# A Review on Reconfigurable Liquid Dielectric Antennas
## 2 Liquid-Based Antennas
### Liquid Solutions and Other Combinations  
A combination of a liquid dielectric with a high permittivity solid dielectric powder can help to control the effective permittivity and to form a liquid dielectric with desired characteristics. For example, in Reference [75], colloidal barium strontium titanate (BSTO formula: \(Ba_{0.6}Sr_{0.4}TiO_{3}\), \(\epsilon\sim 500\), \(\tan\delta\sim 0.01\)) was dispersed in hydrotreated naphthenic oil (\(\epsilon=2.1\), \(\tan\delta=0.001\)). This colloidal dispersion was used to provide a frequency tuning for liquid-based monopole antenna with a tuning range between 2.75 GHz to 4.22 GHz. A practical range of the liquid height between 8 mm and 24 mm was found that provides good 2:1 SWR ratio. Outside these limits, the monopole parameters must be changed to maintain an acceptable performance, for example by having a matching network [39] or variable height probe. A combination of two or more liquids can be used to improve the radiation characteristics even further. For example, in Reference [76], a combination of pure and saline water was used to improve the bandwidth of a monopole antenna. In the frequency range from 50 MHz to 300 MHz, sea water can be considered as a good conductor. On the other hand, the pure water can be considered as an imperfect dielectric [46]. As was suggested in Reference [77], a dielectric ring around a monopole antenna can improve its bandwidth. In Reference [78], it was demonstrated that by placing a dielectric ring of pure water around a seawater monopole antenna, the bandwidth can be improved by 73% compared to a single sea water antenna. By changing the shape of the dielectric ring, the bandwidth can be improved even further, resulting in a frequency range from 54.5 MHz to 251.4 MHz (129%). For some applications the high freezing temperature of water can become an issue. That is why some researches are exploring other alternatives such as sea water (mixture of water with salts, for example, NaCl or KCL) and mixing water with liquids that have lower freezing point, such as ethanol. However, ionised water has a significant conductivity, for example, sea water has the conductivity of around 4.7 S/m. Thus, one of the main concerns of using ionised water for liquid frequency tuning is the efficiency [41]. Another solution is to mix water with propylene glycol (PG) which is a kind of commonly used antifreeze that has good solubility in water. The freezing point of water with 5% PG is \(\sim\)3 \({}^{\circ}\)C which meets the lower limit for temperature range of commercial electronics (0-85 \({}^{\circ}\)C). In Reference [79], the complex permittivity of a liquid solution of water and PG was investigated numerically and experimentally as a function of frequency (0-18 GHz), concentration (0\(\sim\)70%) and temperature (\(-\)10\(\sim\)70 \({}^{\circ}\)C). In Reference [80], a DRA is realized with a combination of solid and liquid dielectric materials. The basic monopole DRA is made of a K9 glass (\(\epsilon=6.85\)) and the pure zone can be filled with a liquid dielectric ethyl acetate (\(\epsilon=7.1\)). The inner glass-based DRA is excited in its broadside HEM\({}_{11\delta}\) mode, and it is switched to the conical TM\({}_{01\delta}\) when ethyl acetate is added. This antenna operates in the range of 3.75-5.37 GHz with a 35.5% impedance bandwidth, and with a \(>\)80% efficiency in the first state and 50-63% efficiency in the second state. Table 4 summarises the liquid-based antennas that utilise other liquid types and liquid solutions. By carefully choosing the liquid type, antennas performance can be improved, in particular in terms of the lower operation temperature and losses.