## 1 Introduction  
There is no doubt that the appearance of the Internet has changed the conception of the world and our lives. Today, there are more devices connected to the Internet than people in the world. With this premise in mind, it seems logical to think that the Internet of Things (IoT) is called to be one of the major future revolutions [1]. Of course, the possibility of interconnecting embedded devices implies that the number of applications in which IoT has a place is practically unlimited. Sensors are going to play a crucial role in this scenario, as shown in Figure 1. They are a way of monitoring and extracting information from the environment that surround us. Evidently, IoT is clearly committed to the use of autonomous and self-sustainable sensors, such as those described in [2; 3; 4]. Power consumption is a key factor in this context, since it is difficult to combine self-sustainabilityand large operation times without the sensor being connected to the power grid. Nevertheless, the constant growth in the number of radiofrequency (RF) transmitters and early studies on the matter [5] have turned RF energy harvesting into a realistic and reliable power source. An extended overview of the topic can be found in [6]. In terms of power, almost all significant bands within the radiofrequency spectrum are located between 80 MHz and 5500 MHz. In particular, special attention should be paid to 800, 900, 1800, 2100, 2400, 3500 and 5200 MHz, where the mobile, WiFi and WiMAX frequency bands can be found. Another relevant power contribution comes from the FM band, which is typically located from 80 to 100 MHz in most countries. However, at these frequencies, the antenna would be too large. Multiband antennas are typically used as the harvesting element. In general, they are easy to design because of their well-known characteristics. Thus, the triple-band antenna proposed in [7] is capable of operating at 2.1 GHz, 2.4-2.48 GHz, and 3.3-3.8 GHz. Frequently, slots or corrugations [8] are made in the structure to achieve multiple resonances or to reduce the wave propagation in a certain part of the antenna. However, this can be a double-edged sword, since the control of the mutual coupling between the slots themselves can be difficult [9]. On the other hand, wideband antennas can be harder to design, but in return, its broadband behavior ensures that the antenna will certainly cover the entire region of interest. Hence, an increasing number of authors are starting to use wideband antennas as harvesting elements. Wideband printed monopoles [10] and printed spirals [5; 11] are the most common types of wideband antenna used in practice. However, the current bottleneck of RF harvesting systems is the conditioning circuit, which is essentially in charge of rectifying the input signal. This is due to the intrinsic consumption of the diodes placed in the circuit. Since they are not ideal, they dissipate a high relative part of the total power for low input power levels. In consequence, the efficiency of a typical half-/full-wave rectifier, defined as the relation between the output DC power and the input RF power  
\[\eta=\frac{P_{out_{DC}}}{P_{in_{RF}}} \tag{1}\]  
is typically under 25% for input powers of less than 1 mW (0 dBm). In [5], a half-wave rectifier circuit is implemented over an array of equiangular spirals. The results show that the efficiency of the rectifier ranges from 1% to 20%, depending on the input power. Some recent and very advanced studies [12] have achieved efficiencies of up to 74% for an input power of 1 mW. They externally feed the circuit in question and use the self-body biasing technique in a CMOS configuration to change the threshold voltage and turn on the transistor more quickly. Nevertheless, no schemes that have reached high efficiencies in the conditioning circuit make use of a true passive configuration; that is, they externally bias the elements of the circuit. The most used configuration for a conditioning circuit is the half-wave  
Figure 1: Some types of sensors used in IoT.