## 7 RF-EH Circuit
### Antenna Configurations
#### 7.1.1 Antennas Single Band  
Single-band antennas in rectenna systems are designed to operate in a single narrow frequency band. Based on the perspective of performance, various parameters are taken into consideration during the design of the single-band antennas. The CP is a crucial parameter to improve the overall rectenna output power by achieving a steady power output from the antenna [78]. Furthermore, the CP antenna is a vital component to improve the overall efficiencies in rectenna systems, as it is able to receive RF energy with minimal loss in any plane. To date, a number of single-band antenna configurations have been investigated to achieve effective CPs. Moreover, the characteristic mode (CM) theory is applied to the design of polarization reconfigurable antennas that can switch between linear, left-hand circular, and right-hand circular polarization via electronic (RHCP). For example, the main radiator in [79] is considered a square patch with an I-shaped slot. It has two orthogonal CMs with the same modal significance and 90 characteristic angle differences at 2.4 GHz [79]. A new CP antenna design proposes a planar dipole array antenna and a center-fed Gysel power divider. The center-fed Gysel power divider fed two uniform linear array antennas with two planar dipole antennas, meandering the branch lines of the center-fed microstrip Gysel power divider, which allowed it to be reduced in size. It produced an excellent CP wave and a highly symmetric structure [80]. Patch antennas with inhomogeneous substrates are proposed in [81]. To control the inhomogeneity of the substrate, it is composed of ethyl acetate and air. Because of the patch antenna's inhomogeneous substrate, excitation of two orthogonal fundamental TM modes with a 90\({}^{\circ}\) phase difference is simple, resulting in CP radiation. Finally, the CP can be reconfigured using the fluidic control method [81] by dynamically changing the substrate's inhomogeneity. At 2.4 GHz band, a dual-port square patch antenna with polarization diversity (PD) is proposed in [82]. It is possible to generate linear polarization, left-handed CP (LHCP), or right-handed CP with the proper control of p-i-n diodes (RHCP). Dual linear polarized antennas can also be created in LP modes by obtaining two orthogonal waves simultaneously. Reconfigurability in both the patch antenna's frequency and polarization are demonstrated. By employing 12 varactors with two independent voltages, a fractional bandwidth of around 40% can be achieved while allowing the user to select from linear (LP) and circular polarization (CP) [83]. A simple, low-cost, and efficient reconfigurable circularly polarized patch antenna array was demonstrated in [84] for the early detection of brain cancer. Measurements of S11 by single, double, triple, and quadruple antenna elements are used to detect the tumor [84]. In [85], a CP was generated using a semiecentric annular dielectric resonator antenna (DRA) and fractal-shaped DRA. CP can now be achieved using a new method for modifying the DRA geometry. For future 5G applications, a low-cost 16-port nonplanar multiple-input-multiple-output (MIMO) antenna system is introduced [86]. Biomedical applications will benefit from a CP ground radiation antenna with a wide AR bandwidth and low-profile properties. The antenna shown here is proposed to work in the ISM band (2.4-2.48 GHz). Coplanar waveguide feed (CPW) is asymmetric, allowing CP radiation to be realized [87]. Helical slot antenna CP radiation is examined in this letter. High-gain omnidirectional coaxial line-feeding CP helix antennas with simple feeding structures based on the radiation of a slot antenna are presented. To improve CP performance, helix- and rectangle-shaped slots are cut into an outer conductor. In addition, a cone-shaped inner conductor is used to match impedance. The proposed antenna in [88] can be fed with a stepped structure instead of redundant feed structures to simplify the feeding of the CP antenna.