# State-of-the-Art Techniques in RF Energy Harvesting Circuits
## 3 Results
### Design Specification
#### 3.2.1 Receiving Antenna  
The main function of the receiving antenna is to scavenge as much RF radiation-power as possible. The main factors that are taken into account in its design are the reflected power at the input of the antenna, the impedance, the gain, and the antenna efficiency, as well as  
Figure 2: Papers referring to RF energy harvesting from 2010 until 2020.  
its weight and size. Several types exist in antenna design and various materials are utilized. The most common types are Bowtie antennas [8; 9], log-periodic dipole arrays [10], dipole antennas [11], monopole antennas [12], loop antennas [13; 14], Yagi-Uda antennas [15; 16; 17], planar inverted-F antennas [18], dielectric resonator antennas (DRAs) [19], etc. One of the most famous type of antennas for RF EH is the microstrip antennas, due to their comparative advantages [20; 21; 22; 23].  
In [8] we can find a broadband bowtie antenna, which operates as an energy harvester at 845 MHz and 3.5 GHz. The authors in [9] designed a modified printed bowtie antenna, operating in the frequency band of 5G NR mobile commutation networks. The log-periodic antenna in [10] was manufactured to harvest energy from six bands within the frequency range of 650 MHz to 2500 MHz. A compact rectenna with RF-based energy harvesting for IoT smart sensors is presented in [11]. The antenna of this system is a dipole antenna that operates in the range 902 MHz to 928 MHz. In [12] we can find a monopole antenna for ambient RF energy harvesting at the 600-1500 MHz band. The authors in [13] designed a loop antenna for RF energy harvesting at GSM-1800 band. Another loop antenna that operates at dual bands is described in [14]. The authors in [15] manufactured a rectenna using a Yagi-Uda antenna that can harvest RF power of GSM-1800 and UMTS-2100 bands. In [16; 17] we can also find systems that use Yagi-Uda antennas. The first system operates at 2.45 GHz and the second at GSM-900 and GSM-1800 frequency bands. The authors in [18] designed a planar inverted-F antenna operating in the frequency bands GSM-900, GSM-1800. In [19] a dielectric resonator antenna is presented, which operates as an energy scavenger in the frequency band from 1.67 GHz to 6.7 GHz. The authors in [20] designed a differential microstrip antenna for the GSM900 band (890-960 MHz). Stacking two single-port patch antennas back to back gives us a compact dual-port L-probe patch antenna [21]. The measurement results show that there is an efficiency greater than 40% for the whole rectenna. In [22] we can find a dual-band E-shaped (modified) patch antenna, which operates as an energy scavenger at the frequency bands of two different systems; Narrow-band Internet of Things (NB-IoT) and EGSM-900 Mobile Communication Systems. Also, the same authors in [23] designed, optimized, and fabricated a multiband patch antenna that was operating in the frequency bands of LoRaWAN, GSM-1800 UMTS-2100. The proposed antenna showed acceptable performance for RF energy harvesting applications. Table 1 summarizes different types of antennas that have been utilized in RF energy harvesting systems.  
\begin{table}
\begin{tabular}{c c c} \hline
**Reference** & **Design/Type** & **Frequency Bands** \\ \hline
[8; 9] & Bowtie Antenna & 845 MHz, 3.5 GHz \\  & & EGSM-1800 \\  & & (1800 MHz), UMTS (2100 MHz) \\
[10] & Log-Periodic Dipole Antenna & 650 MHz–2500 MHz \\
[11] & Dipole Antenna & 902 MHz–928 MHz \\
[12] & Monopole Antenna & 600 MHz–1500 MHz \\
[13; 14] & Loop Antenna & GSM1800 \\  & & DTV, Cellular radio waves \\  & & GSM-1800 and UMTS-2100 \\
[15; 16; 17] & Yagi-Uda Antenna & 2.45 GHz \\  & & GSM-900, GSM-1800 \\
[18] & Planar Inverted-F Antenna & GSM-900, GSM-1800 \\
[19] & DRA & 1.67 GHz–6.7 GHz \\
[20] & Differential Microstrip Antenna & 890–960 MHz \\  & & GSM-900, GSM-1800, UMTS-2100 \\
[21; 22; 23] & Microstrip Patch Antenna & (NB-IoT), EGSM-900 \\  & & GSM-1800, UMTS \\ \hline \end{tabular}
\end{table}
Table 1: Type of antennas that have been applied in RF energy harvesting systems.