## 3 Fundamentals of RF Energy Harvesting
### Components of an RF Energy-Harvesting System
#### 3.2.2 Matching Network  
A matching network is a circuit designed to adjust the impedance between two components, typically between an antenna and a rectifier, to ensure efficient power transfer. It plays a critical role in maximizing energy transfer by minimizing signal reflection and ensuring that the maximum possible RF power is delivered from the antenna to the rectifier. Proper impedance matching enhances the overall efficiency of the energy conversion process and significantly improves the performance of RFEH systems [21; 30]. Traditional matching networks utilize passive components such as inductors and capacitors configured in various L, T, or \(\pi\)-network topologies, as shown in Figure 4. An L-network consists of two reactive components (inductors and/or capacitors) arranged in either a series-parallel or parallel-series configuration. There are two basic types of L-networks: the series L configuration, where an inductor is placed in series with the signal path and a capacitor is placed in shunt, and the shunt L configuration, where the positions of the inductor \begin{table}
\begin{tabular}{p{56.9 pt} p{56.9 pt} p{56.9 pt} p{56.9 pt} p{56.9 pt} p{56.9 pt} p{56.9 pt}} \hline
**Antenna Type and Frequency** & **Antenna Structure** & **Size (mm)** & **Antenna Gain (dBi)** & **Circuit Type Frequency** & **Efficiency (\%)** & **Input Power (dBm)** & **Minimum Sensitivity (dBm)** \\ \hline Logarithmic Spiral Antenna (2-18 GHz) & \(60\times 60\) & 3.2 & Half-Wave Rectifier (2.4 GHz) & 20 & +4 & \(-18\) \\ \hline Linear Tapered Slot Antenna (1.85/2.4 GHz) & \(80\times 40\) & 5.1 & Differential Rectifier (1.85 GHz) & 13 & \(-15\) & \(-22\) \\ \hline Concentric Square Patch (2.4/5.5 GHz) & \(45\times 45\) & 4.0@ 2.4 GHz & Full-Wave Rectifier & \(36\%\)@ 2.4 GHz, 5\% @ 5.5 GHz & 0 & \(-16\) \\ \hline Slotted Patch Antenna (2.45 GHz) & \(35\times 30\) & 5.3 & Half-Wave Cockoroff-Walton & 68 & +5 & \(-21\) \\ \hline Meander Antenna (434 MHz) & \(45\times 25\) & 1.2 & Voltage Doubler & 20 & \(-30\) & \(-35\) \\ \hline Fractal Antenna (900 MHz/ 2.45 GHz) & \(25\times 25\) & 2.5 & Doubler/ Full-wave & \(30\)@ – 10 dBm & \(-10\) & \(-20\) \\ \hline Printed Dipole (2.45 GHz) & \(60\times 20\) & 2.0 & Full-Wave Rectifier & 33 & \(-5\) & \(-18\) \\ \hline \end{tabular}
\end{table} 
Table 3: Comparison among different antennas for RFEH systems [9; 23; 24; 25; 26; 27]. and capacitor are reversed. L-networks are simple to design and are typically used for narrowband applications, where the load impedance is either greater or less than the source impedance [31]. A \(\pi\)-network consists of two capacitors connected in parallel (shunt) with the transmission line, while an inductor is placed in series. This configuration forms a structure resembling the Greek letter \"\(\pi\).\"  
The \(\pi\)-network provides a broader bandwidth compared to L-networks and is effective in high-frequency applications requiring better selectivity and attenuation of unwanted signals. Additionally, it allows for greater flexibility in impedance transformation, making it suitable for dynamic RF environments. A T-network consists of two inductors, connected in series with the transmission path, and a capacitor placed in a shunt to ground at the centre, forming a structure that resembles the letter \"T.\" T-networks are commonly used in wideband applications and provide higher flexibility in impedance matching over a broad frequency range. They also help in filtering out unwanted high-frequency components while maintaining efficient impedance transformation [32]. The selection of an appropriate matching network depends on the application requirements, frequency range, and impedance characteristics of the antenna and rectifier circuit. While L-networks offer simplicity and efficiency for narrowband applications, \(\pi\)- and T-networks provide improved bandwidth performance and greater adaptability for dynamic RF harvesting scenarios.