Coaxial cable

(For the helicopters with coaxial principal rotors, to see contrarotating Rotor)

The coaxial cable or coaxial line is a Line of transmission, used in high frequencies, composed of a cable to two conducting S. the central heart, which can be mono-bit or multistranded (in Cuivre), is surrounded by a material Diélectrique (insulator). The dielectric one is surrounded by a braided conducting sheath, called shielding, then of an envelope of Plastic, for example of PVC. This type of cable is used for the transmission of analogical numeric signals or to high or low frequency.

For example, you will find a coaxial cable:

between your antenna TV and your Television;
in the urban cable network of television and Internet;
between a transmitter and the sending antenna;
between your CD reader, your receiver FM and your amplifier;
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.

The coaxial cable is replaced by the Fiberoptic on the long distances (higher than a few kilometers).

The advantage of a coaxial cable on a Two-wire line (composed of two parallel drivers separated by dielectric) is that there is creation of a screen (Faraday screen room) which protects the signal from the electromagnetic disturbances and which prevents that the drivers 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 necessary to place, between the exit of the antenna (symmetrical) and the coaxial line (asymmetrical) a balun ( BALanced/UNbalanced , symmetrical/asymmetrical converter).

It is preferable not to use of damaged cable because your waves could overflow in your neighbor.

Characteristics

Mechanical characteristics of the coaxial cable:

the nature of the driver and its dimensions;
the diameters interior of the sheath and outside of the central driver (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 the video, and with 50 ohms for the instrumentation, the ultra high frequencies and the old networks Ethernet;
its Constant of weakening α at a given frequency, which translates the losses in the line.

They are given by the manufacturers.

Losses

It should be recalled that the currents high frequency circulate in a film close to the surface of the drivers. The thickness of this film decreases when the frequency increases. The resistance of a driver 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 at the end of the line; that appears for example by a reduction in power RF radiated in the case of a transmitter. Here some rules:

Plus the diameter of the driver is small, larger will be its resistance, and thus more there will be losses.
Plus the frequency increases, more there will be losses.
Plus one increases the length of the cable, more there will be losses.
19 vatc = loss of 19 db/100 meters at a given frequency.

Moreover, there exists an optimum report/ratio of the diameter of the heart on that of the shielding. This one corresponds to an impedance characteristic of 75 Ω, 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 heart 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 an impedance characteristic of about 30 Ω. the value of 50 Ω corresponds to a compromise between losses in emission and losses in reception.

Characteristic of the principal references of coaxial cables

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