GOOD2USE Knowledge Network Line Attenuation in a Coax Cable

GOOD2USE Knowledge Network Line Attenuation in a Coax Cable

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The Attenuation Function of a Coaxial Cable of length l is given as follows:

α_K(f) = (α₀ + α₁ * f + α₂ * f - √f) * l.

It is important to note the difference between α_K(f) in dB and the "alpha" coefficient with other pseudo–units. The attenuation function α_K(f) is directly proportional to the cable length l, α_K(f) =

α_K(f) / l

is referred to as the 'attenuation factor' or 'kilometric attenuation'. The frequency-independent component α₀ of the attenuation factor takes into account the Ohmic losses.
The frequency proportional portion, α₁ * f of the attenuation factor is due to the derivation losses (crosswise loss) and the dominant portion α₂ is the result of the skin effect, which causes a lower current density inside the conductor compared to its surface. As a result, the resistance of an electric line increases with the square root of the frequency.

The constants for the standard coaxial cable with a 2.6 mm inner diameter and a 9.5 mm outer diameter, the short Coax (2.6/9.5 mm) are:

α₀ = 0.014dB/km, α₁ = 0.0038dB/ (km * MHz) and α₂ =2.36dB/(km * √MHz)

The same applies to the small coaxial cable - short Coax (1.2/4.4 mm):

α₀ = 0.068dB/km, α₁ = 0.0039dB/ (km * MHz) and α₂ =5.2dB/(km * √MHz)

These values can be calculated from the cables' geometric dimensions and are valid for a temperature of 20° C (293 K) and frequencies greater than 200 kHz.

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