To operate aircraft close to thunderstorms in figure 1 they need to be protected against catastrophic failure conditions from lightning [1]. For modern aircraft the susceptibility of electromagnetic interferences is even increased with the employment of composite materials and solid state electronics.
A variety of electronic components for communication, navigation and surveillance is installed in aircraft. The interconnection of these components is established with complex cable harnesses as in figure 2 carrying lots of signal lines. Their length can easily reach several hundreds of kilometers in modern passenger jetliners.
Cable shields allow to reduce the level of induced voltages and currents at cable pins to which electronic components are attached. To explain the cable shield effect the crosstalk model in the figure 3 is considered. On the left the cross section and on the right the 3D model is shown.
Cross section and 3D model
The cross section on the left shows a generator and receptor wire above a reference plane made of a
Perfect Electric Conductor (PEC). Assigning PEC to the common reference plane allows to exclude
common-impedance coupling [2]. The only coupling mechanisms present in the model are inductive and capacitive coupling.
The crosstalk circuit model is shown in figure 4 - 6. The circuit is driven with 1V voltage source. The generator and receptor wire are terminated with low- and high-impedance loads, respectively. For low-impedance loads inductive coupling is dominating and for high-impedance loads capacitive coupling [2].
Three different cable shield bond configurations are considered here: OO (the shield is open at the near and at the far end), SO (the shield is shorted at the near end and open at the far end), and SS (the shield is shorted at the near and at the far end).
The shield is open at the near and at the far end.
The shield is shorted at the near end and open at the far end.
The shield is shorted at the near and at the far end.
The computational results of the three cable shield bond configurations for high-impedance loads of 1kOhm are shown in figure 7. The results show the magnitude of computed near-end cross talk. With high-impedance loads capacitive coupling is dominating for the configuration OO. With shorting the near end of the shield to reference the capacitive coupling contribution is zero and inductive coupling is dominating now. The inductive coupling is affected by the shield only if the shield is shorted to reference at both ends and the frequency is greater than 3kHz.
[1] FAA, Aircraft Electrical and Electronic System Lightning Protection, AC 20-136B, Washington, 2011.
[2] Paul, C.R., Introduction to Electromagnetic Compatibility, New Jersey: John Wiley & Sons, 2006.
