Hybrid TL/MoM Solutions for Cabling EMC Problems

Complex cable bundles/harnesses cannot be solved by full 3-D EM solvers due to excessive computational resource requirements.  Instead, such analysis is done by solving 2-D fields at selected positions, or cuts through the bundles to determine the transmission line (TL) characteristics. These characteristics are then used within a Spice-equivalent network simulation to calculate the currents and voltages along the 3-D cable path.

Cross section through a cable harness.	Cable path inside the chassis of a car.

When solving the complete problem of e.g. a cable bundle running inside an automobile chassis, the 2-D field based approach is used to solve the cable bundle, whereas the effect of the vehicle is solved using a 3-D EM solver, such as FEKO. This interaction is handled through and interface between the two codes.

 FEKO supports an interface with the following multiconductor transmission line analysis codes:

• CRIPTE (by the ESI Group)
• CableMod (by Simlab)

Verification Example

A simple example which illustrates the basic idea of the combination of FEKO and CableMod for the radiation from transmission lines is shown here.

The geometry consists of a flat metallic plate of size 1m 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 also 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 depicted in the results below (green line).

Alternatively the TL/MoM hybridisation can also be used to first determine the current distribution along the transmission line and then, as described above, import the cable geometry with current data versus frequency into FEKO. FEKO uses this information as an impressed source, and computes the induced currents on the plate as well as in the loaded dipole antenna (the result is shown in red and compares well with the full MoM solution).

After the current distribution along the transmission line and on the discretised MoM objects (plate and dipole) has been obtained, the radiated near- and far-fields can be computed with FEKO, taking all currents and charges into account.

Current induced in the dipole: Compare FEKO only with FEKO+CableMod	Horizontal component of H-fields (at 200 MHz) 15cm above the cable.

Although only a simple cable is considered in this example (a single wire), the same principle applies for more complex bundles.