Researchers have developed a novel design approach that improves upon limitations of conventional antennas operating at low frequencies, demonstrating smaller antennas that maintain performance. Impedance matching is a key aspect of antenna design, ensuring that the radio transmits power through the antenna with minimal reflections while in transmit mode and that when the antenna is in receive mode, it captures power to efficiently couple to the radio over all frequencies within the operational bandwidth.

Impedance matching techniques with passive components — such as resistors, inductors, and capacitors — have a fundamental limit, known as the Chu-Wheeler limit, that defines a bound for the maximum achievable bandwidth-efficiency product for a given antenna size. In general, low-frequency antennas are physically large or their miniaturized counterparts have very limited bandwidth and efficiency, resulting in a higher power requirement.

The new approach improves bandwidth and efficiency without increasing size or changing the topology of the antenna. The proposed impedance matching approach applies a modular active circuit to a highly miniaturized, efficient, lightweight antenna, overcoming the Chu-Wheeler performance limit. The antenna enables the integration of power-efficient, low-frequency radio systems on compact mobile agents such as unmanned ground and aerial vehicles.

The ability to integrate low-frequency radio systems with low size, weight, and power (SWaP) opens the door for the exploitation of this underutilized and underexplored frequency band as part of the heterogeneous autonomous networking paradigm. In this paradigm, agents equipped with complementary communications modalities must adapt their approaches based on challenges in the environment for that specific mission. Specifically, the lower frequencies are suitable for reliable communications in complex propagation environments and terrain due to their improved penetration and reduced multipath.

Researchers developed an active matching technique to equip robotic ground vehicles with powerful, miniature antennas. (U.S. Army)

The antenna was integrated on small, unmanned ground vehicles; it demonstrated reliable, real-time digital video streaming between UGVs. By exploiting this technology, the robotic agents could coordinate and form teams, enabling unique capabilities such as distributed on-demand beamforming for directional and secure battlefield networking.

While previous experimental studies demonstrated bandwidth enhancement with active matching applied to a small, non-resonant antenna (e.g., a short metallic wire), no previous work simultaneously ensures bandwidth and radiation efficiency enhancement compared to small, resonant antennas with performance near the Chu-Wheeler limit. The active matching design approach addresses these key challenges stemming from the tradeoff among bandwidth, efficiency, and stability.

The researchers built a 15-centimeter prototype (2 percent of the operating wavelength) and demonstrated that the new design achieves more than threefold bandwidth enhancement compared to the same antenna without applying active matching, while also improving the transmission efficiency 10 times compared to the state-of-the-art actively matched antennas with the same size.

For more information, contact the U.S. Army CCDC Army Research Laboratory Public Affairs at 703-693-6477.


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This article first appeared in the December, 2020 issue of Tech Briefs Magazine.

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