Attenuation loss through CO-AX cable formula

Thread Starter

Engineer11235

Joined Jun 26, 2023
5
Hi,

Unsure if correct forum, please inform if not.

I am trying to understand and determine the correct mathematical formula(s) to calculate overall attenuation loss through a COAX cable. I would like to be able to plug in cable information into a datasheet, and for it to determine the overall loss based on length, frequency, etc.

I understand this information is in datasheets, but only for set frequencies and lengths (usually 100m).

I have so far gathered a bunch of formula (see attached) - this covers attenuation loss for metal conductivity, dielectric loss tangent, dielectric conductivity, and radiation. Some information is missing (such as the loss tangent formula) - any help here would be appreciated.

QUESTION: Taking a random COAX cable here, i try to calculate the attenuation at 100MHz, based on my formulae. I know what I should expect as the attenuation information is in the datasheet. When calculating the loss through metal conductivity alone I get ~10.5dB/100m; the datasheet states 21dB/100m.

This is where I stop - I expect the other attenuation losses (tangent, dielectric etc) to be of some value, but definitely not near 10dB/100m worth; I'd expect the metal conductivity losses to be closer to the total attenuation loss value of the cable.

I also understand that the values given in their datasheets would have been determined under testing of the cable. However I'd expect to get within +/-1dB/100m of accuracy?

Any help is greatly appreciated.
 

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Thread Starter

Engineer11235

Joined Jun 26, 2023
5

sagor

Joined Mar 10, 2019
898
The RG58 ( like the example you posted) that I checked shows 15.7dB loss for 100m at 100Mhz. A lot depends on many factors, like dialectic constant, %shield coverage, etc. It may be that some values you used are off a bit, or have a range of values where you took one value instead of another.
 

Thread Starter

Engineer11235

Joined Jun 26, 2023
5
Hi,

I'm reading through and come across this example -
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I am trying this example however I'm struggling to get the same answer as they do i.e. (17.94 + j51.85).

This should be trivial yet I get an answer far from theirs - for a start, I get a negative real term...

I have looked through the same section and it seems they use it as the base for calculating attenuation loss!
 

Thread Starter

Engineer11235

Joined Jun 26, 2023
5
Update - Managed to read through, and successfully achieve the same answers as the above equations.

Unfortunately applying this to my use-case has yielded incorrect results overall; the above propagation constant formula does not include skin effect, but still the answer is far from the expected case.

I will continue to work on potential solutions and read other material as well, thank you for the help thus far.
 
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