How are the Per-Unit-Length Parameters Calculated for MTL Cables?
Question: How are the per-unit-length parameters, L, C and G calculated for MTL cables (including thick shields)?
Answer: For the per-unit-length parameters calculation, the structure is meshed into a 2D triangular mesh. The parameters (L, C and G) are then solved using a 2D Laplace FEM solver.
The generated mesh considers all boundaries. The inside and outside of the shield are two separate boundaries. Each conductor in this cross-section is considered as if it is a perfect electric conductor (PEC). The solution computes the per-unit-length parameters on a sub-circuit basis (there are no capacitance / inductances between conductors on either side of the shield). The parameters G and R are introduced to model the metallic and dielectric losses respectively. The cable coupling is modelled using the transfer impedance and admittance parameters, Zt and Yt.
For the computation of Zt, you can choose from the Schelkunoff, Kley or Vance formulas or specify the Zt and Yt parameters.
- Low-frequency approximation
- Exact description
- High-frequency approximation
- Practical shields where the wall thickness is small compared to the radius of the tube
If the shield is PEC, then it is known that Zt = 0.
Furthermore, Feko uses a dynamic range of -120 dB to calculate a cutoff distance of N * delta, after which Zt = 0 is set. If the shield is thicker than N times the skin depth, Feko sets Zt = 0 to avoiding numerical noise.