# A Compact Circular Rectenna for RF-Energy Harvesting at ISM Band
## 1 Introduction  
Wireless communication has drastically advanced and subsequently increased the availability of RF signals emitted by RF sources such as digital TV transmitters, Wi-Fi routers, mobile base stations, and radio transmitters [1]. The abundant accessibility of RF energy promotes the ideation of RF-energy scavenging. For the past few years, researchers and academicians have been focused on developing green, safer, and self-sustaining technology. Thus, RF-energy harvesting is the best option to design potentially low-cost and batteryless devices. RF antenna, impedance matching circuit, and rectifier are the key modules of an RF-energy harvesting system. The antenna acts as a transducer and absorbs RF energy from the surroundings. Pairing impedance between the transducer and the rectifier is then established by matching the networks of almost all of the energy absorbed by the antenna, which must reach the rectifier without any loss, and then the rectifier changes the received RF energy to DC power [2]. To extract RF energy from the surroundings, a single-band antenna with high power density and a multiband or wideband antenna with low and medium power density are used to increase the efficiency of the rectenna. In [3], air sandwiched between FR4 substrates is used to obtain more gain at 2.45 GHz, and is designed with an open-stub rectifying network to achieve 0.46 V at 0 dBm power input. In [4], an aperture-coupled pi-shaped slot rectenna is proposed to achieve a good PCE rate of 68.83% with a 0.167 V DC output voltage at 2.45 GHz, and 49.90% with 0.236 V DC output voltage at 5 GHz. In [5], a compact wideband rectenna is designed to obtain a better conversion efficiency by 58%, and 1.6 V DC output voltage at 2.45 GHz with 0 dBm power input. A low-power multiband or broadband antenna array can gather surrounding high RF energy from multiple frequencies. However, the wideband or multiband antenna array is usually very large in size, and its complicated construction may causecoupling between antenna parts, thus reducing their efficiency [1]. A monopole antenna to harvest RF energy could be any shape, i.e., rectangular [6], fractal [7], circular [8; 9; 10], Vivaldi antenna [5], etc., and is a good choice to use as a receiving antenna. In [11], a 3D broadband discone antenna is proposed to harvest RF energy from cellular operating bands alongside a Dickson 2-stage voltage doubler rectifier network to charge a storage capacitor of 330 uF with 10 K\(\Omega\) load resistance. Schottky HSMS 2862 diode is used as a rectifying element due to its low voltage drop, low contact capacity, and sensitivity of 35 mV/uW at 2.45 GHz. This rectenna is bulky and not recommended for portable devices. In [12], a high gain CPW fed with grounded broadband cross-mark slot rectenna is designed to absorb RF energy from 2.2 GHz to 2.6 GHz with 13 dBm input power, and 72.5% highest efficiency is observed at 2.45 GHz with 900 \(\Omega\) load. The rectifier circuit is designed between the coplanar waveguide-fed gap, which causes a mismatch of impedance that reduces the efficiency from 72.5% to 50% in the broadband range. In [13], a coplanar monopole rectangular patch with two engraved U-slots is designed to operate at 2.45 GHz and to make the rectenna compact. The rectifier circuit is combined directly with the antenna, which shifts the operating frequency. The trade-off between antenna miniaturization and performance degradation is observed. The smaller antenna has a maximum gain of 0.8 dBi and the measured power conversion efficiency of 20% is observed at \(-\)20 dBm input power with a load of 4.7 K\(\Omega\). For Bluetooth/wireless local area network applications operating at 2.45 GHz, a compact novel coplanar waveguide-fed rectenna with greater efficiency is presented on the FR4 substrate. Impedance matching and bandwidth are improved by using a tuning stub approach with rectangular slots to obtain a peak gain of 5.6 dBi.