PSpice: coax cable models

In tline library, each co-axial cable (RG58C/U) has two simulation model implementation

One in form of .Model and other in form of .SUBCKT, which uses fixed frequency for model. depending upon how instance parameters in your schematic, one of these model implementation would be picked up.

In both case R is function of SQRT(Frequency); i.e you should get similar attenuation/phase shift for both of these and both of these are active models.

Hope this clarifies.

Hi,

 You've explained why there are two types of model for each coax cable.

My question is different.

Below is a model of RG58C/U from the TLine.lib file:

*******************************************************

*             Z0(Ohms)  vp(%)  F1(MHz) Loss1(dB/100Ft) F2(MHz)  Loss2(dB/100Ft)
* RG58C/U        50       66      100       4.9      1000        20
.model RG58C/U  TRN (r={63.9098u*sqrt(2*s)} l=252.700n
+   g={0.1584230p*abs(s)}   c=101.08p)
*$
* Subckt version uses fixed frequency, frq, to model simple lossy line
*
*                  Near end hi
*                   |  Near end lo
*                   |   |  Far end hi
*                   |   |   |  Far end lo
*                   |   |   |   |
.subckt RG58C/U    A1  A2  B1  B2 params: frq=100Meg len=1
.param PI2 {3.141592654*2}
*.model RG58C/U  TRN (r={63.9098u*sqrt(PI2*frq)} l=252.700n
*+   g={0.1584230p*PI2*frq}   c=101.08p)
t A1 A2 B1 B2 rg58c/u len={len}
.ends
*$

***********************************************

In line 3 of the model above, one can find following definition of R: r={63.9098u*sqrt(2*s)}

Why it is proportional to (2*s) and not to (s), as it should be for "simple" skin effect? What is a physical explanation of it? Where this coefficient 2 came from?

Thanks

Sincerely

Pavel

I can not think of "physical" explanation for "2". R should be function of "SQRT(S)". To me physical interpretation still remain same even if I say "SQRT(2s)". R remains proportional to SQRT(s), however proportionality constant changes in both cases. Hope this clarifies.