# State-of-the-Art Techniques in RF Energy Harvesting Circuits
## 3 Results
### Design Specification
#### 3.2.2 Impedance Matching Network  
The design of an impedance matching network is indispensable for the design of the rectenna because when the impedance of the rectifier does not match the impedance of the antenna, a percentage of the power from the source is not absorbed and is reflected in the environment. Therefore, the impedance of the antenna and the rectifier must be matched at the desired operating frequency so that the impedances are complex conjugates of each other [24]. An ideal circuit of this type for RF energy scavenging must not only provide the desired impedance matching between the antenna and rectifier but must further do so with minimum losses so that the amount harvested can be maximized [25]. The complex source impedance (\(\text{R}_{\text{Source}}+\text{j}\text{X}_{\text{Source}}\)) has to be matched with the load impedance (\(\text{R}_{\text{Load}}+\text{j}0\)). This can be achieved with a matching network (\(-\text{j}\text{X}_{\text{Match}}\)), which has equal and opposite reactance from the source. In this way, the opposite reactance gets canceled thereby matching the source and the load (assuming \(\text{R}_{\text{Source}}=\text{R}_{\text{Load}}\)). Theoretically, if the impedances of the source and the load are completely matched, we will have a transfer without losses, all the energy from the source will be transferred to the load. Therefore, it can be understood that the IMN makes a condition in which we have left only pure resistance value at the source and the load. This can be achieved if the load impedance changes into a complex conjugate of the source impedance. At this point we have to say that the circuit's reactance depends on frequency, hence the impedance match between the source and the load will be achieved at a specific frequency. The frequencies in which the impedance is matched determined by the \"\(Q\)\" factor of the matching network, which depends on the network design. The Bandwidth is given by the following equation, in which \(BW\) is the Bandwidth, \(F\) is the operating frequency and \(Q\) is the measure of energy stored in reactance to that being dissipated. \[BW=\frac{F}{Q} \tag{1}\]  
In many applications, due to the limitation of small sizes of PCBs, used antennas with higher \(Q\). Thus, the design of a high \(Q\)-factor IMN for a high \(Q\) antenna can display performance problems that are caused by the matching components. Therefore, it is important to keep the \(Q\) factor of the IMN at low levels. The value of the \(Q\) factor is a key parameter that should be chosen carefully on the design of the impedance matching network. The interested reader may find additional details in [26]. Figure 4 depicts the four most widely used matching networks, which are the L, the reversed L, the \(\pi\), and the T network. The selection process of an adequate adaptation network depends on the intended goal. One of most popular impedance matching networks is the L network. The L network got its name of the positions of its elements, which form an \"L\" shape. There are also different L network variations like as the inverted L section and the reverse L section networks. The L-network is simple and elegant, and it is the key building block of any other impedance matching network design. But it has a limitation, its \(Q\)-factor. That is the source, and the load impedance are determined but the \(Q\) factor is defined. The designers do not have control over the \(Q\) factor's value because its value is a function of the value of the resistance of the series and the parallel components. So, while designing the L-type network the designers have, as a result, a fixed \(Q\), or a fixed bandwidth, which may not achieve the circuit requirements. According to the Equation (1), the \(Q\) factor defines the bandwidth of the matching network, which is a key parameter when we design an impedance matching network. If a wideband matching network is required, an L-type IMN would be sufficient.