# Coaxial cable power rating

TheEggo
I am trying to calculate the maximum power of a coaxial cable, based on heat transfer parameters and limiting temperatures of cable components. I've determined that a certain heat load on the conductor surface (in Watts per inch of cable) will put the dielectric above its operating temperature.

I am having trouble turning that heat load number into a power rating (I'm looking at attenuation numbers). Would a power rating be dependent on cable length? My calculations suggest that length is a factor, but every data sheet I look at disagrees.

Gold Member
Hi
You would probably find what you need in the detailed spec of the cable. The power rating of a cable will ultimately depend upon the actual operating frequency (the loss is frequency dependent), the thermal environment of the cable (clearance between cables and ambient temperature) and even the VSWR on the cable. But the manufacturer will have a conservative spec, which would be the one to use unless there is a really good excuse - e.g. cost or available duct space.
I couldn't be sure how W/inch would help (except for a very short piece of cable) as the thermal power dissipated externally AND the RF power dissipated internally would both be proportional to length - so the equilibrium temperature would be the same). Needless to say, it would be the start of the cable run where the most power would be dissipated. Once you were 3dB down, the power dissipated would be halved.

TheEggo
Interesting. I had been assuming that the power would be dissipated evenly over the length of the cable. What you said makes sense now that I understand attenuation more. So the front end (power-in end) will dissipate more power.

I'm still hung up on the effect of length. Thermal resistance and power dissipated externally are linearly dependent on length. Attenuation (in dB) is also linearly dependent on length, but it is a logarithmic ratio, so it seems to me that the length dependency doesn't wash out? I can post my math if that is not clear.

Every cable data sheet I can find lists a power rating as independent of length, either I'm doing it wrong or they made an assumption or two.