# A Review on Reconfigurable Liquid Dielectric Antennas
## 2 Liquid-Based Antennas
### Antenna Geometry Variations
When one of them was filled with ethyl acetate (\(\epsilon_{r}\!=\!6.6\)), the resulting polarization was RHCP. When the other container was filled instead, the polarization changed to LHCP. The antenna operates in the range 2.31-2.72 GHz with the radiation efficiency of \(>\)70%. In Reference [65], another compartment geometry was proposed for switching between the RHCP and the LHCP. Measurement results showed that this antennas can achieve a CP reconfigurability in dual band 1.55-1.72 GHz and 2.29-2.52 GHz with wide bandwidths of 10.4% and 9.7%, respectively. In Reference [66], another compartment geometry was proposed to for a wide-band frequency reconfigurable antenna based on pure water. A bow-tie feeding slot is loaded with a central square acrylic container which is surrounded by another eight square acrylic containers. By changing the height of the liquid in these containers, different frequency states can be achieved in the range of 168-474 MHz. A high efficiency of more than 80% was achieved. The authors also analysed the effect of the slot size, ground plane size, and temperature on the radiation characteristics of the antenna. In Reference [63], an optically transparent patch antennas was realized with pure water used for both the patch and the ground. The antenna was excited by a small disk loaded probe positioned in the air between the patch and the ground as illustrated in Figure 4e. It was demonstrated that the usable frequency band can be shifted by changing the height of the ground. In a follow up work [67], the authors modified the previous design by adding an annular water ring around the disk-loaded probe. This addition helped to reduce the size of the antenna (lower operating bandwidth) by effectively introducing an additional shunt capacitance between the patch and the ground. In Reference [68], a low-profile T-shape water based antenna was fed by a small rectangular probe. Two parasitic water-based elements were added on both sides of the fees to improve the impedance matching. A 34% impedance bandwidth was achieved from 3.18 GHz to 4.47 GHz with the radiation efficiency up to 80%. In Reference [69], a water patch was excited by an L-shape probe. Even though the authors did not show the tunability of the system, from the model it is expected that the resonance frequency of the antenna will shift down with the gradual introduction of water. From the perspective of the paper, the main advantage of such an antenna is its transparency which allows its seamless integration with solar cells. Figure 4: Liquid dielectric based antennas of various geometries. (**a**) [54; 55], (**b**) [56; 57; 58; 59; 60; 61], (**c**) [53], (**d**) [62], (**e**) [63]. In Reference [70], a dipole was loaded with a water-based bow-tie shaped structure to lower its electrical length. By applying water-based bow-tie structures of different dimensions, frequency tuning within 0.9 to 1.4 GHz can be achieved. The authors also compared the difference between the pure (\(Im(\epsilon)=2.4\)-6.2) and sea water (\(Im(\epsilon)=40\)-105). From the simulations, the radiation efficiency of the pure water-based antenna is stable throughout the whole frequency range and above 95%, while for the sea water-based one the efficiency changes from 75% to 90%. In this work, the water-filled bow-tie structures of different dimensions were fabricated separately and then installed one after another. It is a very slow reconfiguration process, and a more practical one could be to use a flexible PDMS-based water holder which can be gradually stretched. Table 3 summarises the performance of the liquid-based antennas with various geometry designs. It can be seen that liquid dielectrics can be used to realise many types of antenna geometry that provide discrete or continuous frequency tuning, radiation pattern and polarization reconfiguration.