1: EMA HIRF Background (IEL)
Chapter 1: Background
The development and proper application of computational electromagnetic (CEM) models to the evaluation of high intensity radiated fields (HIRF) across its entire frequency band is very challenging. The objective of this document is to outline model development requirements and the accompanying analysis techniques that assure accurate HIRF coupling assessments. This is accomplished through the development of understanding of the underlying HIRF coupling processes and the modeling elements and procedures necessary to model them based on canonical models, system models and comparison with measurements. Here, finite-difference time-domain (FDTD) is the computational technique of choice due to its unique ability of producing broadband result from a single simulation. A final objective of this document is to provide a definitive roadmap and process for the evaluation of HIRF coupling and the relationship this evaluation has to the certification process.
1.1 The Need For CEM HIRF Evaluations
The certification process for an aircraft is a lengthy and expensive process dependent on design and installation features, testing and analysis. The HIRF environment forms one segment of the certification requirements.
It is not possible to perform certification testing on the entire aircraft for all incident field orientations to determine a true worst case response for the aircraft. For instance, on typical commercial aircraft, it is not possible to perform tests with field illumination from below as it might occur in-flight. A CEM model can provide a much more detailed view of the HIRF coupling processes and levels throughout the aircraft than is generally possible in testing due to time and cost constraints with any desired incidence angle and polarization.
A validated CEM model can provide essential supporting data needed for certification. These data can be used to reduce or even eliminate some aspects of an expensive test program. Further, the CEM model can be used to perform early evaluations of the HIRF response of the aircraft to aid in the design process.
The CEM evaluations also provide a way to investigate in detail the effect that proposed design changes may impact the HIRF response for an aircraft. This can be extended into the sustainment phase for the aircraft where the validated model can be used to reassess coupling issues based on aging or design changes.
Finally, SAE ARP5583A (page 77) recommends the use of 3D simulation when the aircraft is less than 2 meters off the ground. Most HIRF testing find the elevation of the aircraft difficult to achieve.
The application of CEM models and analysis to HIRF evaluations is an integral part of the design, certification and long term maintenance of HIRF protection features of an aircraft.
1.2 Computational Tools
EMA3D is a powerful 3D numerical solver of Maxwell’s curl equations based on the time-domain finite-difference method in Cartesian coordinates. The code has applications to nearly any EM coupling, radiating, or interaction problem. EMA3D is also capable of capturing the detail of entire aerospace or ground systems down to individual electronic interfaces in a computationally accessible manner. EMA3D can also be run on computational clusters in a parallel fashion, and was chosen for computations of the HIRF fields coupling.
MHARNESS is a transmission line solver whose models are capable of containing multiple conductors, shields, and branches to capture the actual cable routing of real buildings, vehicles, aircraft and spacecraft. Each branch segment can contain multiple layers of shields, wires, and conductors, all immersed in a variety of media. The latest version of EMA3D integrates MHARNESS cables into its computation such that the EM fields in the 3D model interact with the harnesses down to the pin level, and vice versa. This integrated version of EMA3D/MHARNESS is used for computations of HIRF coupling to cables.
A methodology for modeling and interpreting HIRF predictions using EMA3D and EMA3D/MHARNESS is provided in this document. The intent is to lead the analyst through the entire process of HIRF CEM analysis, providing the basic outline for the successful completion of a system CEM HIRF assessment. This assessment is completed through the integration of three principal components of HIRF modeling:
- Model requirements definition based on the certification documents and general FDTD modeling constraints
- Understanding measurement requirements, procedures and limitations and their impact on modeling and analysis
- Post processing of the raw HIRF predictions to integrate the effects of measurements into the final results
1.4 Summary of Contents
While this document should be treated as an integrated whole on the topic of computational HIRF analysis, the reader may choose to skip to a particular chapter, each of which may be viewed as a separate document in its own right. The following summarizes the chapter contents.
Chapter 2 gives a brief overview of the various documents used in HIRF certification. Most of these documents are not requirements, but guidelines. The key points of each existing document are presented.
Chapter 3 delves into the various methods of modeling wire harnesses for HIRF evaluation of cable currents. A canonical aircraft model was created for this purpose, and several different cable variations were modeled and compared for cable currents.
Chapter 4 introduces the methods of modeling aircraft for HIRF evaluation of EM fields. A simplified Boeing 707 model was created, and the effects of absorption losses and aperture losses evaluated. A discussion of the applicability of statistical electromagnetics to the evaluation of HIRF fields at high frequencies is also presented.
Chapter 5 discusses the history of HIRF measurements. Most of the measurements presented are from the research of National Institute of Standards and Technology (NIST), where they tested a variety of aircraft including Boeing 707, Boeing 737, and Bombardier Global 5000. Some data from the Cessna 680 Citation Sovereign are also presented. Results from Nigel Carter on the spread of EM coupling characteristics between aircraft are discussed.
Finally, Chapter 6 summarizes the requirements for creating a proper model for CEM HIRF evaluation.
1.5 Contributors to This Document
The entire technical staff at EMA has contributed to the development of this document. Key aspects of the contents were completed by the following people in alphabetical order:
Overall integration of the document was performed by Robert Fisher and Jennifer Kitaygorsky.