conductor.
r1 = radius of the conductor.
r2 = radius of the insulator.
We are everyday being bombarded by
electromagnetic waves from all angles. There are lots and lots of waves
passing through our homes all the times. These waves could be radio
waves propagated from a nearby radio station, microwave, waves from our
cell phones, infrared ray and lots more.
There is a possibility that the waves
propagated by the cables will have the same frequency or wavelength
which can cause interference. Not only will the cables leak signals and
cause interference and reduce the signals’ intensity, it can also absorb
a signal that is passing close by.
With our cell phones switched on, our
radio on and our TV set also on, if the coaxial cables were not highly
insulated you hardly can hear a sound due to interference. Also notice
that when you are close to a radio and a signal comes into your cell
phone, the radio set begins to make unwanted noise due to interference
of the waves.
Coaxial cables are highly
insulated in order to prevent leakage of signals and prevent signals
from the surrounding from penetrating to the inside of the cables.
The cable do not only prevent leakage of signals but also electrical
energy seeing that the materials used in insulating the cables are
Non-conductors of heat and electricity.
The coaxial design makes the magnetic
and electric field that are propagated along the conductor to be
confined to the dielectric which serve as the insulator, giving room for
little or no leakage of signal outside the shield.
Coaxial cables are good choice when it
comes to choosing cables that can convey weak signals and cannot
tolerate interference from the surrounding or for higher electrical
signals that must not be allowed to radiate or couple into adjacent
structures or circuit.
