# A Review on Reconfigurable Liquid Dielectric Antennas
## 3 Liquid-Assisted Antennas
### Cavities
This method allows to use liquids as a mechanically tunable dielectric layer, instead of air. As liquids have higher permittivity, the antenna size can be efficiently reduced [95]. In Reference [96], a similar idea is realised on a polyurethane (PU) substrate. Compared to PDMS, PUs can be more easily metalized due to a lower thermal expansion coefficient. Moreover, the Young's modulus of PUs is higher, thus PUs are more stiff compared to PDMS which means better mechanical stability but less stretchability [96]. The fabricated patch antenna had the dimensions of \(30\times 30\,\mathrm{mm}^{2}\) and demonstrated a \(32\%\) efficiency and a \(3.9\%\) frequency agility. In Reference [101], it was proposed that by patterning the metallic patch (e.g., as in Reference [102]), the flexibility of such antenna can be improved. In Reference [103], the resonance frequency of a bent dipole antenna was modified by the presence of castor oil (\(\epsilon_{r}=2.7\)) or ethyl acetate (\(\epsilon_{r}=6\)). The antennas was incorporated into a sealed plastic box (acrylonitrile butadiene styrene (ABS) body and clear polycarbonate lid \(\epsilon_{r}\sim 2.7\)). A flexible diaphragm made of a \(1\,\mathrm{mm}\) latex rubber sheet was placed across the box. One of the liquids was pumped in bounded by the diaphragm which expanded as the volume of liquid was gradually increased such that the diaphragm touches the whole enclosure eventually. Due to the presence of the latex diaphragm the antenna parts and the ABS box are protected from the direct contact with the liquid. A frequency tuning range of \(0.9\) to \(1.44\,\mathrm{GHz}\) was demonstrated with ethyl acetate with a high radiation efficiency of \(69\%\)-\(92\%\). Table 6 summarises the liquid-assisted antenna designs that utilize cavities inside the substrate for radiation control. As in this case the liquid can be gradually introduced into the cavity, a continuous tuning can be achieved. \begin{table}
\begin{tabular}{l l l l l l l l l l l} \hline \hline
**Ref.** & \begin{tabular}{l} **Antenna** \\ **Type** \\ \end{tabular} & \begin{tabular}{l} **Pla-Nar** \\ **mm\({}^{2}(\lambda_{0})\)** \\ \end{tabular} & \begin{tabular}{l} **Dimensions** \\ **Deconfig.** \\ **Type** \\ \end{tabular} & \begin{tabular}{l} **Liquid** \\ **Type** \\ \end{tabular} & \begin{tabular}{l} **Tuning** \\ **Range** \\ **(GHz)** \\ \end{tabular} & \begin{tabular}{l} **Frequency** \\ **Equation (1)** \\ \end{tabular} & \
\begin{tabular}{l} **TR** \\ **Equation (3)** \\ \end{tabular} &
\begin{tabular}{l} **X** \\ **Edition \\ **Editionency** \\ \end{tabular} \\ \hline
[91] & patch & yes & \(168\times 168\) (\(1.1\lambda_{0}\)) & frequency & transform-er & contin. & 1.42-1.96 & 31\% & 1.38 & \textgreater{}87\% \\ \hline
[92] & spiral & yes & N/A & frequency & air & contin. & 0.426-0.542 & 24\% & 1.27 & 2–55\% \\ \hline
[93] & patch & yes & \(10\times 23\) (\(4.2\lambda_{0}\)) & frequency & air & contin. & 51.00–55.35 & 8\% & 1.09 & 36–39\% \\ \hline
[94] & patch & yes & \(16\times 16\) (\(0.3\lambda_{0}\)) & frequency & air & contin. & 5.7–6.1 & 7\% & 1.07 & N/A \\ \hline
[95] & patch & yes & \(3.85\times 3.85\) (\(0.3\lambda_{0}\)) & frequency & silicone oil & contin. & 23.28–23.53 & 1\% & 1.01 & N/A \\ \hline
[96] & patch & yes & \(30\times 30\) (\(0.9\lambda_{0}\)) & frequency & air & contin. & N/A & 3.9\% & - & 32\% \\ \hline
[103] & bent & no & \(120\times 65\times 40\) & frequency & ethyl acetate & contin. & 0.90–1.44 & 46\% & 1.6 & 69–92\% \\  & dipole \({}^{a}\) & & \((0.58\lambda_{0}\)) & & & & & & & \\ \hline \hline \end{tabular}  
* data provided here are for the case of the ethyl acetate.  
\end{table} 
Table 6: Planar antennas with substrate cavities.