# A Review on Reconfigurable Liquid Dielectric Antennas
## 2 Liquid-Based Antennas
### Basic Design  
The most common water-based antenna design is a monopole antenna shown in Figure 2. Such antenna typically consists of a cylindrical holder mounted on a dielectric substrate, a feed protruding through the substrate into the holder, and water deposited into the holder. By varying the water height in the holder, the resonance frequency of the antenna can be modified. By modifying the dimensions of the holder, the substrate, and the feed, the radiation parameters of such antennas can be optimized. Many modifications of this basic design have been proposed in the literature and will be discussed here. It is considered that [PERSON] was the first one to propose a so-called \"ionic liquid antenna (ILA)\" in 2002 [42]. The website [42] is no longer supported, but for those interested the data can be recovered using an internet archive tool [43]. The first scientific publication on the topic did not appear until 2005, when [PERSON] and [PERSON] presented their work at a conference [37] and later published a letter in 2006 [38]. In References [37; 38], a water-based monopole antenna was proposed, where saline water was used as the dielectric material. A monopole antenna was constructed using a polyvinyl chloride (PVC) tube mounted vertically on a ground plane. A round steal electrode of diameter 0.65 mm was protruded through the base of the tube and connected to an sub-miniature A (SMA) connector. It was found that with the increase in the salinity of the water from 0 ppt to 0.8 ppt, the bandwidth of the antenna increases from 10% to 16%. However, by further increasing the salinity of the water up to 6 ppt and further, the efficiency of the antenna reduces significantly and it essentially stops resonating. The ratio of the tube diameter to the water height affects the operating frequency of such a resonator. In Reference [39], the authors studied the performance of a DRAs based on pure water as a function of the water height. By changing the water level in a cylindrical \(550\times 300\) mm\({}^{2}\) plastic holder from 134 mm to 175 mm, the resonance frequency changes from 50 MHz to 96.8 MHz. The authors confirmed that the frequency behavior agrees well with the analytical Equation (4) for the HEM\({}_{11\delta}\) (magnetic dipole) mode in a cylindrical half-space DRA [44]. \[f_{0}=\frac{19.972\times 10^{8}}{2\pi a\sqrt{e_{r}+2}}\left[0.27+0.36\left( \frac{a}{2h}\right)+0.02\left(\frac{a}{2h}\right)^{2}\right], \tag{4}\]  
where \(h\), \(a\), and \(e_{r}\) are the height, radius, and permittivity of a cylindrical dielectric, respectively, and \(f_{0}\) is the resonant frequency of the corresponding HEM\({}_{11\delta}\) mode. In Reference [39] the authors also found that the feed size is an important parameter, that is, the feed should be shorter than the water depth \(h\) and comparable to \(h\) for high and low \(h/a\) ratios, respectively. Later on, [PERSON] et al. further investigated the sea water monopole antenna in terms of the effects of the dielectric layer between the water and the ground and the conductivity of the water [45]. It was found that the dielectric layer has a significant impact on the resonant frequency and radiation efficiency of such an antenna, that is, smaller permittivity and higher thickness will provide better  
Figure 2: Schematic of a basic water-based monopole antenna [37; 38; 39; 40; 41]. radiation efficiency. They also further studied the effect of the water conductivity (the percentage of salt). It turned out that depending on the conductivity of the water, such antenna can be regarded as a dielectric antenna, a conducting antenna or a combination of the two. In a following paper [40], the authors further investigated the radiation characteristics of this antenna with respect to the ambient temperature, diameter and height of the water layer, and the size of the feed and the ground plane.