Electromechanically Shifting Antennas Changing Communication

For military wireless applications, avoiding jamming and ensuring the most reliable frequency and protocols of operation are essential in highly critical implementations. For IoT/M2M applications, tuning a device toward a less burdened band of frequencies rapidly and reliably can ensure carrier-grade quality in a noisy and crowded wireless world. Researchers from NC State University in North Carolina, have made a significant breakthrough in creating purely electromechanically controlled liquid metal antennas that have the ability to adjust their resonant frequency over several gigahertz.

Many material and technology innovations are necessary to usher in the connected age and for creating seamless wireless connectivity. A main focus of research and industry effort has been in developing RF communications systems that can be digitally reprogrammed, and even make decisions on how best to operate. These devices, known as cognitive radios, and a component of them, software-defined radios (SDRs) -- like the ADC34J22 from Dallas Logic or Analog Device's ARRadio -- leverage sophisticated computer techniques to enable intelligent radio systems. Though these communication systems can adapt over a variety of modulation and electromagnetic (EM) environmental variables, the limits in adjustable hardware ultimately restrain the performance of the RF transceivers critical in these systems.

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The advent of controllable conductive liquids has been excitedly explored by many researchers to create RF components and devices that can be used to create highly tunable systems. Though the filters, frequency selective surfaces (FSS), and antennas developed by these researchers have proven configurable, most of them require pneumatic systems to run. Incorporating a pneumatic system increases complexity, cost, and potential failure modes of a device. But, with an electromechanically controllable antenna, the speed, accuracy, repeatability, and tuning range of a liquid antenna could be greatly enhanced. This was experienced by the researchers at NC State, when there were able to develop a prototype liquid metal antenna from Eutectic Gallium-Indium (EGaIn) within a capillary structure.



Figure 2: The DC bias applied across the capillary structure enables the EGaIn liquid metal to flow into the capillary when voltage is applied, and to retract when the voltage is removed. (Source: J. Appl. Phys. 117, 194901 (2015))

Using a simple low-voltage DC bias, the EGaIn antenna can be continuously and reversibly tuned over a 5:1 tuning range. This is done by adding or removing a thin surface oxide on the EGaIn material using the electric potential, which variably manipulates the interfacial tensions along the capillary. The creation of this oxide is only possible due to the inclusion of an electrolyte within the capillary that prevents permanent development and adhesion of the EGaIn oxide with the capillary walls. The DC potential applied to this system is what draws the EGaIn into, or out of, an EGaIn-filled reservoir.



Figure 3: The electromechanical control process enables completely reversible tuning and consistent behavior, unlike many tuning technologies with memory effects. (Source: J. Appl. Phys. 117, 194901 (2015))

The overall efficiency at 0.66 GHz seen was 41 percent with 70 percent efficiencies seen at 3.4 GHz. This compares poorly to a roughly 95 percent radiation efficiency of dipole antennas with pure metal structures. 

However, radiation efficiency is reduced when an antenna isn’t properly tuned or resonating at the frequency of transmission or reception as well. Though the conductivity of the electrolyte solution—sodium hydroxide (NaOH)—in this prototype reduced the overall antenna radiation efficiency, the potential for future development of electrolytes with less efficiency impact as NaOH may become viable as research developed.



Figure 4: Though the radiation efficiency in the prototype suffered from non-ideal electrolyte behavior, future development could enhance the efficiency and tunability parameters of the liquid metal antenna. (Source: J. Appl. Phys. 117, 194901 (2015))

This development and future advancements have the potential to create antenna structures—either simple or highly complex—that may be used with SDR to develop hardware and software for highly configurable RF and microwave communications systems. For example, the prototype developed by the NC State University researchers focused on a single dipole antenna, where the potential for a reconfigurable array of such antennas could dramatically enhance the utility of current antenna array technology. 

Other possibilities include antennas that change their radiation patterns significantly to accommodate different EM environments, as a form of electromechanical beam steering that doesn’t produce ware in the antenna control elements. Such a system could reduce the cost and complexity of beam-steering applications that may require many power dividers/combiners and amplitude/phase-balancing electronics.

In the current designs of smartphone and portable electronic devices, they usually contain several inefficient antenna designs that are subject to very tight design constraints from the form factor and antenna proximity within the device. A reconfigurable antenna that is adjustable over a wide range of frequencies could be used to optimize for the frequency most efficient for communications and dramatically reduce the power consumption and cost of filter and matching networks on the device.

Though digital reconfigurability has been proven viable in the RF and microwave communication industries, reconfigurable hardware has historically been less accessible. Cost and complexity have been a large factor in the adoption of effective and efficient reconfigurable hardware. Liquid metal antennas with electromechanical control may offer a needed configurability dimension and enable applications from commercial and industrial IoT/M2M to the latest military wireless network and electronic warfare (EW) demands.