# How do you compute for cable loss?

#### kiwi00

Joined Jul 30, 2015
7
Hi! I need to find the overall loss of a 28m long RJ11 telephone cable, how do I compute for the overall cable loss?
If you can give me a formula it would help a lot. Thanks!

#### sailorjoe

Joined Jun 4, 2013
365

#### MikeML

Joined Oct 2, 2009
5,444
Phone cable is not coax being operated at RF, so that calculator does not apply.

To do a calculation of losses, start by studying the specifications for Cat3 cable.
Pay attention to the Resistance per 100m, and the Capacitance per foot.

What frequency/pulse shape/amplitude is your signalling?
What is the source impedance?
What is the load impedance?
How accurately do you need to know the signal shape/amplitude at the receiving end?

#### Papabravo

Joined Feb 24, 2006
21,328
Great for COAX. For twisted pair -- not so much.

#### sailorjoe

Joined Jun 4, 2013
365
The calculator let's you select from a variety of different cable types, including the RJ11 the OP asked about.
However, we don't have the background information to understand exactly how the calculator works, so if it's wrong, we'd need to do some hand calculations to show it. Anyone have some good formulas for RJ11?

#### MikeML

Joined Oct 2, 2009
5,444
RG-11 and RJ11 are not even close to being the same thing...

If the TS ever answers my post #3 questions, we can help...

#### sailorjoe

Joined Jun 4, 2013
365

#### MikeML

Joined Oct 2, 2009
5,444
True. I assumed the OP was referring to typical phone cable. The calculator has a selection for RJ11, and again I assumed that it was for typical phone cable.
@sailorjoe : RG-11 coax cable (Belden 8213 or 8261) has nothing in common with Cat3 phone cable, except that they might both have some Copper in them...

#### sailorjoe

Joined Jun 4, 2013
365
@sailorjoe : RG-11 coax cable (Belden 8213 or 8261) has nothing in common with Cat3 phone cable, except that they might both have some Copper in them...
Yeah, Mike, I know. OK, my bad. I went back and checked the calculator, and I apparently misread the RG-11 entry as RJ-11.
Perhaps the OP can get some insight from this article, plus the one in post #6.
http://www.scatmag.com/technical/techarticle_dec02.pdf

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