The coaxial cable or coaxial line is a line of transmission or asymmetrical connection, used in high frequencies, composed of a cable with two conductors. The central core, which can be mono-strand or multistranded (out of copper or plated copper, even out of coppered steel), is surrounded by a dielectric material (insulator). The dielectric one is surrounded by a conducting braid (or aluminum foil rolled up), called shielding, then of a insulating sleeve and protective.
This type of cable is used for the transmission of analog numeric signals or to high or low frequency. The invention is allotted to American Hermann Affel (1893-1972) whose patent is accepted on December 8th, 1931.
For example, it is possible to find a coaxial cable:
between an antenna TV (rake TNT or satellite parabola) and a television receiver
in the urban cable network
between a transmitter and the sending antenna, for example an electronic card Wi-Fi and its antenna
between equipment of processing of the sound (microphonous, amplifying, CD player…)
in the transmissions networks of data such as Ethernet in its old versions: 10BASE2 and 10BASE5
for the telephone interurban connections and in the underwater cables.
for the transport of a video signal, telegraphic camera example off-set, at significant distances > tens of measure
The coaxial cable is now replaced by optical fiber on the long distances (higher than a few kilometers).
The advantage of a coaxial cable on a two-wire line (composed of two parallel conductors separated by dielectric) is that there is creation of a screen (Faraday screen room) which protects the signal from the electromagnetic interferences and which prevents that the conductors do not produce themselves of the disturbances. A coaxial cable can be placed along the walls, gutters or hidden because the presence of objects does not influence the propagation of the signal in the line. The losses are constant with the wire of time, the particles of dust settling on the insulating support not having an influence on the propagation of the signal.
It is sometimes necessary to place, between the output of the antenna (symmetrical) and the coaxial line (asymmetrical) a balun (BALanced/UNbalanced, symmetrical ⁄ asymmetrical converter) to optimize the transfer of the energy between the antenna and the cable (in reception as in emission).
It is preferable not to use of damaged cable because its characteristics and its properties are then degraded and the waves could overflow in your neighbor.
Connection to a coaxial cable must be realized by the use of coaxial connectors adapted to the cable and assembled while respecting the indications provided to preserve at the unit the characteristics desired in terms of quality of transmission. For the Digital terrestrial TV, cards known as IEC are indicated, whereas for the TV by satellite are the cards F " to screw, although they are assembled on the same cable general public
Mechanical characteristics of the coaxial cable:
the nature of the conductor and its dimensions
the diameters interior of the sheath and outside of the central conductor (this one is sometimes hollow)
the nature of the dielectric one.
Electric characteristics of the coaxial cable:
its characteristic impedance Zc, standardized with 75 ohms for TV (SAT & TNT), FM radio station, it video or the audio, and with 50 ohms for the instrumentation or the connection of Wi-Fi antennas, the ultra high frequencies and the old Ethernet networks
its attenuation constant has at a given frequency, which translates the losses in the line.
They are given by the manufacturers.
It should be recalled that the currents high frequency circulate in a film close to the surface of the conductors. The thickness of this film decreases when the frequency increases. The resistance of a conductor increases like the square root of the frequency, it is what is called the skin effect. The losses produce a reduction in the amplitude of the signal in end-of-line, that appears for example by a reduction in power RF radiated in the case of a transmitter. Here some rules:
The smaller the diameter of the conductor is, larger will be its resistance and thus more there will be losses.
The more the frequency increases, the more there will be losses.
The more one increases the length of the cable, the more there will be losses.
19 vatc = loss of 19 db ⁄ 100 meters at a frequency of reference of 800 MHz.
17 vatc = loss of 17 db ⁄ 100 meters at a frequency of reference of 800 MHz
In satellite reception (B.I.S 950/2150 MHz) the cable 17 Vatc or Patc is recommended, like for the reception (< 860 MHz) of the Numerical Terrestrial TV (TNT) delicate. The losses in satellite mode at the maximum frequency of 2150 MHz, oscillate around 31 dB/100 m
Moreover, there exists an optimum report ⁄ ratio of the diameter of the core on that of the shielding. This one corresponds to an impedance characteristic of 75 Ohm, which explains why this value is employed for the cables of reception which must minimize the losses, all things being equal in addition.
For the transport of power, one would tend to think that to maximize the diameter of the core decreases resistance and thus the losses. This is true uninterrupted, but in high frequency, the reduced thickness of dielectric involves a weaker tension of breakdown, and thus a limited acceptable maximum power. The optimum is carried out for a characteristic impedance of about 30 Ohm. The value of 50 Ohm corresponds to a compromise between losses in emission and losses in reception.
Characteristics of the principal references of coaxial cables