Cable Coupling

Introduction

The challenge of cable coupling analysis: The electromagnetic coupling between cable bundles and their environment represents a very important and challenging class of EMC problems. For instance, consider the problem of a radiating source coupling to complex cable bundles mounted inside a car, truck or bus. In such a case, the coupling is determined by the whole geometry of the vehicle, consequently a full-wave analysis of the vehicle as a whole is required.

Using FEKO: FEKO is well suited to the full-wave analysis of large structures (such as a vehicle chassis with mounted antennas). It features a built-in capability to include shielded cables within its models, for irradiation analysis. This option is useful when simple cables are considered, with a library of standard cable types available to the user.  Various cable configurations are supported in FEKO:

• Single conductor above ground
• Coaxial cable above ground
• Multiple widely spaced conductors above ground
• Multiple shielded conductors above ground
• Ribbon cables
  

However, for coupling problems involving more advanced cable geometries or complex cable bundles, interfacing FEKO with a dedicated multi-conductor transmission line (TL) analysis tool is suggested.

The FEKO cable analysis interface: The interface provided by FEKO is supported by two multi-conductor TL analysis tools, namely CableMod (SimLab) and CRIPTE (ESI Group).  The combination of these computational tools enables the user to efficiently deal with this very challenging and complex class of problems, which otherwise would be intractable.

About the Cable Modelling Tools

The cable modelling tools interfacing with FEKO are based on multi-conductor transmission line theory. 2-D field solvers are used to determine transmission line characteristics which are then used within network simulators for voltage and current analysis of a full 3-D system of cables. In this way the geometry of cables and their immediate surroundings is taken into account in a very accurate manner. Cables and cable bundles of practically arbitrary complexity can be handled by these tools, e.g. single wires, coaxial cables, twisted pairs, ribbon cables, complex cable harnesses, etc. However, the full external geometry can clearly not be incorporated in this way, which is exactly why hybridisation with FEKO is such a crucial enabling feature.

CableMod: CableMod has a professional and easy-to-use GUI, featuring push-button importing of 3-D CAD geometry data (e.g. a car chassis in Nastran format) and external wiring data and cable harness geometry, as well as supporting direct specification of cabling and export options. Solution options and the post processing are also done through the GUI. CableMod capabilities include a geometry library management system, automatic cable bundling, parametric variation and Monte Carlo analysis, multiple simulation capabilities in time and frequency domain. Analysis options include voltage measurements at specific points, current flow through components, S-parameters and impedance calculations.

CRIPTE: Input data can be managed through the PRE-CEM pre-processing module, while the stand-alone use of CRIPTE is also possible. CRIPTE capabilities include the management of realistic cable bundles featuring hundreds of individual wires, internal and terminal junctions, splices, sub-networks, frequency dependent lossy dielectric coatings, buried wires, and many more. The CRIPTE tool is continuously refined based on industrial needs, with the aim of optimising the related computer resources when dealing with fully realistic networks exceeding a thousand cables.

Analysis options 

Full 3-D MoM (FEKO only):

• All details of the TL has to be discretised for the MoM analysis.
  
• Fine mesh required for surfaces close to the TL (both these requirements result in many unknowns).
  
• Complex cable bundles are generally not feasible .
  

Multiconductor TL analysis only (stand-alone usage of cable modelling tools):

• Coupling with external structures only through static L/C matrices, no coupling through radiation.
  
• Limited incorporation of external geometry. 

Hybrid TL/MoM analysis of cable radiation:

The case of cables being exited and consequently radiating, with the radiating fields then coupling to external structures such as antennas or circuits. Cables are analysed in CableMod/CRIPTE, taking all loads and connections into account . Computed currents and charges on the cables are passed to FEKO. FEKO uses these as impressed sources and does a 3-D, full wave solution of the structure. Near-fields, radiated far-fields, induced currents on the structure, or coupling to elements such as receiving antennas or waveguide ports can be computed.

Hybrid TL/MoM analysis of cable irradiation:

The case of electromagnetic fields due to external sources, coupling into cables. FEKO obtains the geometry of the cable paths from CableMod/CRIPTE.  The 3-D structure is modelled in FEKO, using the desired excitation (e.g. a voltage source on a transmitting antenna, an incident plane wave, etc.). FEKO calculates the electric and magnetic near-fields at sample points along the cable paths specified by CableMod/CRIPTE. CableMod/CRIPTE imports this information and transforms it into equivalent distributed voltage and current sources based on modified transmission line theory. CableMod/CRIPTE then computes the resulting currents along the cables and in the connected elements.

Example

Transmission line radiation coupling into an adjacent dipole antenna. A simple example which illustrates the basic idea of the combination of FEKO and CableMod for the radiation from transmission lines is illustrated here. The geometry consists of a flat metallic plate of size 1 m x 50 cm. Above the plate, at a distance of 5 cm, there is a straight transmission line, which is excited at one side by a voltage source of 1 V, and which is terminated on both sides with 50 Ω resistors connected to the plate. The task is to determine the radiation from this transmission line in the frequency range 10 ... 300 MHz, and to compute the coupling into a 20 cm long dipole antenna, which is placed 20 cm above the plate and loaded with 50 Ω. This configuration can be solved with FEKO alone by applying the MoM, and the resulting induced dipole current versus frequency is plotted in green on the following graph.

Alternatively, the TL/MoM hybridisation may be used. First determine the current distribution along the transmission line with CableMod (taking the plate into account during the computation of L/C matrices), and then as described above, import the cable geometry and current data versus frequency into FEKO, to be used as an impressed source to calculate the induced currents on the plate and dipole. The result is shown in red on the output graph and compares well with the full MoM solution (green). After the current distribution along the transmission line and on the discretised MoM objects (plate and dipole) have been obtained, the radiated near- and far-fields can be computed with FEKO, taking all currents and charges into account. An example for the resulting magnetic near-field distribution (horizontal H-components in a plane 15 cm above the plate for 200 MHz) is shown here.