Cross-Coupling Between a Vehicular 3G Transmitter and a Cable Bundle
This white paper demonstrates the FEKO GUI elements that are useful in setting up an investigation into the cross-coupling between transmitters and cable bundles in a vehicle. In this case the vehicle is fitted with a 3G booster antenna on the rear roof of the vehicle and a cable that connects control electronics in the engine bay to the left rear tail light assembly.
Questions involving the cable that are answered include:
- Irradiation case: How much energy couples into the cable when the 3G antenna is transmitting?
- Radiation case: How much energy is coupled into the 3G antenna when signals are transmitted through the cable?
Definition of the Cable and Its Shielding
The case described in this white paper considers a cable bundle with 2 insulated cores, surrounded by a braided shield. The specification of this cable is performed with the following steps in CADFEKO:
- Specify the dielectric properties of the insulation material for each core of the cable. This is done in the standard way for defining dielectric materials, or by selecting an existing material from the material library.
- Define a single conductor of the cable. The dialog allows the user the freedom of specifying the core metallic medium, core radius, insulation medium and insulation thickness.
- Define the shield for the cable. The shield can be specified with the industry standard Kley formulation, Schelkunoff formulation for a solid shield or simply as a list of points that define the transfer impedance from the outside of the shield to the inside.
- Define the cable bundle. Any number of predefined cores can be grouped into a single cable, which may be surrounded be a predefined shield.
|Definition of cable components|
|(a) Conducting core||(b) Cable shielding||(c) Bundle|
Routing of the Cable
The next task is to define the path that the cable follows in the model. CADFEKO offers users various options for doing this.
In cases where the position of the cable is known and the coordinates available in a text format file, these coordinates can be imported into the dialog that defines the cable path. KBL format files that define cable paths may also be imported into CADFEKO.
An interactive specification of the cable path is also possible. CADFEKO allows users to click on the coordinates for the cable path in the usual point list creation method. An innovative tool in CADFEKO recognizes that a cable is typically located at a certain distance from bodywork of the vehicle. This tool allows the user to set a "snap offset" variable and then click on the appropriate points on the body panels of the vehicle to set the cable points at the predefined distance from the clicked points.
|Definition of cable paths|
Connection of Cable Elements
CADFEKO features a 2D schematic view of the cable paths to assist users in the connection of the different conductors in the cable to connectors, lumped circuit elements (e.g. capacitors, inductors, resistors) and Spice probes to measure currents and voltages on the cable.
|2D cable schematic view in CADFEKO|
Results of Cross-Coupling Simulations
In the radiation case the cable is excited at the connector in the car's engine bay to compute how much energy will couple from the cable to the receiver at the base of the 3G antenna on the roof of the vehicle. The following images demonstrate the voltage induced in the 3G antenna, a cut through the near-field radiated from the cable and the currents induced on the vehicle as a result of the radiation escaping from the cable.
The very low levels of voltage induced in the 3G antenna demonstrate that radiation from the cable is unlikely to have an effect on the operation of the 3G radio.
|Results of cable radiation investigation|
|(a) Voltage induced on 3G antenna terminals|
|(b) E-field radiated by cable||(c) Currents induced on body of vehicle|
The irradiation case examines the opposite direction of errant energy excitation, i.e. the 3G antenna transmits and the energy that couples into the cable is under investigation. The following images demonstrate the voltage that is excited in the load that represents the light bulb at the end of the cable, the near-field in the vicinity of the cable and the currents that are excited on the vehicle by the transmitting 3G antenna.
These images demonstrate that at this high frequency, a much higher amplitude voltage is excited in the cable termination compared to the voltage excited in the antenna for the radiation case. Whether this high (but still low) level of induced voltage will cause any interference with the electronics connected to the cable is a question to be evaluated be the responsible engineering team.
|Results of cable irradiation investigation|
|(a) Voltage induced on load terminating the cable at vehicle rear|
|(b) E-field near cable path||(c) Currents induced on body of vehicle|