# Fractal Cardioid Slot Antenna for Super Wideband Applications
## 1 Introduction  
Considering the expansion of mobile communication systems and the forecast that there will be 38 billion connected devices within the IoT and 1.5 billion within 5G by 2025, it is clear that electromagnetic radiated power will be much higher in the future than it is now, as the number of devices connected wirelessly will be greater and it is desirable that antennas for energy harvesting (EH) are designed to cover all bands assigned for those communications. In view of the dependence of the operating frequency on antenna dimensions and the purpose of the antenna, it was concluded that it is optimal to use bands above 1.8 GHz for EH, although the emitted power is higher in the lower bands. Networks such as 3G (UMTs), 4G (LTE) and the future 5G cellular network; ISM (Industrial, Scientific and Medical); and the bands of satellite communications are of particular interest. Electromagnetic energy can be generally collected using rectennas [1, 2, 3, 4, 5, 6, 7, 8, 9, 10] (that consist of a single or multiple antennas integrated with a rectifier) or using meta-surfaces [11, 12, 13]. Numerous studies of the level of electromagnetic radiation in different cities can be found in the literature. The results presented in [2, 3] confirm that bands between 0.3 GHz and 3 GHz could be used for energy harvesting systems and also include the bands planned for 5G and IoT and the bands for satellite communications. The main requirements for energy harvesting and IoT antennas are the following: simple (planar) structures, low cost, miniaturized without gain degradation and efficiency (electrically small antenna), wide operating bandwidth, wide beamwidth and good broadband matching [4, 5]. Printed antennas generally exhibit these characteristics, since they are planar, small in size, have a high degree of reproducibility and since they are easy to make--they do not require complex fabrication processes, and they are compatible with integrated circuitry [6]. However, widespread patch antennas are of the narrowband type. A large number of energy harvesting antennas can be found in the literature, with good characteristics, but they are mostly designed with narrow bandwidth for singlefrequency operation, most often with GSM or Wi-Fi [6]. The second group of antennas includes multiband antennas that are designed on expensive dielectric substrates in order to reduce their overall dimensions, which makes them unsuitable for EH applications in IoT and WSN and for integration with other electronics [4; 6]. With the advancement of the 5G network, the use of the millimeter frequency bands began, especially the K-band (24.25-27.5 GHz); thus, it is necessary to develop systems for collecting electromagnetic radiation in those bands. It would be optimal to satisfy the stated requirements with the use of only one ultra-broadband antenna. A comparative study of the antennas proposed for EH can be found in [4; 5; 6; 7; 8; 9]. Energy harvesting systems in the microwave band can operate with frequencies up to 30 GHz in [10; 14; 15]. An antenna designed for energy harvesting application on satellites is proposed in [10]. Generally speaking, antennas that are designed for energy harvesting can also be used in communication systems. It is desirable that one antenna can cover all services used in mobile communications [16]. In the case of using ultra-wideband antennas in mobile communications, there is often a need for filtering unwanted frequency bands, i.e., adding notch filters to the antenna itself as in [17; 18; 19; 20; 21], which turns this antenna into a multi-resonant one and separates certain bands. Numerous printed antennas designed on a low-cost FR-4 substrate can be found in the literature. The wideband antenna proposed in [22] has very small dimensions (25 mm \(\times\) 25 mm), an operating range of 7.7:1, efficiency of 82% and gain of 5 dBi. It is made on a low-cost FR-4 substrate, but with substrate thicknesses of 1.2 mm, which makes it difficult to integrate with other electronics.