# A Review on Reconfigurable Liquid Dielectric Antennas
## 3 Liquid-Assisted Antennas
### Cavities  
Another approach to changing the effective permittivity of the substrate is to introduce a cavity into it, which can be filled with a dielectric material. The substrate then can be rigid or flexible. For example, in Reference [91], a rectangular cavity between a patch antenna and the ground was filled with a low-loss transformer oil (\(\epsilon_{r}=2.24\), tan\(\delta\) = 0.005). By varying the height of the oil in the cavity, the volume ratio of air to liquid (oil) changes, as illustrated in Figure 6a, thus changing the effective permittivity of the substrate. The achieved tuning range is from 1.42 GHz to 1.96 GHz with a high radiation efficiency due to the low-loss transformer oil. In Reference [92], a planar spiral antennas made of liquid metal incorporated into a flexible PDMS membrane was proposed. The membrane is positioned over a rigid planar ground plane and fixed with an annular FR-4 substrate. A microstrip feed is coupled to both arms of the spiral. When the air pressure under the membrane changes, it inflates the structure to achieve a spherical cap, as illustrated in Figure 6b. A tuning range of 526-542 MHz was achieved with this technique. However, the efficiency of such antenna is very low at the planar state, with the maximum efficiency of 55% reached at the fully inflated state. In Reference [93], a millimeter-wave microstrip patch antenna was printed on an ultrasoft elastometric polydimethylsiloxane (PDMS) substrate with a membrane, such as in the illustration on Figure 6c. The technological feasibility of PDMS membranes as millimeter-wave antenna substrates was previously demonstrated in Reference [97]. The main technological difficulty of fabricating metal patterns on PDMS is its high thermal expansion coefficient and weak adhesion to metal [98; 99]. Thus, PDMS cannot be used in conventional evaporation and sputtering metallization techniques. In Reference [97], the authors suggested a transfer technique with the help of a Ti + SiO\({}_{2}\) adhesion layer that mitigates the aforesaid issues. With the help of pneumatic actuation, air can be introduced into the cavity and thus change the distance between the patch and the ground. By changing the distance between the ground and the patch, a continuous change in the resonant frequency can be obtained, that is, from 55.35 GHz to 51 GHz (8%) with the dimensions as in Reference [93]. In this experiment, the cavity was filled with air using an electronic syringe pump such that the total maximum air volume was 55 \(\upmu\)L. The matching level of less than \(-17\) dB and stable radiation characteristics were achieved  
Figure 6: Planar antennas with a cavity. (**a**) [91], (**b**) [92], (**c**) [93; 94; 95; 96]. at all resonance positions [93]. A \(4\times 2\)-element microstrip antenna array based on this technology was also demonstrated [100]. However, the authors did not provide any data on the stability of the electrical conductivity of the patch antenna after several cycles of inflating/deflating. In Reference [94], a similar antenna design was suggested. In this work, multi-wall carbon nanotubes (MWNT) were used to facilitate the adhesion of gold to the PDMS substrate. A \(16\times 16\) mm\({}^{2}\) square patch antenna was simulated and fabricated. When air was injected into the cavity, the distance between the patch and the ground changed, resulting in the frequency shift from \(6.1\,\mathrm{GHz}\) to \(5.7\,\mathrm{GHz}\). Moreover, the authors conducted stretchability tests where the resistance of the patch was measured at each height change as the air was injected into the cavity to verify the repeatability of the experiment. Similar to Reference [93], the resonance of a patch antenna can be changed by reshaping the the volume of the dielectric between the patch and the ground [95]. In Reference [95], [PERSON] et al. came up with a liquid-encapsulation technique that allows to deposit polymer directly on a liquid surface. The encapsulated liquid can then be deformed using electrostatic force.