A View from Christopher Mims
Introducing the Electromagnetic Bazooka
A technique borrowed from acoustics could lead to a super-powerful amplifier of microwave radiation.
Non-nuclear electromagnetic pulses (EMPs), featured in movies like Goldeneye and The Matrix, are the stuff of electrical engineers’ nightmares. Imagine a conventional explosive that sends out a shockwave of electromagnetic radiation so powerful that it short circuits computers, stops cars dead in their tracks, and causes airplanes to drop out of the sky. Such devices might also have more prosaic uses: for example, two Texas congressman just proposed using EMPs to stop smugglers at the U.S.-Mexico border.
Previously, it was thought that creating an EMP of sufficient power to disable vehicles or infrastructure would require, at the least, a conventional chemical explosion. Work funded by the French Ministry of Defense, published earlier this year, describes an early prototype of an all-electric version of what the researchers describe as a potential “electromagnetic bazooka.”
The device borrows a technique invented in 2004 for audio signal processing by French physicist Mathias Fink, known as Time Reversal Signal Processing. The technique uses a Time Reversal Mirror to receive a short pulse of electromagnetic energy at an antenna and then shoot back toward the initial transmitter the same signal, but with its wave-form reversed in time. The technique is enabled by the use of an Arbitrary Waveform Generator, which can generate any waveform you like, including a backwards version of the waveform an antenna just received. It’s a bit like responding to a given signal by playing the same signal backward, although it happens in milliseconds.
When used on either audio signals or electromagnetic waves, a Time Reversal Mirror allows engineers to exploit what’s known as the “pulse compression property” of time reversal to create an amplified version of the signal at a point outside the reverberation chamber housing the transmitter and the time reversal mirror. Therefore, at some distance from the device, a significantly amplified version of the initial signal is generated.
Using this setup, the researchers discovered a linear relationship between the number of antennas (from one to eight) used in their time reversal mirror and the resulting amplitude of the microwave pulse they generated.
Further amplification of the signal before retransmission by the mirror, using a technique called “one-bit time reversal,” allowed the team to achieve 46 dB of amplification. Because decibels are represented on a logarithmic scale, that corresponds to amplification of the original signal by a factor of more than 10,000.
It’s clear that this work is early-stage because the paper did not make explicit the power requirements of the device or the destructive capacity of the resulting amplified electromagnetic signal, if any. However, as our infrastructure and our military becomes ever more dependent on microchips and electronics in general, any developments in the ease with which an EMP can be developed should be of considerable interest to military and security professionals, or even, if this device is as straightforward as it appears to be, enterprising attendees of next year’s Maker Faire.