By blocking “backdoor” access, an antenna developed by University of Wisconsin-Madison engineers may enable both military personnel and ordinary citizens to keep their communications more private and secure.
Called a “low-probability-of-intercept/detect antenna,” the device marries advances in software and hardware and is easy to swap with existing antennas. The engineering team described its advance in a paper published in IEEE Transactions on Antennas and Propagation in August 2024.
Antennas are the backbone of all sorts of communications networks. Tiny antennas are inside cell phones, Wi-Fi base stations, satellites, radar systems and more. They’re also at the heart of military communications—and they’re becoming increasingly important as 5G networks and the internet-of-things connect more and more of the world wirelessly.
But antennas have fundamental vulnerabilities. Because their electromagnetic signals are radiated in many different directions, they are subject to eavesdropping or jamming and, by following a signal backwards, can pinpoint a transmitter’s location.
The new antenna could eliminate these shortcomings. It’s the brainchild of Nader Behdad and John Booske, professors of electrical and computer engineering at UW-Madison, and PhD student Jiahao Zhao, who led the research.
Nader Behdad
To understand how the antenna works, imagine its signal as light from a flashlight. “The antenna focuses most of the energy in the direction it’s trying to communicate with—but it also radiates smaller amounts of energy in other directions,” says Behdad. “Those are called side lobes.”
Those side lobes can serve as a back door for people who want to interfere with or intercept those transmissions. “Someone who wants to eavesdrop can point a very sensitive receiver at an antenna and pick up those little bits and pieces that radiate in other directions,” explains Behdad.
It’s also possible for adversaries to overwhelm and jam communications by sending high-energy signals through the side lobes. And in some cases, side lobes can help an adversary detect and pinpoint the source of a private signal, which can be catastrophic in intelligence or military situations.
The new antenna can reduce all these vulnerabilities: It uses a bank of physical switches, guided by a sophisticated algorithm, to scramble and reduce side lobe signals to nearly undetectable levels. As switches turn on and off hundreds of millions of times per second, they scramble the side lobe signals and send them off in random directions while maintaining the integrity of the main signal.
John Booske
Under the supervision of Behdad and Booske, Zhao designed and fabricated one of these antennas and showed the technique works very well on a small-scale prototype and for narrowband signals. Now, they hope to continue to develop the technology for use with wideband signals and larger antenna arrays, as well as for more specific applications.
“For example, on the battlefield, this technique could conceal communications from the enemy’s detection, thus preventing giving away the location of transmitters and protecting the transmitted signal,” says Zhao.
The team says these antennas are compatible with legacy communications systems, making them commercially attractive. “You can drop these in and upgrade systems that are 20 or 30 years old with our antenna, which brings communications security and jamming resilience capabilities while leaving everything else unchanged,” says Behdad.
While the antenna seems like a perfect fit for intelligence and defense-related applications, the team also believes that securing antennas is also critical to many future commercial technologies. For instance, the engineers’ antennas could help prevent autonomous vehicles and self-driving car signals from being hacked or jammed or could keep personal communications on wireless networks from being intercepted.
Nader Behad is the McFarland-Bascom Professor. John Booske is the Keith and Jane Morgan Nosbusch Professor Emeritus. Other UW-Madison authors include Shichen Qiao.
Featured image: Graduate student Jiahao Zhao built this small-scale prototype of a low-probability-of-intercept/detect antenna. Credit: Joel Hallberg.