# A Review on Reconfigurable Liquid Dielectric Antennas
## 1 Introduction
The change in a material is achieved by changing its permittivity or permiability under different voltage levels (liquid crystals) or a static applied electric/magnetic field (ferroelctrics/ferrites). Liquid crystals are widely used at optical frequencies, however their property of changing the dielectric constant can be used for reconfigurable antennas at microwave frequencies as well [6]. Their power consumption is relatively low, however, they have to be kept at the temperature range between 20 \({}^{\circ}\)C to 35 \({}^{\circ}\)C in order to stay in the liquid crystal state [6]. The major advantage of the ferrite-based reconfigurable antennas is their high permittivity (\(\epsilon_{r}>10\)) and permeability (\(\mu>1000\)) values that allow for antenna miniaturization and wide tuning range. However, the main disadvantage is the relatively complex biasing network  
Figure 1: Categorization of antenna reconfiguration techniques [4]. required to achieve the maximum tuning range in bulk material and higher DC power consumption [6]. Compared to ferrites, ferroelectric thin films require less bias voltage and have smaller losses [6]. Liquid metal-based reconfigurable antennas are widely used for flexible, stretchable, and wearable electronics and sensors. They offer intrinsic reconfigurability and conformability, as well as high electrical conductivity and low loss. However, they have a relatively low switching speed, limited power handling, and repeatability issues [9]. The research in liquid metal application for reconfigurable antenna is relatively new, however several review papers and books have been already written [9; 11; 12; 13]. Liquid dielectric antennas have drawn a significant amount of attention for a number of reasons, (1) conformability: any antenna shape can be achieved due to the nature of liquid; (2) reconfigurability (both physical and chemical): it is easy to change the resonance frequency and bandwidth by changing the height/width of the liquid stream and the chemical composition; (3) low cost: liquid dielectrics are cheap and easily available compared to more costly liquid metals (e.g., mercury (Hg) or eutectic gallium indium allow (EGaIn)), (4) transparent and biocompatible, (5) high permittivity which helps to miniaturize antennas. Besides antennas, liquid dielectric materials have been applied in the field of metamaterials and metasurfaces [14; 15; 16; 17], for the design of reconfigurable frequency selective surfaces (FSS) [18], absorbers [19; 20; 21; 22; 23], sensors [24], reflect-arrays and array lenses [25], and polarization converters [18]. However, these topics fall beyond the scope of the present review and the details can be found in the review papers [15; 26]. There are a few books [1; 27] and several review papers published [4; 6; 14; 28; 29; 30] up to date. They provide comprehensive overview and comparison of modern antenna reconfiguration techniques. However, a systematic review on liquid dielectric reconfigurable antennas is still missing although it is an important promising category. To the best of our knowledge, the only review covering liquid dielectric reconfigurable antennas was done in Reference [31], however it was not comprehensive enough. In the present review we summarize the recent progress on this liquid dielectric reconfigurable antennas. The specific focus of this review paper is to systematise and analyse the research on active antennas that utilize liquid dielectric for radiation reconfiguration (both frequency, pattern, and polarization). It should be noted that even though this review is focused specifically on reconfiguration of active antennas by means of a liquid dielectric(s), the concepts discussed here can be applied to other electromagnetic applications such as sensors, filters, arrays, frequency selective surfaces, and meta- surfaces.