Frequency-Reconfigurable Dipole Antenna Using LiquidMetal Pixels

Reconfigurable antennas offer an adaptive solution in a dynamic communication environment, demonstrating the ability to change radiation pattern, polarization, and operational frequency [1, 2]. Although reconfigurability is typically achieved using PIN diodes [3], varactors [4], or MEMS switches [5], liquid metal has also recently been shown to implement reconfigurable antennas. Recent demonstrations in liquid-metal reconfigurable antennas include monopole [6, 7], dipole [8], planar inverted F [9], Yagi-Uda [10–12], patch [13, 14], and slot [15] antennas. Reconfigurability is achieved either by altering the physical dimensions of the radiating element with liquid metal or by configuring an associated liquid-metal parasitic element. This paper achieves frequency reconfiguration using liquid metal in the form of square pixels. Pixelated antennas have been demonstrated before [16–19], but this is the first paper to implement antenna pixelation using liquid metal. To turn on a pixel, a discrete amount of liquid metal is electrically actuated from a reservoir buried below the antenna. To turn off a pixel, the liquid metal retreats to the buried reservoir [20]. 2. Design


Introduction
Reconfigurable antennas offer an adaptive solution in a dynamic communication environment, demonstrating the ability to change radiation pattern, polarization, and operational frequency [1,2].Although reconfigurability is typically achieved using PIN diodes [3], varactors [4], or MEMS switches [5], liquid metal has also recently been shown to implement reconfigurable antennas.
This paper achieves frequency reconfiguration using liquid metal in the form of square pixels.Pixelated antennas have been demonstrated before [16][17][18][19], but this is the first paper to implement antenna pixelation using liquid metal.To turn on a pixel, a discrete amount of liquid metal is electrically actuated from a reservoir buried below the antenna.To turn off a pixel, the liquid metal retreats to the buried reservoir [20].

Design
2.1.Liquid-Metal Pixelated Dipole.The resonant frequency of a half-wavelength dipole antenna depends on the electrical length of the dipole arms.Using liquid-metal pixels to adjust the dipole length results in discrete changes in the antenna's operating frequency.Figure 1 illustrates the concept.The liquid-metal pixelated antenna is based on the dimensions of a 64 mm long baseline planar copper dipole on a 0.787 mm thick Duroid 5880 substrate, shown in Figure 1(a).The liquid-metal pixelated antenna shown in Figure 1(b) replaces a section of both dipole arms with a 1 × 4 pixel array.The walls of the pixel array are made of polyimide.The top side of the array is covered in polystyrene, and the bottom side of the array is covered in polydimethylsiloxane (PDMS).Adjacent pixels on the top side are interconnected with stainless-steel connectors embedded between the pixel walls.The pixel array connects to the copper section of the antenna through a soldered stainless-steel wire.This is necessary as gallium-based liquid metals such as Galinstan [21] used in this antenna amalgamates with copper, compromising actuation.Although Galinstan reacts with some materials, it has no known adverse effects on the human  International Journal of Antennas and Propagation body [21].The interface between the copper and pixel array is covered in a watertight polymer, which is not shown in Figure 1.

Liquid-Metal Pixel.
A layout of a pixel is shown in Figure 2. Liquid metal moves between the top and bottom reservoirs by applying a voltage on the electrodes.Both electrodes are fed through the bottom reservoir, which is covered with a layer of PDMS.The electrodes are electrically isolated from each other on the bottom side of the pixel.A pixel is considered "on" when the liquid metal is actuated to the top-side reservoir of the antenna.The pixel is turned "off" when the liquid metal is actuated to the bottom-side reservoir.

Liquid-Metal Actuation Mechanism.
Liquid metal is actuated by manipulating its surface tension using continuous electrowetting (CEW) [22].Liquid metal is immersed in a 1 M solution of sodium hydroxide (NaOH), forming an electrical double layer (EDL) at the metal-NaOH interface.
A voltage acting on the EDL creates a surface tension imbalance on the liquid metal.This results in a pressure differential, actuating the liquid metal.Figure 3 demonstrates actuation in an early 4 mm × 4 mm liquid-metal pixel prototype.A 1.2 V square wave with a +1 V DC offset is applied to the electrodes to actuate the liquid metal from a reservoir buried below.The liquid metal is then actuated back to the reservoir by swapping the applied voltage polarities on the electrodes.The pixels built for the liquid-metal pixelated dipole utilize a 3 mm × 3 mm design.The actuation voltage for this design is a 30 Hz 4 V square wave with a +1 V DC offset, which induces a larger actuation force than the 1.2 V actuation voltage used in the 4 mm × 4 mm prototype pixel.The larger force acting on a smaller body of liquid metal within the pixel significantly increases actuation speeds, being able to switch the pixel between the on and off states within 0.03 to 0.09 seconds.

Experimental Results
The antenna is tested by incrementally turning on one pixel on each dipole arm and then measuring the resulting resonance frequency and radiation pattern.The measured resonance frequencies agree with simulated values obtained from an ANSYS HFSS model (Table 1

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As expected, lengthening the dipole by adding liquidmetal pixels on each arm decreases the resonant frequency of the antenna.As the antenna becomes longer, the incremental frequency shift decreases as the inverse square of the antenna length, as expected from the derivative of where f is the frequency, l is the antenna length, and v is the propagation velocity.
The frequency bandwidth and antenna efficiency of the pixelated dipole antenna are compared to those of a planar copper dipole antenna (Table 2).The performance of the pixelated dipole does not deviate significantly from that of the baseline planar copper dipole.The measured radiation patterns are that of a typical dipole antenna, with nulls at θ = 0 °and 180 °in the E-plane and an omnidirectional pattern in the H-plane (Figure 4).The variation in peak gain between the 1-and 4-pixel-perarm cases is approximately ±3 dB.The cross-polarization ratio is between 10 and 20 dB.
The effect of pixelating the baseline dipole was also investigated.Figure 5 compares the measured radiation pattern of the baseline planar copper dipole to that of a pixelated copper equivalent.Both antennas are tested at 2.1 GHz.This figure shows that pixelation of the dipole antenna presented in this paper has negligible effects on the radiation pattern at the resonant frequency.

Conclusion
This paper demonstrates the first implementation of a pixelated antenna using liquid metal.Pixels actuate liquid metal with a 4 V signal to increase the length of a dipole antenna.This allows a pixelated dipole antenna to resonate at 2.51 GHz, 2.12 GHz, 1.85 GHz, and 1.68 GHz.It has also been found that pixelation of the dipole antenna presented in this paper has negligible effects on the radiation pattern at the resonant frequency.

Figure 1 :
Figure 1: (a) Baseline planar dipole antenna to compare to liquid-metal pixelated equivalent.(b) Liquid-metal pixelated dipole prototype filled with liquid metal.(c) Top side with zoom-in of pixels.The left pixel is in the "on" state, with liquid metal present and interfacing with stainless steel placed within the walls between pixels.(d) Bottom side with zoom-in of pixels.The right pixel is in the "off" state.(e) Side view.

Figure 2 :
Figure 2: Layout of pixel in the "off" state.(a) Top side of pixel.(b) Bottom side with liquid metal present.(c) Side view with liquid metal residing on the bottom side.(d) Application of voltage on the electrodes moves the liquid metal from the bottom side to the top side.

Figure 3 :Figure 4 :
Figure 3: (a) Layout of the prototype pixel bottom side with an actuation circuit.(b) Fabricated pixel with an outlined pixel and electrodes.Stainless-steel syringes (not shown) are used to puncture the PDMS.(c) Application of 1.2 V to actuate liquid metal from a reservoir buried below.(d) Liquid metal actuating.(e) Completion of liquid-metal actuation.(f) Swapping voltage applied to electrodes.(g) Liquid metal retreats to the reservoir below.(h) Completion of liquid-metal actuation.

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