## 7 RF-EH Circuit
### Antenna Configurations
#### 7.1.3 Antennas with Multiple Bands  
The multiband antenna exhibits a wide range of advantageous frequencies suitable for various situations once used in rectenna applications. Even though the antenna receives more power from the surrounding environment when using this method, it is less efficient than the previous method. One of the most difficult challenges for researchers working in the field of radio frequency energy harvesting is the low RF-to-direct-current conversion rate that is necessitated by the low available RF power density. The antenna arrays are ideal in terms of output power because they have a large output area. On the other hand, they are difficult to integrate and take up a significant amount of chip space. In order to increase output power, it may be preferable to use a system that has three or more band frequencies. Consequently, the development of compact multiband circularly polarized rectennas for environmental RF energy harvesting is required [117]. A compact multiband rectenna for WLAN, Wi-MAX, GSM, and satellite communication bands is demonstrated in [117]. The rectenna system is fractal in geometry and has six radiating bands: GSM 1.8 GHz (1.6-2.1 GHz), GSM 0.9 GHz (0.8-1.2 GHz), Wi-MAX 3.5 GHz (3.1-4.0 GHz), WLAN 2.5 GHz (2.2-2.8 GHz), WLAN 5.5 GHz (5.3-6.4 GHz), and 7.35 GHz (7.0-7.8 GHz). In [118], the author describes a dual-band rectifying antenna that can be reconfigured for 5.2 and 5.8 GHz rectification. Using an input power of 15 dBm, the proposed rectenna can achieve maximum conversion efficiency (CE) of 65.2% at 4.9 GHz and 64.8% at 5.9 GHz. The presented design is an example of novel ultrawideband energy harvesters in the UHF and low microwave spectrum with octave and decade bandwidths; this is accomplished by optimizing the dimensions of a broadband impedance matching nonuniform transmission line (TL) [119]. The dual-band slot antenna proposed in [120] has a novel symmetrical structure and is small in size. Antenna feeding is accomplished via a coaxial cable with a structure composed of two metal plates. The printed slot antenna structure consists of a single metal surface on a FR-4 substrate with a thickness of 1.6 mm. In terms of WLAN/WiMAX applications, this slot antenna is ideal [121]. To address the dual-band omnidirectional antenna's lower gain and bandwidth, as well as the challenge of low efficiency, a new CRLH dual-band rectenna has been demonstrated [122]. For the two ISM bands (2.4 and 5.5 GHz), the authors in [123] propose a dual-band microstrip antenna driven by differentially driven microstrip antennas. The proposed dual-band antenna has a peak efficiency of about 90% in either band or is constructed with low-loss materials. In the GSM900 band (890-960 MHz), the antenna has an 8.5 dBi gain. Then. There is the 5.8 GHz antenna, which is compact and has a 10.1 dBi gain [124]. In [125], a circular patch antenna (CPA) with a gain of 8.3 dBi and 7.8 dBi was created to cover the frequency bands of 1.95 GHz and 2.45 GHz. In reference [126], a tree-like receiving antenna with a gain of 2.16 dBi and 1.92 dBi was introduced to operate in the 2.45 GHz Wi-Fi and 3.5 GHz WiMAX bands. Log-Periodic Dipole Array antennas are triangular-shaped dipole antennas [127]. A 3.5 GHz and 5.8 GHz dual-band antenna has been developed and optimized. To maximize the antenna's gain, a genetic-algorithm-optimized rectangular unit cell has been etched on the patch radiating element. A receiving antenna collects energy from two high-gain bands [128]. A high-gain multiband antenna is able to increase the receiving power of the antenna. Various techniques have been reported, such as the air gap technique [129], differential-feeding [130], the aperture-coupled feeding technique [131], and the antenna array technique [132, 133], and are affected either by the large antenna size or design complexity. Incontrast, a small antenna size used in a multiband rectenna enables the increase of the rectenna applications.