# State-of-the-Art Techniques in RF Energy Harvesting Circuits
## 3 Results
### Design Specification
#### 3.2.2 Impedance Matching Network
But if we have to limit the bandwidth, it is not to be possible to achieve a high \(Q\) matching network with L-type network architecture. In that case, we want a network with three elements that provides an additional freedom degree. Thus, with a network with three elements, the designers can choose any value of the Q factor that is higher than that can be possibly achieved by an L-type network. There are two possible configurations, the \(\pi\)-type network and the T-type network [27]. A common application of the \(\pi\)-network is matching a high antenna impedance to the low rectifier impedance. The T-network got its name because the elements form a T-shape as shown in Figure 4c. The T-network is the dual of the \(\pi\)-network. The T-network can be considered as a series combination of two back-to-front L networks. The performance of the system depends on different features such as the quality of components, frequency, cost, the size, hence we could not select one of these topologies for every application. The networks that are looking to maximize the absorption from the antenna and the output voltage gain have to be designed taking into account the tradeoffs created by the components in the specific circuit [3]. The L matching is frequently used and has only two components, which makes the design and control process match more simple. On the other hand, for example, if a band-pass HF filter needs to be designed, it is better to choose a T or \(\pi\) impedance matching network that achieves wider bandwidth than the L network. In more detail, if we want to design efficient filters, it is important to understand which type of IMN is better for each application. The LPF (Low Pass Filter) configuration appears a low or high impedance at the harmonic frequencies in order to suppress them. If the LPF presents a high-impedance, then it reflects the harmonics to the source, while if it presents a low-impedance, then it diverts the harmonic energy to the ground plane. In high-impedance LPF commonly a T-type network configuration is used, where the starting element is a series inductor. On the other hand, a low-impedance LPF uses a \(\pi\)-type network configuration, the starting element is a parallel Capacitor. Thus, the most suitable IMN type is determined by the load impedance and/or the source impedance at the harmonic frequency. As it is recommended in [26] commonly we use a T-type network to match low-value impedances (<50 \(\Omega\)) and an \(\pi\)-type network to match high-value impedances (>50 \(\Omega\)) [26]. Depending on the circuit requirements, we can create several configurations of impedance matching networks as a combination of the simple types (L, \(\pi\), T), either with lumped elements, such as capacitors and inductors (Figure 5) or with distributed elements, such as stubs and microstrip lines (Figure 4). Distributed elements are preferred at microwave frequencies (3-300 GHz), while lumped elements are generally used at lower frequencies (below 3 GHz). The lumped elements parasitic reactance is avoided in the design and they are operated below the self-resonance frequency [7, 28]. Figure 4: Different types of impedance matching networks with distributed elements: (**a**) L-network, (**b**) Reversed L-network, (**c**) T-network, (**d**) \(\pi\)-network [1]. For instance, in [29, 30] a self-startup RF power scavenging system with storage capability was designed an impedance matching network with lumped elements (LC). The authors in [31] presented two rectifiers, one half-wave rectifier and one voltage doubler rectifier with the maximum efficiency of the voltage doubler up to 70%. To achieve this, they use an L-matching network. In [32] though, a triple-band rectifier with open stubs operates at 1800, 2100, and 2600 MHz with conversion efficiency to reach 35% at an input power of \(-\)20 dBm. The authors in [33] designed an N-stage rectifier. The key research area of this work is the impedance matching network that was used. In addition to the L matching network used, a microstrip line network was designed.