Practical HERF: No longer an urban legend?

Nov 14, 2007

For years, HERF weapons (high energy radio frequency) have been the stuff of science fiction and urban legends of the hacker underground. The underlying premise is simple: Integrated circuitry is vulnerable to various forms of radio frequency emissions, and such interference can either disrupt the functioning of or outright destroy circuitry. In theory, these weapons are relatively easy to construct with a decent grasp of electronics and high voltage electrical engineering with readily available parts, but actual examples of such are rarely verified. Personally, I've heard some tales coming out of a certain hacker con in the west (which was disproven) and one or two tales from some friends who are police officers on the eastern seaboard, but that's certainly not the same thing as watching such a device operate firsthand.

Until now, if reports are to be believed.

A company called Eureka Aerospace has developed under contract HPEMS, High Power Electromagnetic System, which is a weapon designed to disable vehicles by interfering with the operation of the computers controlling the engine and electrical system. The HPEMS units are designed to be attached to patrol vehicles and powered from the alternator and were designed primarily with land-based facilities in mind (namely, factories, government buildings, and military bases) though there is also a variant that can be worked into a helicopter and used to protect offshore assets, such as oil rigs. Testing is complete and full-scale HPEMS weapons will be released on the open market (with a hefty price tag, no doubt) within eighteen months' time.

The basic idea is that an oscillator that produces signals in the radio frequency spectrum (the online documentation says between 350 MHz and 1.35 GHz, which are then directed via a number of possible antennae (the examples given were horn, spiral, and impulse radiating antennae) toward lightly protected areas of the target vehicles. Because metal is an excellent shield for RF interference, you can't just blanket an area with a pulse and hope to take down a target; the user would have to aim at locations that permit access to the engine, such as the radiator grille, the windows, or a gap in the chassis of some kind so that the RF pulse could penetrate. Effective range at these power levels is in the neighborhood of 15 meters, though they're working on beefing up the range to 50 meters without impacting the usability of the HPEMS device.

Geeky pontification underneath the cut... There are a couple of ways in which this principle works. First of all, microwave radiation tends to cause things to heat up; this is the principle by which a microwave oven operates (water, fats, and other compounds in food absorb microwave energy through dielectric heating, adjust their rotation due to their dipolar nature, and produce heat as a result). As most of us have seen when the cooling fans in a computer stop, that extra heat causes integrated circuitry to stop working and crash, if not outright ruining the components. Secondly, eddy currents may be induced in the circuitry through exposure to microwave radiation. Too big a charge passing through circuitry designed to handle five volts of direct current at most would be enough to blow some of the parts and break the device.

It really would not be too difficult to shield electronics from a weapon like this: What you would need is metal plates or metal mesh with holes smaller than the wavelength of the microwave pulse generated (which would be between 0.8565 meters and 0.2221 meters, if I did the math right) to prevent the waves from getting through. Faraday cages are relatively easy to construct given time and patience. Shielding vehicles would not be too difficult because their chassis are mostly made of metal anyway, but the ventilation and exhaust louvers would have to be taken into account. The problem with adding shielding involves both mass (which can throw off the handling of the vehicle if balance isn't taken into account, as well as intertia-related problems) and weight (more fuel is necessary and handling is, once again, more tricky). However, avionics systems built into modern aircraft have been RF hardened for years to minimize the possibility of a stray radar pulse causing flight problems (yes, this does seem to contradict why they tell people not to use transmitters of any kind while in flight) so these aren't nearly so difficult with some engineering know-how and a little common sense.

A more pressing concern is what will happen to people who are exposed to microwave radiation. As we've seen before microwave antipersonnel weapons are in both the research and deployment phases (yes, this would be an excellent excuse to migrate more of my older articles over to this new system). The thing about how they work is that they cause the water molecules in the targets (and people's bodies are mostly water, after all) to generate heat, which can cause burns or internal trauma (imagine someone focusing a microwave beam on a thin portion of your skull and causing your favorite three pounds of organic material to suddenly spike in temperature). Pluswhich, people who have pacemakers or implants of some kind (for the sake of argument, a cranial plate or a couple of bone pins) could be maimed by the side effects. I don't see this ending very well under those circumstances.

Things to think about.