# A Review on Reconfigurable Liquid Dielectric Antennas
## 2 Liquid-Based Antennas
### Feeding Technique Variations  
The basic monopole design shown in Figure 2 can be modified in a number of ways, for example, by changing the configuration of feeding. Figure 3 illustrates several feeding design variations. In Reference [46], a so-called top-loaded probe was used to excite a saline water based monopole antenna, as illustrated in Figure 3a. This probe configuration improves the excitation of the TM mode. An optically transparent monopole antenna was constructed for maritime wireless communications using a clear acrylic (\(\epsilon_{r}\) = 2.7) cylinder. The operating frequency can be tuned over a wide frequency range of 62.5-180.2 MHz with the bandwidth of 26.1-49.2% and radiation efficiency of 50.2-72.3%. In Reference [47], a water-based frequency reconfigurable antenna was realized with a small cone probe loaded with a water container, similar to the one illustrated in Figure 3b. This conical probe was used to excite TE\({}_{211}^{y}\) and TE\({}_{121}^{z}\) modes of a water loaded rectangular holder for omnidirectional beam pattern with a wide bandwidth [48]. By varying the water height, the operating frequency was shifted from 700 MHz to 165 MHz. The efficiency of such antenna varies between 75% to 82% while the bandwidth is around 1.2-6.3%. However, the size of the antenna and the amount of water required for tuning is large. Another variation of a cone-fed monopole antenna was presented in Reference [49] where a larger size conical feed with spiral slots etched on the cone was used to change the radiation field \begin{table}
\begin{tabular}{c c c c c c c c c c c} \hline
**Ref.** & **Antenna** & **Pla-Nar** & **Dimensions** & **Reconfig.** & **Liquid** & **Tuning** & **Frequency** & **TR** & **TX** & **Radiation** \\  & **Type** & & **mm\({}^{3}\)(\(\lambda_{0}\))** & **Type** & **Type** & **Range** & **Equation** & **(1)** & **Equation** & **(3)** & **Efficiency** \\ \hline
[38] & mono- & no & 21 \(\times\) 25 \(\times\) & frequency & saline & contin. & 1.597–1.604 & - & - & 40\% \\  & pole & & 25 (0.13\(\lambda_{0}\)) & & water & contin. & 0.050–0.097 & 64\% & 1.94 & N/A \\ \hline
[40] & mono- & no & 25 \(\times\) 100 & frequency & saline & contin. & 1.40–1.88 & 29\% & 1.34 & 46–58\% \\  & pole & & (0.63\(\lambda_{0}\)) & & & & & & & \\ \hline
[41] & mono- & no & 35.4 \(\times\) 11 \(\times\) & frequency & saline & contin. & 2.29–4.58 & 67\% & 2 & 40–70\% \\  & pole & & 11 (0.54\(\lambda_{0}\)) & & water & \({}^{a}\) & & & & \\ \hline \end{tabular} \({}^{a}\) not explicitly demonstrated.  
\end{table} 
Table 1: Basic liquid-based monopole antennas. from an omnidirectional (no slots) to boresight type (with slots) with a wide frequency coverage from 1.6 GHz to 5 GHz. A low loss ionic liquid (trihexylletradecylphosphonium chloride (TPC), formula: [P(nC6H13)3(nC14H29)][CI]) was used as the dielectric in this work. TPC has a very small electric conductivity of 0.00025 S/m and remain in a liquid state at temperatures as low as \(-69.8\,^{\circ}\)C.  
In Reference [50], the probe was modified by introducing a one-turn spiral extension which helps to achieve a wider bandwidth. An illustration of such a feed is shown in Figure 3c. This feeding structure would be hard to realise in a traditional solid DRAs. Moreover, a metasurface lid was introduced to further improve the bandwidth and circular polarization of the antenna. Choline L-alanine was used as the ionic liquid with a wide liquid range from \(-56\,^{\circ}\)C to \(186\,^{\circ}\)C with a low electrical conductivity. Even though the authors did not demonstrate frequency tuning, it is expected that by changing the height of the liquid such tuning can be realized. In Reference [51], a water-based monopole antenna was fed through a conducting ring with one side connected to the ground plane and the other side connected to a \(\Gamma\)-shape feeding arm. This design allows the incorporation of a matching network which can be tuned to obtain a good reflection coefficient.