Learning Forum

[wetheyd]

Hello

I am new to Ansys Electronics Desktop and it is my first time using Q3D, so I thought I would start simple.

I am trying to create a Coax cable, specifically RG58, TE part number 5021D1331, https://www.te.com/en/product-440005-000.html

TE has in the drawing specified the following properties:

Inner conductor: 0.0355", copper
Dielectric = 0.116", Rayolin F
Shield = 0.137", copper
Z0 = 50R
Capacitance = 30.8pF/ft
Resistance = 11.7 Ohms/1000 ft

I created the 3D model following this tutorial:

And from the Q3D via tutorial "Q3D Extractor A 3D PCB Via Model.pdf"

Now I got the Er of the dielectric from putting the values into this calculator: https://www.pasternack.com/t-calculator-coax-cutoff.aspx (Er = 2.155) which provies the correct capacitance at 30.8pf/ft, the Z0 is 48R and L = 72.2nH/ft

Now Ansys Q3D calculates the capacitance for 1 foot as 30.9pF which seems correct
Q3D calculates the inductance at 369nH which is quite far off 72.2nH

Is it because I have not specified the shield as a return path?, what have I done wrong here?

Furthermore if I try and export the circuit, the transmission line looks like the following...

* BEGIN ANSOFT HEADER
* node 1    Conductor:Source1
* node 2    Conductor:Sink1
*  Project: Cable_Coaxial
*   Design: Q3DDesign1
*   Format: Ansys Nexxim
*   Topckt: Cable_Coaxial
*  Creator: Ansys Electronics Desktop 2024.2.0
*     Date: Fri Jan 02 18:15:30 2026
* END ANSOFT HEADER

.subckt Cable_Coaxial 1 2
XZhalf1 1 3 Cable_Coaxial_half
XY1 3 Cable_Coaxial_parlel
XZhalf2 3 2 Cable_Coaxial_half

.subckt Cable_Coaxial_half 1 2
V1 1 3 dc 0.0
R1 3 4 0.706636968706
L1 4 2 1.84528382349e-07
.ends Cable_Coaxial_half

.subckt Cable_Coaxial_parlel 1
R1_0 1 0 2273086.54966
C1_0 1 0 3.15863088431e-12
.ends Cable_Coaxial_parlel

.ends Cable_Coaxial

 

C1 is 3.15pF which I cant tell where it got this number from as its not in the SPICE matrix, and not near the 30.91pF I was expecting?
L1 is half the AC inductance, which is as expected.

 

EDIT: I added 2 more sinks, to each end of the shield, now my inductance is correct, but the capacitance is incorrect. Can I setup the sources and sinks so that I can calcualte both L and C at the same time?

 

Many thanks

Details below:

Dom