# A Review on Reconfigurable Liquid Dielectric Antennas
## 2 Liquid-Based Antennas
### Other Liquids  
Even though water is so widely used as a dielectric liquid of choice for frequency tuning, it has a number of limitations such as low radiation efficiency at higher frequency bands (>1 GHz) due to a frequency dependant permittivity [71] and small working temperature range (due to the phase change below 0 \({}^{\circ}\)C and above 100 \({}^{\circ}\)C). These drawbacks limit the application of water-based antennas. However, other dielectric organic solvents can offer smaller loss tangent and lower freezing point. In particular, in Reference [49], an alternative ionic liquid was used as a dielectric loading for a linearly polarized monopole antenna. The liquid of choice was trihexyltetradecylphosphonium chloride (TPC) (formula: \([P(nC_{6}H_{13})_{3}(nC_{14}H_{29})][Cl]\)) which retains its liquid state at temperatures as low as \(-69.8\,^{\circ}\)C and has a small loss tangent of <0.001. Such ionic liquids are meant to replace water as the commonly used dielectric liquid of choice. In the proposed design the ionic liquid lowered the resonance frequency from around 7 GHz to around 4 GHz and improved the fractional bandwidth (from 20.7% to 44%) of a monopole antenna. In Reference [50], another ionic liquid was used called Choline L-alanine. It also has a wide liquid range from \(-56\,^{\circ}\)C to \(186\,^{\circ}\)C with a low electrical conductivity (\(\sigma=0.00021\,\mathrm{S/m}\)). This ionic liquid was used for a liquid-based monopole antenna. 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. Electrical properties of other ionic liquids that can be used for antenna design were systematically studied in References [72; 73]. A recent book [74] on modern functional organic liquids my be helpful for further improvement of the liquid reconfigurable antennas. \begin{table}
\begin{tabular}{l l l l l l l l l l} \hline \hline
**Ref. ** & **Antenna Type** & **Pla-Nar** & **Dimensions mm\({}^{\gamma}(\lambda_{0})\)** & **Reconfig. Type** & **Liquid Type** & **Tuning Type** & **Frequency** & **TR Equation (i)** & **TX** & **Radiation** \\  & & & & & & & & & **Range (GHz)** & **Equation (i)** & **Efficiency** \\ \hline
[54] & helical \({}^{\star}\) & no & \(70\times 100\times 185\) (\(1.5\lambda_{0}\)) & freq., rad, pattern & saline water & contin. \({}^{3}\) & 1.70–2.44 & 36\% & 1.44 & N/A \\ \hline
[55] & helical & no & \(180\times 70\times 10\) (\(1.77A\)) & polariza-ten & pure water & discrete (N = 2) & 1.17–2.95 & 86\% & 2.52 & 40–80\% \\ \hline
[63] & patch & no & \(299\times 299\times 37.6\) (\(3\lambda_{0}\)) & frequency & pure water & contin. & 1.8–3 & 50\% & 1.67 & \(\leq\)82\% \\ \hline
[67] & patch & no & \(154\times 154\times 30.5\) (\(1.2\lambda_{0}\)) & frequency & pure water & contin. & 1.5–2.4 & 46\% & 1.60 & \(\leq\)78\% \\ \hline
[66] & Yagi-Uda & no & \(58\times 32\times 10.9\)(\(3\lambda_{0}\)) & freq., rad, pattern & water & contin. & 1.22–2.08 & 52\% & 1.71 & 75\% \\ \hline
[67] & Yagi-Uda \({}^{\star}\) & no & \(30\times 300\times 20.1\)(\(0.6\lambda_{0}\)) & freq., rad, pattern & saline water & contin. & 0.64–1.45 & 78\% & 2.27 & \(>\)90\% \\ \hline
[68] & circular patch & no & \(60\times 60\times 20.1\)(\(1.5\lambda_{0}\)) & rad. pattern & pure water & discrete (N = 3) & 5.68–5.72 & - & - & N/A \\ \hline
[61] & Yagi-Uda & no & \(102\times 90\times 30\) (\(0.5\lambda_{0}\)) & rad. pattern & sea water & discrete (N = 2) & 1.08–1.55 & 36\% & 37\% & 1.44 & 1.46 & 50\% & 51\% \\  & & & & & & & 1.00–1.46 & & & \\ \hline
[60] & MIMO & yes & \(200\times 20\times 7.8\) (\(0.63\lambda_{0}\)) & rad. pattern & pure water & discrete (N = 0) & 0.85–0.95 & - & - & \(>\)40\% \\ \hline
[62] & loaded probe \({}^{\star}\) & no & \(50\times 50\times 30\) (\(0.9\lambda_{0}\)) & polariza-ten & pure water & contin.