## 4 RF Energy-Harvesting Techniques
### Hybrid Energy Harvesting
\begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline
**Aspect** & **Single-Band** & **Multi-Band** & **Narrowband** & **Broadband** & **Passive** & **Active** & **Hybrid** \\ \hline \multirow{3}{*}{Operating Frequency} & Single fused frequency & Multiple distinct frequency (e.g., 2.4 GHz, 5.5 GHz) & Very narrow frequency & Wide range (bunders) & Ambient RF & Ambient RF signals + & Combination of RF, \\  & (e.g., 2.4 GHz, 5.8 GHz) & 900 MHz, 1.5 GHz, 2.4 GHz & centre frequency & Wide range (bunders) & Ambient RF & Ambient RF signals + & split/synthesis, and \\  & & 2.4 GHz & & centre frequency & Wide range (bunders) & Assisted with amplifiers & thermal energy sources \\ \hline \multirow{3}{*}{Typical Antennas Used} & Microwing patch, & Fractal antennas, multi-band patch, & High-Q microstrip & Virvild, spiral, LPDA, & Develop, simple path, & Recontinguishable & Flexible, transparent, \\  & dipole, monopole & multi-band patch, helical & patch, helical, & bowtie, ultra-sideband & Directional antennas & electroistically stream & principle antennas \\  & & antennas, IPFAs & loop antennas & (UWB path) & & & & \\ \hline \multirow{3}{*}{Design Complexity} & Single and optimized & High (requires separate tuning or) & Conplex (requires & & Moderate to high & High (multiple) \\  & broadband matching) & Relatively single & broadband matching & Simple & (additional power) & harvesting units and \\  & & broadband matching) & & and rectification) & & (circuit) needed) & \\ \hline \multirow{3}{*}{Conversion Efficiency} & High at target frequency & Varies across bands, & Very high at & Moderate to high & Moderate (depends on & Higher than passive & Varies based on \\  & & needs optimization & centre frequency & (depending on design) & ambient signal strength) & & RF conditions & source combination \\ \hline \multirow{3}{*}{\begin{tabular}{} \end{tabular} } & Low (only effective at & High (can capture different signals) & Low (best in controlled & Very high (suitable for & Depends on & More adaptable to & Very high (suitable for \\  & & different signals) & frequency environment) & dynamic environments) & environment & power amplification & or hardware and \\  & & & & & & & \\ \hline \multirow{3}{*}{Power Output} & Depends on presence of single RF source & Generally better (due to & & & & & \\  & single RF source & Travelinging source) & Stable RF source & & & & & \\  & multiple sources) & & is constant & & RF availability & & \\ \hline \multirow{3}{*}{Typical Applications} & RFID tag, dedicated & Smart (sidiidi) & IoT devices in & Urban RF networking, & Low-power IoT nodes, & Medical implants, & Remote sensing, \\  & wireless sources & IoT, dynamic & conventional & smart sensors, IoT devices in dense areas & environmental sensors & wearable electronics & biomedical implants \\  & & & & & & & \\ \hline \multirow{3}{*}{Challenges} & Limited flexibility to & Complexity in circuit & & Difficult impedance & & & & \\  & changing environments & and impedance & Limited to known RF & & & & \\  & & matching & source environments & & & & \\  & & & & & & & \\ \hline \multirow{3}{*}{\begin{tabular}{} \end{tabular} } & Typical Antenna & & & & & & \\  & Gain (dB) & 2-4 dB & 4-8 dB & 5-10 dB & 8-15 dB & 2-3 dB & 5-10 dB & 3-8 dB \\ \hline \multirow{3}{*}{\begin{tabular}{} \end{tabular} } & Circuit Performance & & & & & & \\  & (Difference) & 85-90\% & 75-85\% & 90-95\% & 60-80\% & 30-50\% & 60-80\% & 70-85\% \\ \hline \multirow{3}{*}{\
\begin{tabular}{} \end{tabular} } & 20-40 cm\({}^{2}\) & 50-100 cm\({}^{2}\) & 10-20 cm\({}^{2}\) & 100-200 cm\({}^{2}\) & 5-20 cm\({}^{2}\) & 20-50 cm\({}^{2}\) & 50-150 cm\({}^{2}\) \\ \hline \multirow{3}{*}{
\begin{tabular}{} \end{tabular} } & Energy Efficiency & 85-90\% & 70-80\% & 90-95\% & 60-75\% & 30-50\% & 60-80\% & 70-85\% \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Comparison of RFEH techniques.