# A Review on Reconfigurable Liquid Dielectric Antennas
## 2 Liquid-Based Antennas
### Antenna Geometry Variations  
Due to the nature of liquids, they can take the shape of any given container. This property is widely used in the design of various liquid dielectric reconfigurable antennas. Figure 4 illustrates a few examples of this type of antennas with various shapes. In Reference [54], a thin and flexible tube was used as a liquid holder instead of previously proposed rigid designs. The tube can be freely shaped into a helical antenna of various height and pitch sizes as illustrated in Figure 4a. In their work [54], the authors systematically analysed the effect of various parameters (such as the length of the probe, the substrate thickness and material, liquid height and diameter) on the radiation characteristics of such an antenna where saline water was used as the liquid. A similar design was evaluated in Reference [55] and compared with a metallic helix of an equivalent size. It was found that the impedance bandwidth of the water-based helix is reduced compared to the metal helix from 107.7% to 73.1%, while the maximum radiation efficiency is comparable (92.5% for the water-based one compared to the 98.3% for the metal-based one). It was found that by increasing the number of turns of the water arm, the higher is the maximum realized gain. Moreover, by combining two water-based helical antennas of the same dimensions but opposite wound directions, it is possible to reconfigure the antenna polarization from RHCP to LHCP. With the introduction of a parasitic water cylinder, such water-based monopole antenna can obtain a unidirectional far-field radiation pattern [56], which is essentially a Yagi-Uda antenna. By changing the water height in both cylinders, a frequency range from 1.22 GHz to 2.08 GHz can be covered. An illustration of such antenna is shown in Figure 4b. In another variation of a Yagi antenna presented in Reference [57], a central saline water monopole is surrounded by four parasitic water filled cylinders that can act as reflectors or directors depending on the amount of liquid inside. In Reference [58], a circular monopole antenna was surrounded by three sets of seven resin-based reflectors that can be filled with water to steer the beam into three distinct directions. This antenna operates in the 5.7 GHz band. Similarly, in Reference [59], a sea-water based monopole antenna was surrounded by 12 parasitic cylinders which can be individually filled with water to realize a 360\({}^{\circ}\) beam steering. The proposed antenna operates within the range of 334 to 488 MHz with a radiation efficiency of more than 60%, although no frequency tuning was demonstrated. In Reference [60], a multiple-input multiple-output (MIMO) antenna based on two perpendicular dipoles was coupled with four parasitic elements (Yagi-Uda) that can be loaded with water to increase their electrical length. When not loaded, the parasitic elements act as directors, and when loaded--as reflectors. By combining loaded/unloaded parasitic elements, six distinct pattern configuration can be achieved. In Reference [61], [PERSON] et al. proposed a methodology for analyzing sea-water based monopole Yagi-Uda antenna. It is based on the assumption that the sea-water can be treated as an imperfect conductor. By applying the 3-term theory and voltage-current relation, an analytical prediction of the radiation pattern can be obtained. In Reference [53], a rectangular container was divided into several compartments as schematically illustrated in Figure 4c. Each compartment was gradually individually filled with pure water thus providing a greater degree of frequency tunability from 2.3 to 2.92 GHz. In Reference [62], another compartment geometry was proposed where four quadrants where used as the dielectric load. A schematic illustration of such design is shown in Figure 4d. The water level in each quadrant can be controlled individually, which affects the radiation pattern created by an L-shape probe (LHCP, RHCP, or LP). In Reference [64], two containers were placed near a single probe.