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.

 

Hello,

Has the skin-effect been taken into account?

Perhaps this page will tell you more:
https://www.microwaves101.com/encyclopedias/coax-loss-calculations

Bertus

 

Hello,

Has the skin-effect been taken into account?

Perhaps this page will tell you more:
https://www.microwaves101.com/encyclopedias/coax-loss-calculations

Bertus

Hi Bertus,

Thanks for responding - yes, funnily enough this is where I gathered most of my information. You can see on the same topic, but different thread, that they go into the skin effect and include it in the formula, found here.

Thanks

 

Hello,

You might want to have a look at the following book:
https://archive.org/details/microwave-and-rf-design.-vol-2-3ed-2019.-transmission-lines

There are also other RF books in that series:
https://archive.org/details/folkscanomy_electronics?query=microwave+rf+3ed&sort=-publicdate

Bertus

 

Hello,

You might want to have a look at the following book:
https://archive.org/details/microwave-and-rf-design.-vol-2-3ed-2019.-transmission-lines

There are also other RF books in that series:
https://archive.org/details/folkscanomy_electronics?query=microwave+rf+3ed&sort=-publicdate

Bertus

Hi Bertus,

Thank you - will have a read and see where I get!

 

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.

 

Hello,

You might want to have a look at the following book:
https://archive.org/details/microwave-and-rf-design.-vol-2-3ed-2019.-transmission-lines

There are also other RF books in that series:
https://archive.org/details/folkscanomy_electronics?query=microwave+rf+3ed&sort=-publicdate

Bertus

Hi,

I'm reading through and come across this example -



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!

 

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.