The
power loss caused by a coax cable is referred to as attenuation. It is defined
in terms of decibels per unit length, and at a given frequency. Obviously the
longer the coax cable, the greater the loss, but it is also found that the loss
is frequency dependent, broadly rising with frequency, although the actual
level of loss is not linearly dependent upon the frequency.
Attenuation of coax-cables is
described as the attenuation of the individual parts. Inner conductor,
dielectric and outer conductor attenuation form the overall attenuation of the
cable according equation [1].
αtot = α i+
α foam+ αo [1]
The individual components are
described with equation [2], [3]and [4].
Attenuation of inner conductor
α i = 36.1*ki
*√f / Zc*de [2]
Attenuation of outer conductor
α o = 36.1*ko
*√f / Zc*De [3]
Attenuation of the dielectric
layer
αfoam = 9.096 *√ εr* tan δ *f [4]
α i - attenuation
inner conductor [dB/100m]
α o - attenuation
outer conductor [dB/100m]
αfoam - attenuation
dielectric layer [dB/100m]
Zc - characteristic
impedance [ohm]
f - frequency
[MHz]
εr - dielectric
constant
ki - shape
factor inner conductor
ko - shape
factor outer conductor
de - electrical
equivalent inner diameter
De - electrical
equivalent outer diameter
The electrical equivalent diameter
considers the skin effect, which occurs on high frequency signals where the
current tends to flow only in a very thin skin layer. The depth of penetration
is given by following formula.
‘δ =
15.9 / √(σ*f )
δ
- conducting layer [mm]
σ
- conductivity [m/Ω mm2]
f - frequency [kHz]
With above relation
de = di CU – 2* δ
De = Do CUinner + 2* δ
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