Integracja rozwiązań dla instalacji telekomunikacyjnych i operatorów kablowych

The basis of any RTV/SAT installation is the antenna and coaxial cable connecting it with the receiver.  Every additional element introduced to such an installation (connectors, active/passive elements) can only cause further degradation of the transmitted signal.   That's why coaxial cables, right after the receiving antenna, are the most important elements of RTV/SAT installations. Even the best receiving antenna will not fulfill its task if the installation is based on a low-quality cable. Add to it an inaccurate installation, problems with proper reception of TV signals are almost certain.

Coaxial cables are built from several elements that determine their properties and quality.  Usually the information about them is included in the specification or directly on the cable. However, we do not always have the opportunity to check them.  To make matters worse, this information is not always reliable!

So how do you check the quality of a cable?

All it takes is basic knowledge and the use of our senses - below we suggest 7 practical ways.

1. Verification of cable intended use. 

The outer jacket is made of a plastic layer to protect the conductor from moisture. This outer insulation not only provides resistance to external conditions and mechanical damage - the material used for the outer jacket determines the properties of the cable.

The most commonly used are:

• PVC - for indoor use;
• PE - for outdoor use. In addition, the external cables have a layer protecting them from weather conditions and UV radiation - they are gelled. By removing a part of the outer jacket, we can see if there is a jelly-like sticky substance underneath, protecting the inside of the cable from moisture.

2. Checking the strength of the outer jacket

To find out, just try removing the jacket without using any tool. If the PVC/PE jacket is loose and easily slides off the braid or film layer, and at the same time elongates and stretches, it does not guarantee adequate strength of the cable.

3. Screen quality control

The shield is usually an aluminum or copper film wrapped over a dielectric. Its task is to isolate the signal flowing inside the cable from external electromagnetic waves that would cause interference.  The screen also does not allow the signal to penetrate to the outside so as not to lead to signal attenuation. For both functions to be fulfilled, the film covering the dielectric or braid should be "overlapping" wrapped. Additionally, the film wrapping the dielectric should be glued to it.  This is easily checked by removing part of the jacket and exposing the screen. If the film directly covering the dielectric is loose, shifts, it means it has not been glued. There is therefore a danger that when laying the cable, when we have to bend it properly, the film will expose the dielectric and there will be an unprotected "gap" through which signals will penetrate.

4. Checking the density of the braid

The braid is usually a copper or aluminum mesh made of wires with a diameter of about 0.16 mm. The higher the braid density (more wires), the higher the electromagnetic integrity of the cable.  It is another barrier (after the screen) that protects the transmitted signal from external interference.
It is difficult to judge the density of the braid at first glance. To do this, it is worth noting the angle at which the wires intertwine. If they are laid loosely, more along the cable, then we are dealing with a thin braid. If, on the other hand, the wires form a dense braid and are arranged more across the cable, the density of braid coverage is high. Thus, our signal will be better protected.

5.Checking the dielectric

A dielectric is a plastic layer, usually made of polyisobutylene (PIB). It is important that the material is physically foamed.  Chemical foaming does not give permanent results - after a few years the dielectric will become brittle. The brittleness of the dielectric maximizes the risk of displacement of the core relative to the film (first shield stage), which can result in a loss of cable concentricity and cause a change in cable impedance. Unfortunately, the type of foaming is difficult to check without having the right tools. Instead, we can verify that the dielectric is tight and adheres well to both the inner conductor (core) and the film (shield). This is critical when bending the cable and determines its allowable bend radius.

6. Verification of electromagnetic integrity / cable class

The class of the cable is indicated by the degree of its electromagnetic integrity. We distinguish between several classes of coaxial cables: B, A, A+, A++. The higher the class, the higher the cable integrity  (leaktightness) and resistance to external interference.  Greater integrity is achieved, among other things, by selecting the type and number of screens and braids used. We distinguish cables:

• TWINSHIELD - have two layers inside: film + braid;
• TRISHIELD – have three layers inside: film + braid + film;
• QUADSHIELD – have four layers inside: braid + film + braid + film.

Of course, the greater the number of layers, the higher the electromagnetic integrity and the better the cable quality. To see what kind of cable we are dealing with, it is enough to "peel" it layer by layer, reaching the dielectric.

7. Checking the type and thickness of the inner conductor (core)

The type of inner conductor used and its thickness have a significant effect on the cable's signal attenuation. Attenuation determines how much the transmitted signal will weaken as the distance increases.  The value of attenuation of coaxial cables is usually given per 100 running meters of cable.  Low attenuation cables maintain signal quality along the entire length of the cable, all the way to the receiving device.

The core of a coaxial cable can be made of copper (Cu) or copper-clad steel (CCS).  Copper has better conductive properties and offers less resistance to electrical charges. Therefore, it is more commonly used in various types of power cables and conductors. In turn, the diameter of the conductor increases the active conductive surface area and decreases the resistance to these charges.

If the core of the cable is soft, and after slight scraping off or cutting it off it basically does not change colour, then we can be sure that we are dealing with a copper conductor. But if the core is harder, and after scraping off the top layer of copper the silvery color of steel clearly emerges, then we have a cable with copper-clad conductor.

So remember - if:

• the thickness of the inner conductor exceeds 1mm (e.g. 1.02; 1.13);
• the film, which acts as a screen, is bonded to a physically foamed dielectric;
• it is a TRISHIELD or QUADSHIELD cable, i.e. with triple or quadruple insulation;
• braid has a density of min. 67% (densely arranged wires with a thickness of min. 0.16mm);
• the type of sheath is adapted to the purpose of the cable (gel-filled for outdoor use);
• maximum attenuation at limiting frequencies of 862MHz and 2150MHz is respectively less than: 21dB and 32dB (parameter for 100mb of cable).

then we can be sure that we are dealing with a good quality cable. We can rely on such cables when designing and building SMATV/SATV systems that have to work reliably for many years.