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Problems with coax attributes

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Wild Life

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May 4, 2011
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Hello ev'body, I'm using Pspice for circuit design, I have to insert a Coax model... I need these four attributes: R, C, L, G of RG-174/U Coax Cable.

So, R is resistance per length unit (Ω/m), so I found it using:

R=(d/2)*(d/2)*π*ρ [ρ is COPPER static resistivity and d is inside cable diametrer]

C is requested by my project

the problems are in L and G parameters.

L is inductance per length unit (H/m), and I have to use this:


D is external diameter
d is internal diameter
μ0 is magnetic constant
μr is magnetic permability

As I have every paramter, I can't find ANYWHERE the μr! I know I have a RG-174/U, with Polyathylene as Dielectric, but I didn't find anywhere PE μr!!!

Another problem is about G, I don't know how I can get it!

Can someone help me?

For the time being, just assume µr = 1 and G = 0, which correctly represents an isolating and non-magnetic medium. If you expect any benefit from the lossy cable model, you should however put in a frequency dependent R value for the intended operation frequency, which will be surely much higher then the DC resistance you calculated above. If the objective of your simulation is wide band or impulse transmission of coax cables, the model still won't help you much because it can't represent the actual behaviour, neither frequency dependent attenuation nor pulse distortion. It's most likely more suitable to refer to the ideal coax cable model for most cases. There has been a previous edaboard discussion about more exact tranmission line modelling, but I guess, it's not really needed for your purposes.

Thank you very much, only one thing's missing: what's the TLOSSY attribute's meaning?

TLOSSY attributes could be parameters needs to simulate TLOSSY device. These are

T<name> <A port (+) node> <A port (-) node>
+ <B port (+) node> <B port (-) node>
+ [ <model name> [electrical length value] ]
+ LEN=<value> R=<value> L=<value>
+ G=<value> C=<value>

Hope this helps.

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