COAXIAL CABLES STRUCTURE

The coaxial cable forms a transmission line through which a signal, intercepted by the receiving antenna, is transported to the television set.

The crossing signal undergoes an attenuation proportional to the work frequency and to the length of the cable itself.

The technology utilized in the construction of coaxial cables strains to curb attenuation as much as possible trying, in the meantime, to optimize their physical-mechanical characteristics and product life.

For a system to continuously operate in top condition, the cable must :

• curb attenuation;
• protect the signal from external interference;
• resist mechanical stress exposed to especially in the installation phase;
• be indifferent, as much as possible, to agents in the atmosphere;
• maintain its characteristics as time passes.
       

The aim of the inner conductor is to transport the antenna’s signal to the televison.
 
The inner conductor, or rather, its surface, is responsible for over 80% of a coaxial cable’s attenuation level. Radiofrequency, in fact, runs along the outer layer of the wire, for the so-called skin effect.
The bigger the section of wire, the bigger the surface and the lesser the attenuation.
 
Today the inner conductor is made with bare copper wire which is electrolytic copper with a purity level over than 99.9%.
 
Since the skin-foam-skin technology is used for insulation, the outer surface of the conductor is protected by an extremely thin layer of natural polyethylene. Therefore, the inner condutor of bare copper is secured from the oxidation, which was a property of the previously used tin-plated or even silver-plated copper.
 
To guarantee the quality of its cables, CAVEL Italiana Conduttori’s inner conductors are developed with their own wire-drawing system.
     

The dielectric is the layer of material placed around the inner conductor and serves:

• to keep the conductor exactly concentrical with respect to the screen
• to protect the conductor from atmospheric agents.

Previously, the expansion of the dielectric was obtained by means of chemical additives. Today, productive technology uses the gas injected system which causes expansion of the dielectric by means of physical injections of nitrogen gas into the polyethylene. In addition, skin-foam-skin technology is employed: the first skin insulates the inner conductor, the expanded section gives way for the electromagnetic signal to quickly spread out, and subsequently the second skin seals the insulation. 
 
With this type of expansion, in fact, the best mechanical, electrical and durability features are obtained.
 
The gas injected dielectric also ensures a better life span and stable attenuation values. In accordance with IEC 68-2-3 requirements which prescribes an ageing test of 21 days at 40°C and relative humidity of 93%, checks have shown that attenuation increases less than 5% over original values.
 
The graph shows the attenuation increase of two equivalent cables, one with chemical foam dielectric and the other with physical foam dielectric. The attenuation differencies have been tested at 1750 Mhz.

The screen functions is to protect the antenna’s signal running along the inner conductor from external interference as well as to avoid the radiation of electromagnetic signals outward.

The great amount of electromagnetic signals that crowds the atmosphere today makes it necessary to use only cables with a screen that guarantees a high immunity to interference, or in other words, a screening effectiveness of at least 75 dB.
A screen level of this value is obtained only with a double screen, first made up of a metalic film foil laminate and then of a copper wire braid.

The screens of standard drop coaxials are generally done with an aluminum film foil and a tin-plated copper braid.
Bare copper foil, on the other hand, is used more for distribution cables, often installed outdoors, underground or generally in environments that are more hostile than indoors.

To install cables in locations that are especially polluted by strong interference signals, there is a range of coaxial cables available that guarantees a screen efficiency of more than 90 dB.
This result is still obtained with a metallic film foil laminate but with higher optical coverage braids, actually formed by a greater number of wires in the braid.

DG series cables should be used for all applications of digital transmissions which are particularly sensitive and subjected to electromagnetic interference.
   

The sheath protects the coaxial cable from the external environment.

The thermoplastic compound most widely used to develop the protective sheath of a coaxial cable is PVC, for its flexibility, durability and relatively low cost.

This material corresponds to the applicative specs for indoor installment even if, in practice, the quality of PVC used nowadays, both black and white, is such to render it resistent to UV rays and thus usable even for outdoor installment.

Good policy is, instead, to use cables with sheathing in black PE (containing black carbon to better resist UV rays) for outdoor installation that, in addition, has a better degree of impermeability respect to PVC.

Suitable cables for underground installation are further provided with a petrol-jelly flooding compound filling, placed between the screen and the sheath of black polyethylene. We suggest that they are installed in any environment where the presence of water and/or a high ratio of moisture are likely.

In all cases in which there is the danger of fire, PVC, although a self-extinguishing material, is not used because it emits corrosive and toxic gasses, which are dangerous when inhaled. Similarly, PE isn’t used since it burns easily.

We suggest that all installations in public areas to be done with cables provided with a security sheath in case of fire, which is the cable Series ZH.

Such a sheath is flame retardant, low smoke, zero halogen and UV resistant.