expert insights
Military and Aerospace EMC
High Intensity Radiated Fields (HIRF)
By Patrick André
U

nder the heading of radiated susceptibility (RS) testing is the category of High Intensity Radiated Fields, or HIRF. What is HIRF and why does anyone need to test to these high levels?

NASA/TP-2001-210831, In-Flight Characterization of the Electromagnetic Environment Inside an Airliner[1], has this definition (emphasis mine):

HIRF encompasses man-made sources of electromagnetic radiation generated external to the aircraft considered as possibly interfering with safe flight. The easiest way to distinguish HIRF from other types of EMI is to state what it is not. HIRF does not include interference among on-board systems; this type of interference is referred to as an Electromagnetic Compatibility or EMC issue. HIRF also does not include EMI effects caused by portable electronic devices (PEDs) carried by passengers, such as cellular telephones, laptop computers, and portable radios. … HIRF does not include the effects of lightning, nor the effects of static electricity generated on the airplane; this is called Electrostatic Discharge or ESD. The effect of lightning on aircraft and avionics systems is similar to that produced by low frequency HIRF (kHz frequency range).
HIRF sources are intentionally generated, man-made radio frequency emissions. They are very intense and often come from ground radars, radio transmitters, and the like. These transmitters are typically friendly, however, HIRF has military applications that are used to intentionally disrupt electronics.
The Trestle, Kirtland Air Force Base, with B-52
Figure 1: The Trestle, Kirtland Air Force Base, with B-52
HIRF effects are real. And they are dangerous. HIRF has been known to cause many incidents, several crashes, and, sadly, loss of life. Reports of HIRF effects go back to the early 1980s and likely before. In 1972, the U.S. Air Force built ATLAS-I, the world’s largest all-wood structure known as “The Trestle,” at Kirtland Air Force Base in Albuquerque, New Mexico. Designed for EMP testing, the high frequency and high intensity fields had application to HIRF design and shielding.

Research into HIRF effects was performed in parallel by several agencies. These include the military and the FAA, working with the SAE, the EUROCAE and others. The Federal Register on HIRF states:

In 1987, the FAA contracted with the Department of Defense Electromagnetic Compatibility Analysis Center (ECAC) (currently the Joint Spectrum Center) to research and define the U.S. HIRF environment to be used for the certification of aircraft and the development of Technical Standard Orders. In February 1988, the FAA and the Joint Aviation Authorities (JAA) tasked the Society of Automotive Engineers (SAE) and the European Organization for Civil Aviation Equipment (EUROCAE) to develop guidance material and acceptable means of compliance (AMC) documents to support FAA and JAA efforts to develop HIRF certification requirements.[2]
The SAE sub-committee AE-4R with the EUROCAE Working Group WG-33 recognized the contribution of over 500,000 radio, television, radar, and other transmitters in just Western Europe and the USA which were causing the issues which were found.

The next step was to document the contribution of each of these transmitters – their frequency range, transmitting power, beamwidth, and similar information. With the data provided, the frequency ranges for each of the spectral envelopes were created. The following 17 ranges are now commonly used to define each of the HIRF environments:

10 kHz – 100 kHz
100 kHz – 500 kHz
500 kHz – 2 MHz
2 MHz – 30 MHz
30 MHz – 70 MHz
70 MHz – 100 MHz
100 MHz – 200 MHz
200 MHz – 400 MHz
400 MHz – 700 MHz
700 MHz – 1 GHz
1 GHz – 2 GHz
2 GHz – 4 GHz
4 GHz – 6 GHz
6 GHz – 8 GHz
8 GHz – 12 GHz
12 GHz – 18 GHz
18 GHz – 40 GHz
Some assumptions used to derive the amplitude in each range included that any reflection, say off the ground from an airport radar, was in phase with the direct transmitted amplitude. The amplitude was assumed to be at the closest approach to the transmitter by the aircraft or helicopter. The maximum amplitude in each of these frequency bands was used to determine the peak and average field strength. The average field strength was derived from the modulation (typically pulse modulation) for that transmitter, but it was noted that a transmitter with a wider pulse width might have a higher average energy than a high peak amplitude transmitter with a very short pulse width. Thus, the maximum amplitude for either condition was used, though it may not be from the same transmitter. What was not considered was the total illumination time, the frequency of the modulation (although the pulse width was used), nor the cumulative effect from multiple transmitters.
close up of fiberglass and wood bolts and nuts
Figure 2: Fiberglass and wood bolts and nuts
In the next article, we will continue with HIRF, what considerations were used to define these environments, and how commercial and military applications use them.

Fresh out of college with my physics degree, I had an opportunity to visit the structure, even before I knew how to spell EMI. They were changing out the metal bolts and holding them together, one at a time, to fiberglass and wood since the metal bolts were resonant structures at higher frequencies.

Endnotes
  1. Karl J. Moeller et al., In‑Flight Characterization of the Electromagnetic Environment Inside an Airliner, Langley Research Center, Hampton, Virginia, March 1, 2001. Work of the US Gov. Public Use Permitted.
  2. Federal Register, Volume 71, No. 21, Wednesday, February 1, 2006, Proposed Rules, Docket No. FAA-2006023657; Notice No. 06-01, page 5555.
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