## 7 RF-EH Circuit
### Antenna Configurations
#### 7.1.1 Antennas Single Band
An innovative CP-configurable liquid dielectric resonator antenna is being studied in this communication--the author in [89] proposed resonator antennas with circular polarization reconfigurability. The fluid dielectric used in this design is Ethyl Acetate (er = 6.6), held in a 3D-printed container excited by one probe. Left and right zones are created in the container in order to implement the CP reconfiguration. A new circularly polarized (CP) origami antenna design has been proposed [90]. It is folded into an origami tetrahedron to serve as the antenna's base. Triangular monopole elements perpendicular to each other make up the antenna's structure. Both elements are excited with equal magnitudes and a 90\({}^{\circ}\) phase difference, whereas circular polarization characteristics can be achieved. A polarization-adjustable reconfigurable eccentric annular slot (EARS) antenna has been suggested. Two identical perturbation slots (PSs) are loaded beneath the eccentric ring-shaped slot to generate orthogonal circular polarization (CP) radiations. In order to switch between LHCP, RHCP, and LP, the proposed antenna incorporates a PIN diode switch across each PS [91]. However, an antenna that can be reconfigured by using gravity to control the self-circulation of liquid metal (LM) is explained in [92]. For reconfiguration, the patch is loaded onto the microstrip antenna with a polydimethylsiloxane (PDMS) structure, which has a square ring channel controlled by gravity [92]. Another unique design has been proposed in [93] with both a DGS-equipped and a nonequipped cylindrical slot antenna integrated with a circular cavity. The antenna's DGS and non-DGS design results are compared extensively [93]. In contrast, Helical antennas are typical antennas with two primary modes of operation: normal and axial. Helix axial mode radiation is most substantial in the direction of the helix axis [94]. Rectennas for energy harvesting, such as directional ones [95; 96], circularly polarized ones [97], and dipole ones [98], are currently being developed in large numbers. To harvest radio frequency energy in the GSM-1800 band, [99] has developed a multiport pixel rectenna comprised of a triple-port pixel antenna optimized for use with a triple-port rectifier. Furthermore, 19% of the efficiency can be achieved when the input radio frequency power is at \(-\)20 dBm. A global rectenna device with an RF to DC conversion efficiency ranging from 16.3 to 45.3%, depending on the amount of harvested power, is created by synthesizing two-port grid-array (TWR) antennas at 2.45 GHz with a coplanar, strip-line-based rectifier, as demonstrated in [100]. Rectenna synthesizers operating at 2.45 GHz can be created using a new four-cascaded element array of orthogonal patches [101]. The rectenna's maximum RF to DC efficiency is 77.2%. Therefore, the use of a single-band, high-gain antenna suits well in the design of the rectenna. The techniques that reported the increase in the gain [102; 103; 104; 105; 106; 107; 108] also increased the rectenna's dimension or the design's complexity. [PERSON] et al. [109] reported that a multilayered structure was the most preferable technique to obtain high gain. In order to broaden the application of the rectenna, the antenna size must be small, followed by the rectenna size.