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CST – Computer Simulation Technology

Modeling HIRF Effects on Aircraft

Electromagnetic environmental effects (E3) [1] can cause electronic devices to malfunction or fail altogether. High Intensity Radiated Fields (HIRF) [2], from sources such as TV and radio stations, radar and satellite communication systems, may disturb the safe operation of aircraft electronics. The behavior of aircraft when exposed to a HIRF environment can be simulated effectively using CST MICROWAVE STUDIO® (CST MWS). With CST MWS, surface current and field distributions can be calculated and the coupling into shielded structures predicted.

Figure 1 shows a virtual aircraft and its physical counterpart. The computer model includes the fuselage, the furniture and a cable harness. The virtual aircraft is assumed to be in free space, and exposed to a plane wave [2].



Figure 1: Virtual aircraft (a) and its physical counterpart (b)...

The HIRF frequency spectrum is divided in three frequency bands: Low Frequency (LF: 10 kHz – 50 MHz) band, Medium Frequency (MF: 30 MHz – 400 MHz) band and High Frequency (HF: 100 MHz – 18/40 GHz) band [4]. Each frequency band produces different electromagnetic field distributions around and inside an aircraft. In the LF band, the aircraft acts like an antenna and the electromagnetic field penetration into the fuselage is weak, while in the HF band, shown in Figure 2(b), strong field penetration into the fuselage is seen. In the MF band, both effects — antenna-like behavior and field penetration — are superimposed, as shown in Figure 2(a). External fields in the LF and MF band induce currents on the fuselage skin. These may couple into cable harnesses and, as a result, into the aircraft electronics themselves. The field that penetrates into the fuselage in the HF band may even exceed the external field, due to constructive interference of reflected waves within the fuselage. These strong electromagnetic fields can potentially affect the aircraft electronics directly.

The simulation of electromagnetic field coupling into the cable harnesses can be carried out more effectively using CST CABLE STUDIO®. For more information see [3].



Figure 2: Magnetic field strength distribution of the aircraft in Figure 1 at (a) 70 MHz and (b) 1000 MHz

One goal of a physical HIRF test is to estimate transfer functions (TF) [4]. From these transfer functions, the potential impact of an electromagnetic environment on an aircraft can easily be estimated. Although transfer functions can be derived from measurements, it is also possible to calculate them numerically from electromagnetic simulations. Figure 3 shows the simulated normal electric field En at the point STP1 on the aircraft fuselage skin. As the field amplitude of the plane wave excitation is 1V/m the transfer function, measured in dB(V/m), is simply:

TF = 20 log(En(STP1)/1 V/m)

The impact of an HIRF threat Ethreat can then be estimated as:

THREAT = TF + Ethreat

A broadband simulation of the aircraft model in Figure 1, up to a max frequency of 6.5 GHz, was carried out in full scale. Both its length and wingspan exceed 14 meters, equivalent to over 300 wavelengths in each direction at the highest frequency. The simulation was performed using the MPI-parallelized version of the CST MWS transient solver with a hexahedral mesh of 10.5 billion mesh cells. The total simulation took only 19.5 hours to solve down to an energy dissipation level of -40 dB on a cluster with 131 nodes. Note, that the selected size is not the possible upper limit term of the electrical size. Numerical problems with more than two times larger electrical size – corresponding to more than 100 billion mesh cells - can be solved. For the selected example this would mean an upper frequency of more than 14 GHz, which is already close to 18 GHz, the often maximum required frequency for HIRF tests [5].



Figure 3: Simulated normal E-field on the fuselage skin

References:

[1] https://acc.dau.mil/CommunityBrowser.aspx?id=21848

[2] M. Kunze, et al., "Solving Large Multi-Scale Problems in CST STUDIO SUITE® — An Aircraft Application," ICEAA, pp. 110-113, Oct. 2011.

[3] D. Johns and P. DeRoy, “Simulating Crosstalk and EMI in Cables,” Microwave Journal, Cables and Connectors Supplement, vol. 53, no. 3 sup

[4] M. Lindback, “Optimisation of aircraft transfer function measurements,” M.Sc. Thesis, Lund University, in coop. with Airbus France, 2004.

[5] SAE ARP5583 / EUROCAE ED 107, “Guide to Certification of Aircraft in a High Intensity Radiated Field (HIRF) Environment”.

Acknowledgment:

The aircraft geometry presented in this paper has been provided by EVEKTOR, spol. s r.o. and the HIRF SE consortium of the European project HIRF SE. The EM simulations described in this paper have received funding from the European Community's Seventh Framework Programme (FP7/2007-2013), under grant agreement no 205294 (HIRF SE project).

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