Abstract:
High-frequency (HF) communications are critical for beyond-line-of-sight (BLOS) and highly resilient tactical links. However, because of the long wavelengths in the HF band, practical platform integration requires electrically small antennas (ESAs). Traditional passive, linear-time-invariant (LTI) ESAs are inherently bound by fundamental physical limits (such as the Chu limit), resulting in high quality factors and steep bandwidth-efficiency trade-offs. This talk presents the design, system integration, and over-the-air (OTA) measurement of a non-linear-time-invariant (non-LTI) ESA that effectively circumvents these traditional passive limits. By intentionally violating the passive LTI assumptions used to derive these fundamental bounds, our system achieves bandwidth-efficiency products that significantly exceed those of conventional antennas. Specifically, we introduce an integrated non-LTI ESA architecture that combines an electrically small radiator (a 1-meter monopole) integrated with a class-E switch-mode power amplifier utilizing a GaN transistor switch. Instead of relying on a traditional transmitter chain and impedance-matching networks, our system consolidates RF carrier generation, power amplification, and digital modulation directly at the antenna stage. Modulation of the radiated waveform is achieved by applying pre-defined, pulse-width-modulated gate-driving signals (GDS) to the switch-mode circuit, allowing precise control of amplitude, phase, and frequency. During the presentation, we will detail recent outdoor OTA measurement campaigns conducted at 10.665 MHz. We will demonstrate the successful transmission and demodulation of wideband digitally modulated waveforms—including OOK, BPSK, QPSK, and 8PSK—at symbol rates of up to 3 MS/s and radiated power levels of up to 9.1 W using this non-LTI ESA, achieving 16×–36× (12.0–15.6 dB) bandwidth-efficiency product improvements over competing LTI ESAs. To quantify the capability leap, we will also present direct comparative data against a conventional LTI ESA transmitter utilizing the same radiating element and a commercial power amplifier. Ultimately, this talk will highlight how this non-LTI ESA architecture can drastically improve the size, weight, power, and cost (SWaP-C) as well as the performance of next-generation, bandwidth-hungry HF systems. While defense applications such as resilient, beyond-line-of-sight tactical links are a natural fit, the same approach benefits any platform where antenna size is constrained and bandwidth is at a premium, including maritime and aeronautical communications, skywave links serving remote and polar regions, and disaster-response and emergency backup communications. More broadly, we will make the case that relaxing the LTI assumption opens a rich and largely unexplored design space for electrically small antennas across the RF spectrum. Biography:
Nader Behdad (S’98-M’06-SM’12-F’17) received the B.S. degree in Electrical Engineering from Sharif University of Technology in 2000 and the M.S. and Ph.D. degrees in Electrical Engineering from University of Michigan-Ann Arbor in 2003 and 2006, respectively. Currently he holds the Harvey D. Spangler and the Vilas Distinguished Achievement Professorships in the Department of Electrical and Computer Engineering of the University of Wisconsin-Madison. His research expertise is in the area of applied electromagnetics with particular focus on electrically small antennas, phased-array antennas, microwave periodic structures, high-power microwaves, and biomedical applications of RF and microwaves. He has 25 issued U.S. patents in these areas, with three additional patent applications filed with the USPTO. Dr. Behdad has served as a consultant on topics related to designing antennas and phased arrays for industry. He has also served as a consultant and an expert witness for different U.S. law firms on topics related to intellectual property disputes as well as cell phone record analysis and historical cell site analysis. Over the years, his research has been sponsored by various U.S. Federal agencies including the U.S. Navy, U.S. Air Force, U.S. Army, National Science Foundation, and the Defense Health Agency among others. Dr. Behdad has graduated 33 Ph.D. and 16 M.S. students so far and served as the research advisor of 32 other post-doctoral research fellows and visiting scholars. He is the recipient of the 2025 John Kraus Antenna Award, the 2021 H. A. Wheeler Prize Paper Award, the 2014 R. W. P. King Prize Paper Award, and the 2012 Piergiorgio L. E. Uslenghi Letters Prize Paper Award of the IEEE Antennas and Propagation Society. He also received the Byron Bird Award for Excellence in a Research Publication, McFarland-Bascom Professorship, Harvey D. Spangler Faculty Scholar Award, the H. I. Romnes Faculty Award, and the Vilas Associates Award from the University of Wisconsin-Madison. In 2011, Dr. Behdad received the CAREER award from the U.S. National Science Foundation, the Young Investigator Award from the United States Air Force Office of Scientific Research, and the Young Investigator Award from the United States Office of Naval Research. He served as a member of the Fellow Election Committee of IEEE Nuclear and Plasma Sciences Society (2022-2025) and served as the 2020 chair of the paper awards committee of the IEEE Antennas and Propagation Society. He also served as an Associate Editor for IEEE Antennas and Wireless Propagation Letters (2011-2015) and as the co-chair of the technical program committee of the 2012 IEEE International Symposium on Antennas and Propagation and USNC/URSI National Radio Science Meeting.
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