attenuation (ATT)

Attenuation is the reduction of the transmitted signal energy when passing through a quadripole or transmission path. Since every transmission path, like every transmission medium, is subject to frequency-independent and frequency-dependent losses, it attenuates the signal to be transmitted to varying degrees in its frequency components.

1. Attenuation is expressed as the logarithmic ratio of input to output power of any quadripole, which can be a cable, in units of decibels( dB) or sometimes nepers( Np).
   

   Attenuation of an input signal

   For voltages (U1 and U2) one defines `U(dB) = 20 log *(U1)/(U2)`, for powers (P1/P2) one defines `P(dB) = 10 log * (P1)/(P2), where U1 or P1 is measured at the input and U2 or P2 at the output of the cable or quadripole.
   

   Calculation of power and voltage related dB values

   The attenuation of a cable has different causes and is influenced by different parameters. The attenuation of a metallic cable depends, for example, on its length, the material and the design. It is frequency-dependent due to the capacitive coupling of the two cores and their inductance; this means that higher frequencies are attenuated more than lower ones.
2. In optical fibers, attenuation is the energy loss of the light beam that occurs as it passes through the fiber in the form of scattering and absorption. Attenuation depends on the wavelength used. Favorable wavelengths for fused silica are 850 nm, 1,300 nm and 1,550 nm. The attenuation is specified per kilometer and is referred to as the attenuation coating. Typical attenuation values are about 3 dB/km for 850 nm wavelength. For comparison, coaxial cables have an attenuation of about 17 dB/km at frequencies up to 1 GHz. At higher frequencies, this increases rapidly.
3. For radio waves and in acoustics, we speak of propagation or free- space attenuation. For radio waves, attenuation is the reduction in field strength due to absorption and reflection. Certain physical obstacles, such as walls, attenuate microwaves very strongly so that they can no longer be received. Propagation attenuation also depends on the topography of the terrain and weather conditions. The way propagation attenuation affects satellite TV reception can be seen when there is heavy cloud cover, rain or snowfall.In acoustics, propagation attenuation is the decrease insound energy from the sound source to the listeners or recording equipment. Sound energy decreases by 6 dB for each doubling of distance. In addition, propagation attenuation is frequency dependent and increases with humidity and air temperature.
4. Another significant form of attenuation is caused by electromagnetic interactions of a metallic conductor with its environment, e.g. neighboring conductors. This is referred to as crosstalk (for the cause) and crosstalk attenuation (for the effect). The practical value of a conductor is not only determined by its theoretical attenuation, but by the combination of signal and crosstalk attenuation.