It quantifies the loss of signal and is expressed in dB (decibels). In terms of voltage (reception) 6 dB of attenuation halve the signal, in power (transmission) the signal is halved every 3 dB.
ATTENUATION RATIO IN RECEPTION
Attenuation in a coaxial cable depends on the frequency and length of the cable itself. Higher is work frequency greater will be the attenuation. By convention the length is set to 100 meters, as shown in the following chart.
The cable attenuation is determined by:
- Diameter of the center conductor
- Quality of copper and its drawing
- Dielectric quality
The central conductor (with dielectric) is the weakest part of the cable. For this reason it must be pulled into the cable pipes, joining together the conductor, the braid and the foil . Pulling it from the sheath certainly represents the best option because it's possible to exert a force of at least 25 Kg (for example on the 5mm DIGISAT 122 Expert model).
Expresses the opposition of a conductor to the flow of electrons in alternating current, in other words it is the ratio between the voltage and the current absorbed by a cable (Ohm) in alternating current. This value is not affected by the cable length. For TV cables (as for all other system components) the standard value adopted is 75 Ohm.
The impedance in a coaxial cable is given by the distance between the central conductor, the shield and the dielectric constant of the insulator. In order to have a constant impedance it is necessary that these two conditions are maintained for the whole length of the cable. It is easy to understand that any damage suffered by the dielectric inevitably affects the impedance. In order for a system to work perfectly, the impedance must remain ''harmonized'' along the entire line.
An impedance mismatch creates reflected waves or in the worst case causes "notches", very steep attenuation peaks that lead to the uncontrolled collapse of some frequencies in the TV band.
Attenuation peak caused by an impedance mismatch.
Wave returns caused by impedance mismatch.
The most common causes of impedance mismatches in a TV system are:
- bending and crushing of the coaxial cable
- extension of the coaxial cable due to excessive traction during the passage in the cable duct
- unopened distribution lines with appropriate resistive load (resistances)
- short circuits in the wiring of the splitters (caused by the involuntary contact between some wire of the braid and the conductor in the clamping phase of the terminals)
- Improper use of the coaxial cable (for example as a hoist to lift the work equipment on the roof).
It stands for Structural Return Loss (cumulative losses of reflection). The SRL measures the intensity of the reflected waves, so greater is their attenuation, better will be the coaxial cable, because the reflected waves are very harmful. The SRL strongly depends on the mechanical imperfections inside the coaxial cable and it is no coincidence that impedence imperfections have strong consequences on the SRL.
Note how the peaks reported coincide at the same frequency on impedance and SRL diagrams.
The shielding efficiency generally indicates the ability of the shield to prevent electromagnetic interference from "contaminating" the signal inside the coaxial cable and vice versa. In the 30-3000 MHz band this is expressed in shielding attenuation (SA or Screening Attenuation) and the unit of measurement is the Decibel (dB).
At high frequencies the waves, being shorter, are more susceptible to shielding imperfections. Unlike long waves, these can penetrate into microscopic imperfections of the screen and can reach the central conductor creating disturbances on the same wave length. Generally these disturbances come from transmitting devices like Radar airports, TV broadcasters, amateur radio, mobile repeaters, WiFi, Wii, etc..
At low frequencies (5-30 MHz) it's used the transfer impedance (the unit of measurement is the milliOhm per meter). We remind that this band is particularly important for some years today because it is used for the return signals of digital TV (Return Path, video on demand, for example in the hotel the sign of consent of the guest via remote control to buy the vision of a movie).
Long waves (low frequency), charge all metal surfaces with electricity, making them behave like an antenna. This also happens in the external conductor of the coaxial cable: the electricity formed on the screen, creates disturbing signals between this and the central conductor, degrading the signals carried by the same even in different bands. Generally they are disturbances generated inside of the home network or in the immediate nearby (poorly shielded switches, refrigerator motors, elevators, petrol engines, hydraulic zone valves, street lamps, thermostats, fluorescent lamps, radio microphones, etc.). Especially for digital then, such electromagnetic pollutants could be fatal, going to drastically increase the number of errors in the data flow (BER).
Tests on screening efficiency have been performed in our Test and Measurement Laboratory using the "TRIASSIAL TUBE" method, a sophisticated instrument designed exclusively for the analysis of screening efficiency on conductors and junction elements, able to provide accurate and reliable data according to EN50289.
It quantifies the electrical charge accumulated between the central conductor and the screen in one meter of length.
The unit of measurement is the Farad. In coaxial cables the value is expressed in pF / m (F x 10-9). Capacitance, like impedance, depends directly on the dielectric properties of the insulator and the distance between the two external / internal conductors. The best coaxial cable is that one designed in order to have the correct impedance with the lowest possible capacitance.
VELOCITY OF PROPAGATION RATIO
It's the speed at which the signal travels inside the coaxial cable. It is a percentage of the speed of light, therefore it is expressed in "%". Higher is the value expressed and better will be the coaxial cable.