## 3 Reflection Loss (\(\mathrm{R_{L}}\)) and Microwave Absorption  
As per the transmission line theory, when an electromagnetic wave of fixed-frequency impinges perpendicularly on the surface of a single-layer absorbing material, the power reflection of the material can be calculated with the reflection loss coefficient, \(\mathrm{R_{L}}\) (dB), given be the following equation [91]:  
\[\mathrm{R_{L}}(\mathrm{dB})=20\log\left|\frac{Z_{\mathrm{in}}-Z_{0}}{Z_{ \mathrm{in}}-Z_{0}}\right| \tag{8}\]  
\[Z_{\mathrm{in}}=Z_{0}\sqrt{\frac{\mathrm{\mu_{r}}}{\mathrm{\varepsilon_{r}}}} \mathrm{tanh}\left[\mathrm{j}\left(\frac{2\pi\mathrm{fd}}{\mathrm{c}}\right) \sqrt{\mathrm{\mu_{r}}\mathrm{\varepsilon_{r}}}\right] \tag{9}\]  
where \(Z_{0}\approx 377\)\(\Omega\), i.e., free space impedance, \(\mathrm{\mu_{r}}\) = complex permeability (\(\mathrm{\mu_{r}}=\mathrm{\mu^{\prime}}-\mathrm{j\mu^{\prime\prime}}\)), \(\mathrm{\varepsilon_{r}}\) = complex permittivity (\(\mathrm{\varepsilon_{r}}=\mathrm{\varepsilon^{\prime}}-\mathrm{j\varepsilon^{ \prime\prime}}\)), f = frequency, d = thickness of the material and c = speed of light. The percentage of microwave absorption and reflection loss, \(\mathrm{R_{L}}\), is governed by the following equation [92]:  
\[\mathrm{Microwave~{}Absorption~{}(\%)}=100-\left[10^{(\frac{\mathrm{R_{L}}}{ 10})}\times 100\right] \tag{10}\]  
The absorption efficacy of any EM absorber depends on the frequency of the EM wave, the thickness of the shield and the relative complex permeability and permittivity of the material. Another factor for enhanced EM absorption is better impedance matching. The condition for impedance matching is given by \(Z_{\mathrm{in}}=Z_{0}\), which means that the impedance at the shield surface should be made close to that of air so as to facilitate the passage of EM waves into the shield [93]. This can be made possible by introducing porosity and also fine-tuning the material properties such as permeability and permittivity. The \(\mathrm{R_{L}}\) value is usually negative, and the larger the absolute value of \(\mathrm{R_{L}}\), the smaller the energy of the electromagnetic wave reflected by the material surface. This indicates that the majority of the EM energy is being absorbed and the material has better absorption efficiency. Table 1 presents a comparison between shielding effectiveness (dB) with shielding efficiency (%), and reflection loss (\(\mathrm{R_{L}}\)) with microwave absorption. For instance, \(-10\) dB shielding effectiveness corresponds to 90% attenuation of incident electromagnetic radiation. Similarly, a \(-20\) dB EMI SE represents 99% blockage. Whereas an \(\mathrm{R_{L}}\) value less than \(-10\), \(-20\) or \(-30\) dB infers that more than 90%, 99% or 99.9% of the incident EM energy is absorbed. Usually, the bandwidth with \(\mathrm{R_{L}}\) values less than \(-10\) dB is considered as an effective electromagnetic absorption bandwidth (EAB) [94]. In practice, excellent microwave absorber materials should showcase an \(\mathrm{R_{L}}\) value less than \(-10\) dB within a broad frequency bandwidth at very low thickness. For commercial applications, a minimum of \(-20\) dB shielding and \(\mathrm{R_{L}}\) values lower than \(-10\) dB over a wide range of bandwidth with minimal thickness is considered as an effective EMI shielding material.