# A Review on Reconfigurable Liquid Dielectric Antennas
## 1 Introduction
The purpose of this paper is to thoroughly review state-of-the-art reconfiguration techniques for antennas in order to establish a general classification for the reconfiguration techniques which helps to systematically analyse this research field and draw conclusions on the research perspectives, gaps, and limitations. For the review, several parameters were used to evaluate and compare the performance of a frequency reconfigurable antennas. The most obvious one is the frequency tuning range, which can be defined as follows [6]  
\[TR=\frac{2(f_{H}-f_{L})}{(f_{H}+f_{L})}100\%, \tag{1}\]  
where \(f_{H}\) and \(f_{L}\) correspond to the highest and lowest resonant frequencies of a given antenna, where the resonant frequency is defined as the location of the \(|S_{11}|\) minima. On the other hand, the total spectrum defined in Equation (2) can be used to asses the total antenna working range [6]  
\[TS=\frac{2(f_{max}-f_{min})}{(f_{max}+f_{min})}100\%, \tag{2}\]  
where \(f_{max}\) and \(f_{min}\) correspond to the maximum and minimum usable frequencies defined as location of the \(|S_{11}|\leq 10\) dB. Another parameter that can be used to quantify the spread of frequencies is the tuning ratio, as given in Equation (3) [6]. \[TX_{1}=\frac{f_{H}}{f_{L}},\quad TX_{2}=\frac{f_{max}}{f_{min}}. \tag{3}\]  
In terms of the tuning type, frequency reconfigurable antennas can provide discrete or continuous tuning mechanism. A discrete tuning mechanism will only provide a fixed number of resonant states, while in the case of a continuous tuning mechanism, the resonant frequency can be of any value within a tuning range \(TR\). Radiation efficiency is another important parameter to be considered. The radiation efficiency of frequency reconfigurable antennas may vary through the tuning range \(TR\), that is why it is important to provide the radiation efficiency values for the whole \(TR\). Moreover, it is best if the radiation efficiency of a frequency reconfigurable antennas can be compared to that of an equivalent antenna with a fixed operational frequency (not reconfigurable). The radiation efficiency is normally compromised by the reconfigurability technique used. The last parameter is the physical size of the antenna. In this review, the total size in millimeters (mm) and the longest dimension with respect to the shortest wavelength is calculated where possible for comparison between different designs. In terms of the classification, generally there are two types of frequency reconfigurable antennas that use liquid dielectrics for reconfiguration. The first type of antennas, called liquid-based antennas in this review, have a liquid dielectric as the main radiating part, for example, liquid-based monopole antennas. Such type of antenna design originates from dielectric resonators [32], where liquid dielectrics are used instead of solid ones. Liquid-based antennas are discussed in Section 2. The second type of liquid dielectric antennas uses dielectric liquid to locally modify the currents of a metal-based antenna, such as for example, patch and slot antenna. This type of antennas is called liquid-assisted antennas in this review. When most of the liquid-based antennas are three-dimensional antennas that typically require large quantities of liquid dielectric material, the liquid-assisted antennas typically have a planar and compact design and require less liquid to switch between different frequency states. Liquid-assisted antennas are discussed in Section 3. Following Section 3, discussions are presented in Section 4. Lastly, conclusions are drawn in Section 5.