# Low-Power Rectennas in Microwave Wireless Power Transmission
## 2 Low-Power Rectifiers
### Matching Network Enhancement
#### 2.2.1 Robust Matching Network Deployment  
With the increasingly widespread application of wireless power transfer (WPT), rectennas' operating frequency band and input power range requirements are becoming more rigorous. The ambient RF energy constantly varies and may come from diverse sources \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Ref.** & **Diode Type** & **Junction Capacitance** & **Responsivity** & **Frequency Band** & **Rectification Efficiency** & **Technology Readiness Level** \\ \hline \multirow{4}{*}{[45]} & HSMS-285B & 180 fF & 18.25 A/W & 2.4 GHz & 1.1\% @ \(-\)40 dBm & TRL9 \\  & SMS7630 & 140 fF & 18.42 A/W & 2.4 GHz & 2.1\% @ \(-\)40 dBm & TRL9 \\  & Backward Tunnel Diode & 4.5 fF & 21.65 A/W & 2.4 GHz & 14.6\% @ \(-\)40 dBm & TRL8 \\  & NC-TFET & Not Given & 41 A/W & Not Given & Not Given \({}^{\star}\) & TRL4 \\  & MI\({}^{2}\)M & Not Given & 36.8 A/W & Not Given & Not Given \({}^{\star\star}\) & TRL4 \\  & MI\({}^{2}\)M & Not Given & 0.5 A/W & 28.3 THz & \(1.7\times 10^{-8}\) @ & TRL4 \\  & & @THz Band & & \(-\)16.2 dBm & TRL4 \\  & Spin Rectifier & Not Given & 10,000 mV/mW & 2.45 GHz & 7.8\% @ \(-\)27 dBm & TRL4 \\  & & @\(-\)62 dBm & & & & \\ \hline \hline \end{tabular}  
* The rectification ratio of the device is higher than \(10^{5}\) under negative gate voltage, indicating excellent low-power rectifying characteristics. \({}^{\star\star}\) The max nonlinearity of the device is 15.8, exhibiting strong rectification characteristics.  
\end{table} 
Table 1: Key parameters and rectification performance of high-nonlinearity devices. Figure 4: A photograph of the low-power rectifier using a backward diode [45]. such as FM, 4G cellular, ISM, and the latest 5G NR bands. To harvest more energy, it is essential to broaden the operating bandwidth. However, the input impedance of diodes changes with both input power and frequency, posing significant challenges for matching network design. Adaptive power allocation technology divides the rectifying circuit into two sub-circuits. It uses either frequency-selective or power-selective networks to separate and route the input signals to dedicated sub-circuits, significantly reducing the difficulty of impedance matching [50; 51; 52; 53; 54]. For example, Figure 5 shows a rectifier employing this technology that achieves matching over an input power range from \(-10\) dBm to \(30\) dBm, and maintains rectification efficiency above \(30\%\) from \(-7\) dBm to \(30\) dBm [50]. A method using low-power varactor diodes to compensate for input impedance dynamically is proposed in [55], as shown in Figure 6. Based on adaptive power allocation, the dc output of one sub-circuit is used as the reverse bias for the varactor diode, enabling self-adaptive capacitance adjustment according to the input power and improving impedance matching without any external bias components. This circuit achieved rectification efficiency above \(50\%\) over the input power range of \(2.4\) dBm to \(20.9\) dBm. Similar techniques using varactor diodes for improved impedance matching have also been reported in [56; 57]. For broadband matching design, multi-stage transmission line matching is a simple and effective approach [58; 59]. Impedance compression technology was initially introduced to reduce the variation in the effective resistance of tuned RF inverters under load changes [60]. In recent years, this technique has been widely applied to reduce the sensitivity of diode input impedance, extending the bandwidth and operating power range [61; 62; 63; 64; 65]. By employing short-circuited stubs in parallel with transmission lines to form an impedance compression network, the rectifier designed in [65] achieved peak efficiency greater than \(70\%\) over the frequency range of \(1.0\) GHz to \(2.7\) GHz and the input power range of \(-6.5\) dBm to \(13\) dBm (Figure 7).