Abstract
A tunable liquid-metal antenna demonstrating gain reconfigurability is presented. This antenna uses a reconfigurable stub made with the liquid metal Galinstan. Using continuous electrowetting, a 60 Hz signal with an amplitude of 1 Vpp and 75% duty cycle can actuate the Galinstan continuously along a channel, tuning the stub length and antenna gain. Zero external power is required to maintain the position of the slug. The 5 GHz antenna offers more than 10 dB of analog gain tuning, from −5.90 to 4.43 dB.
1. Introduction
Reconfigurable antennas have become increasingly important for modern wireless communication systems [1, 2]. Reconfigurability has been effectively implemented using PIN diodes [3], varactors [4], and MEMS switches [5]. However, these discrete elements often limit the number and range of states [6].
Liquid metal, capable of assuming arbitrary shapes, offers the potential for the analog tunability of antenna parameters such as frequency, polarization, and gain over a wide range. Liquid metal has been used to implement the three conventional types of antenna reconfigurability [2]: frequency reconfigurability [6–9], polarization reconfigurability [10–12], and pattern reconfigurability [13–15]. This work explores gain reconfigurability, which not only can adjust communication range, but may also find uses in networks with interference. In interference networks, even small adjustments in antenna gain can lead to large gains in communication capacity [16].
The antenna presented in this work is gain reconfigurable in the strict sense that the shape of the radiation pattern remains roughly the same as the gain (defined here as the peak gain within the given measurement planes) varies. Moreover, the presented antenna offers analog gain reconfigurability, rather than gain tuning that is limited to discrete states. The closest example of a gain-reconfigurable antenna using liquid metal is reported in [17]. In that paper, a liquid-metal spiral antenna printed on an inflatable hemispherical elastomer demonstrated a continuous tunable directivity of 2.34 dB.
The liquid-metal antenna presented here is electrically actuated with a 60 Hz, 1 Vpp, 75%-duty-cycle signal only when the gain is tuned. No applied signal is required to maintain the antenna gain after tuning. Previous liquid-metal antennas are either actuated by pressure [7, 8, 13–17], require higher actuation voltages [12, 13], or require a constant bias to maintain its operating state [10].
The antenna presented is novel in that it offers the following three features simultaneously: (1) low-voltage (1 Vpp) electrical actuation, (2) zero external power required to maintain each gain state, and (3) more than 10 dB of analog gain tuning.
2. Design
2.1. Copper-Baseline Antenna
Copper was first used instead of liquid metal to measure baseline antenna performance. The copper antenna consists of a 0.1 mm thick 5 GHz copper patch (23.7 × 19.8 mm2) with feed line insets (1 × 6.96 mm2), a copper stub (15 × 1.5 mm2), a quarter free-space wavelength λo/4 (15 mm) away from the patch, and a copper 50 Ω microstrip feed line (2.35 × 40 mm2) on a Rogers 5880 RT/duroid® substrate (100 × 100 × 0.79 mm3) with a 35 μm thick copper ground plane on the underside (Figure 1). Shifting the stub left and right can continuously tune the gain of the antenna. A total of 15 discrete states were simulated, termed the 0 mm, 0.5 mm, 1 mm, 1.5 mm, 2 mm, 2.5 mm, 3 mm, 3.5 mm, 4 mm, 4.5 mm, 5 mm, 5.5 mm, 6 mm, 6.5 mm, and 7 mm offset states. The midpoint of the copper stub located mm to the right of the center of the microstrip feed line constitutes the -mm-offset state.
(a) 0 mm offset state
(b) 2 mm offset state
(c) 5 mm offset state
(d) 7 mm offset state
2.2. Copper-Baseline Antenna Simulations
Gain simulations and measurements were taken in a plane offset from the -plane by 20 mm, as shown in Figure 2(a). Figure 3(a) shows the simulated gains of all 15 considered states for both a matched 50 Ω inset-fed and unmatched 245 Ω edge-fed copper antenna. All simulations were conducted in ANSYS HFSS. In Figure 3(a), the parabola about each -mm-offset represents the 180° gain pattern above the antenna in the plane shown in Figure 2(b). Both the matched 50 Ω inset-fed antenna and the unmatched 245 Ω edge-fed antenna have radiation patterns typical of a patch antenna, with maximum gain at 90° as defined in Figure 2(b).
(a)
(b)
(a)
(b)
The green and orange lines track the maximum gains of the matched 50 Ω inset-fed (red lines) and unmatched 245 Ω edge-fed (blue lines) copper antennas. The green and orange lines tracking the maximum gains are isolated in Figure 3(b). Note that the unmatched 245 Ω edge-fed antenna demonstrates more gain variability than the matched 50 Ω inset-fed antenna. Thus, for a greater gain tuning range, it was preferable to use an unmatched edge feed.
2.3. Liquid-Metal Antenna: Design and Operation
While the copper-baseline antenna proved that gain tuning is possible, it is not possible to realize the tuning with a static copper stub. Thus, a liquid-metal antenna was designed that uses a liquid-metal slug in place of the copper stub, as shown in Figure 4. The liquid-metal slug is approximately 15 × 1.5 × 0.6 mm3, and consists of a gallium alloy with the trade name of Galinstan. The Galinstan slug has the same dimensions as the copper stub, although it is six times thicker and less conductive (2.3 × 106 S/m for Galinstan [18] versus 5.88 × 107 S/m for copper) than the copper stub.
(a) 0 mm offset state
(b) 2 mm offset state
(c) 5 mm offset state
(d) 7 mm offset state
Gallium-based liquid-metal alloys form a thin oxide skin when exposed to oxygen [18]. The oxidized liquid metal wets to many surfaces, interfering with actuation. To remove this oxide skin, and to enable actuation of the liquid metal by continuous electrowetting (CEW), the Galinstan slug is immersed in a 1 M (4% w/w) NaOH solution [19], which is slightly conductive at low frequencies (~15 S/m [20]) and has a high dielectric constant, εr ≃ 65 [21]. Thus, the simulated results of the liquid-metal antenna are expected to have different trends compared to the simulated results of the copper-baseline antenna.
Figure 5(a) shows the liquid-metal antenna structure. The top four layers form the liquid-metal fixture. The top 12 × 36 × 1 mm3 layer is borosilicate glass (Fisherfinest premium microscope slides) with , and the second and third layers are polyimide (DuPont™ Kapton®) with . The second 12 × 44 × 0.6 mm3 polyimide layer has a 1.5 × 38 × 0.6 mm3 channel for the Galinstan and NaOH. The third 12 × 44 × 0.1 mm3 polyimide layer has a 0.625 mm radius via to allow the Galinstan to contact a 50 Ω copper microstrip feed line below. The fourth 0.1 mm thick polyimide layer serves to level the surface (which has 0.1 mm thick copper on it) for the above layers. The 5 GHz patch antenna is identical to the patch in Figure 1. Figure 5(b) shows the fabricated prototype.
(a)
(b)
In continuous electrowetting (CEW), a voltage applied across an electrolyte submerging a liquid-metal slug induces the motion of the slug [19]. Figure 6 shows the CEW actuation process. The actuation signal only needs to be applied to move the liquid metal; no power is required when the antenna maintains a particular state. Once the Galinstan slug is located at a specific offset state, it remains in position as long as the antenna is not tilted. Although not implemented in this paper, greater stability of the slug position can be achieved by adding minimum-surface-energy states to the channel, as demonstrated in [19].
(a) Empty fixture
(b) 0 mm offset state
(c) CEW actuation
(d) 7 mm offset state
2.4. Liquid-Metal Gain-Reconfigurable Antenna Simulations
Figure 7(a) shows the simulated gains of 15 states for both a matched 50 Ω inset-fed and unmatched 245 Ω edge-fed liquid-metal antenna. The contrast in the gain versus offset in Figure 7 compared to that of Figure 3 is due to the polyimide superstrate, resulting in a change in stub characteristic impedance for the copper-baseline antenna compared to the Galinstan-based antenna. According to Figure 7(b), the matched 50 Ω inset-fed antenna can have its gain tuned from 0.87 dB to 6.25 dB (a 5.38 dB tuning range), and the unmatched 245 Ω edge-fed antenna can have its gain tuned from −6.21 dB to 4.66 dB (a 10.88 dB tuning range). Again, the unmatched feed yields more gain variability than the matched feed. Thus, the unmatched 245 Ω edge-fed liquid-metal antenna was chosen for fabrication.
(a)
(b)
Figures 8(a) and 8(b) show the simulated S11 and the 360° gain pattern corresponding to the four states in Figure 4, respectively. The S11 values of the various states at the operating frequency of 5 GHz are relatively poor for a typical antenna; however, the purpose of this antenna is to maximize gain tunability to increase capacity in a communication system with interference [16]. Thus, although the unmatched edge-fed antenna results in these less-than-ideal S11 values, it was chosen over the matched inset-fed antenna due to the larger range of obtainable gain values. The unmatched edge-fed antenna has radiation patterns typical of a patch antenna, with the maximum gain at 90°.
(a)
(b)
3. Experimental Results
A 60 Hz, 1 Vpp, 75%-duty-cycle signal reliably actuates the 15 mm Galinstan slug back and forth. It takes 2.23 s to actuate the slug from the far left (−7 mm offset state) to the far right (7 mm offset state), a speed of 3.14 mm/s. Slightly overfilling the channel with NaOH significantly boosts the actuation speeds to 19.44 mm/s. Greater voltages can yield higher speeds, e.g., a 60 Hz, 3 Vpp, 75%-duty-cycle signal actuates the slug at 35.0 mm/s in a channel slightly overfilled with NaOH.
Since the actuation was manually controlled by the user, a slower actuation speed was desirable to move the Galinstan slug in submillimeter increments. Thus, the 3.14 mm/s actuation offered by the 60 Hz, 1 Vpp, 75%-duty-cycle signal in a nonoverfilled channel was used.
Figure 9(a) shows the measured and simulated gains of all 15 considered states for the unmatched 245 Ω edge-fed liquid-metal gain-reconfigurable antenna in Figure 5(b). Once the Galinstan slug is positioned at the desired offset state, the actuating electrodes are detached, leaving the slug in place. Gain measurements were then taken with a horn antenna rotated at a constant distance greater than 10λo from the test antenna within the measurement planes indicated in Figure 2.
(a)
(b)
There is strong correlation between measurements and simulation. According to Figure 9(b), the liquid-metal antenna can have its gain tuned from −5.90 dB to 4.43 dB (a 10.33 dB tuning range). This is close to the simulated tuning range value of 10.88 dB discussed previously.
Finally, note that by symmetry, the –-mm-offset state should mirror the performance of the -mm-offset state.
4. Conclusion
This paper demonstrated the first low-voltage electrically actuated liquid-metal antenna with a >10 dB analog gain tuning range (−5.90 dB to 4.43 dB). In practice, it may be necessary to replace the straight channel with a channel that has discrete surface energy minima, such as a channel that consists of interlocking circles [19] or a rail and notch scheme [11, 22]. This can help to hold the liquid metal in place against unintentional motion, although this sacrifices analog tunability for enhanced positioning precision and robustness.
Conflicts of Interest
The authors declare that they have no conflicts of interest.
Acknowledgments
This work was supported in part by the National Science Foundation under Grant no. ECCS-1546980.