2. History:
The real discovery and its actual use dates back to 1929 due to the need for a
more efficient and with less interferences conductor for the transmissions of
many telephone channels on a single "carrier".
Perhaps none of us have never thought that the development of radio
communications would never have been possible without the invention of
coaxial cable, it is continuously used in our applications that we don’t think
about that.
3. Introduction:
Coaxial cables are the most common, basic transmission lines.
They are used to transmit electrical energy, or signals, from one location to
another: to connect a source to a load, such as a transmitter to an antenna.
A coax cable consists of two conductors separated by a dielectric material.
The center conductor and the outer conductor, are configured in such a way
that they form concentric cylinders with a common axis. Hence the term and
name co-axial.
5. Construction
The center conductor may be made of various materials and constructions.
Most common constructions are solid or seven-strand conductors.
Solid conductors are used in permanent and infrequently handled
applications.
stranded conductors are used in flexible cable applications.
Common materials include copper, tinned or silver plated copper, copper clad
steel and copper clad aluminum.
Plated copper is used to aid in solder ability of connectors or to minimize
corrosion effects.
6. Data is transmitted through the center wire.
The outer braided layer serves as a line to ground.
Both of these conductors are parallel and share the same axis. That’s is why
the wire is called coaxial.
7. Because of a phenomena known as skin-effect, copper clad materials may be
used in higher frequency applications ( > 50 MHz) to improve tensile strength
and reduce weight and cost. Skin-effect is the result of higher frequency
signals propagating along the outermost surface, or skin, of the conductor.
8. The insulation, or dielectric material, is used to provide separation between
the conductors. It is desirable that the material has stable electrical
characteristics across a broad frequency range.
9. Temperature range of the cable is often limited by the choice of jacket
material. The jacket material serves as a protective covering from the
environment.
10. Categories of Coaxial Cable:
Coaxial cables are categorize by Radio Guide (RG).
Each RG number denotes a unique set of specifications, including:
The gauge of the inner conductor.
The thickness and the type of the inner insulator.
The construction of shield.
The size and type of outer casing.
12. Types of Coaxial Cable:
Flexible coaxial cable:
Flexible coaxial cable is used where the demand is to fit the cable into a tight
space without performance loss.
13. Semi Rigid Coaxial cable:
Semi rigid cable is using a solid copper outer sheath. This type of coax offers
superior transmission, especially at higher frequencies.
The major disadvantage of this type is that the cable, as its name implies, is
not very flexible.
14. Hand Formable Coaxial Cable:
These cables are between flexible and semi rigid cables. These cables are
good choices when forming at a point of installation is required.
15. Twin-axial cable
Twin-axial cable is a balanced, twisted pair within a cylindrical shield. It
allows a perfect signal which is shielded and balanced to pass through.
16. Triaxial cable:
Triaxial cable is a coaxial cable with a third layer of shielding, insulation and
sheathing. The outer shield is grounded and protects the inner shield from
interference from outside sources
17. Connectors:
The ends of coaxial cables usually terminate with connectors. Coaxial
connectors are designed to maintain a coaxial form across the connection and
have the same impedance as the attached cable.
Connectors are usually plated with high conductivity metals such as silver or
gold.
In the case of computer networks, BNC (Bayonet Niell- Concelman) RF
connectors are used.
18. Electrical Properties:
The most common electrical property of coaxial cable is the characteristic
impedance. Impedance is the total opposition to the flow of electrical energy
within the cable. It is a complex value defined by the cable’s resistance and
capacitance. It is expressed in Ohms. A simple formula to determine the
impedance of a coaxial cable is:
Impedance= 138*Vp*log10(D/d)
Vp= Velocity of Propagation(is the speed at which a signal travels through the
cable with respect to the speed of light.)
D = Diameter of the Dielectric
d = Diameter of the Conductor
19. Capacitance
Capacitance is the ability of the cable to hold a charge. The larger the
capacitance value, the longer it takes a signal to reach full amplitude within
the cable. Therefore, higher capacitance is usually a bad attribute.
20. Physical Properties:
Temperature Rating:
Temperature Ratings provides the limitations on temperature extremes that
the cable material can handle. The safe range is based upon the thermal
properties of the dielectric and jacket materials that assures that the product
will not fracture or melt.
Pulling Tension:
It is typically a safe value well below the break strength of the cable. Staying
below this maximum assures that the conductor will not be stretched in the
cable.
21. Attenuation
Reduction of signal strength during transmission. It is dependent upon the
cable design and is both frequency and length dependent. It increases with
the increase in frequency.
It is most effected by DC resistance of the center conductor and dissipation
factor of the dielectric material.
Attenuation is measured in decibels.
22. Repeaters
Repeaters are used in coaxial cables to reduce attenuation.
A repeater can couple together two 500m segments of coaxial cable. The
repeater occupies one transceiver on each segment and must pass all the
signals from one cable to other.
23. Reflection losses
Reflection losses are based upon signals reflecting back to the source rather
than propagating through the cable.
These reflections are caused by impedance mismatches or variations in the
cable.
Randomly spaced throughout the cable, these mismatches will cause minimal
loss, but when spaced periodically, that is at the same repeat distance, they
add up together causing a large loss corresponding to that period wavelength.
This loss can be minimized by quality cable manufacturing techniques and
proper installation practices.
24. Cross Talk
Crosstalk refers to the amount of coupling between adjacent wire pairs,
which occurs when a wire absorbs signals from adjacent wires. Crosstalk is
measured by injecting a signal into one pair and then measuring the strength
of that signal on each of the other pairs in the cable.
25. Applications
Long distance coaxial cable was used to connect radio networks and
television networks.
It is used by consumers to connect television receivers to external antennas.
Short coaxial cables are also employed to connect home video equipment and
radio systems.
Micro coaxial cables are used in a range of consumer devices, military
equipment, and also in ultra-sound scanning equipment.
Coaxial cable are also widely used in local area networks(LANs).
26. Benefit
The main benefit of using a coaxial cable is that it is the default cable of its
type. This means that most electronics is already compatible with coaxial.
Other benefits of the coaxial cable include its ability to shield television from
outside interference. This can maximize picture quality and help to avoid
static. It is inexpensive also.
27. Advantages of Coaxial Cable
They are most common means of data transmission over short distances.
They are cheap to make
Cheap to install
Easy to modify
Good bandwidth
Great channel capacity
28. Disadvantages of Coaxial Cable
Signals entering the cables can cause unwanted noise and picture ghosting,
making it useless.
A continuous current flow, even if small, along the imperfect shield of a
coaxial cable can cause visible and audible interference.
More expensive than twisted pairs and is not supported for some network
standards.
It is also has high attenuation, have the need to implement repeaters.
