**2. Analysis and design of absorbers for the electromagnetic compatibility applications**

Different types of absorbing materials and geometries are used for EMC applications depending on the operating frequency range. Below 1 GHz, ferrite-based materials having dispersive lossy permeability are used in different planar configurations. Moreover, above 1 GHz, electric losses or magnetic losses provided by lossy dielectric or lossy magnetic materials, respectively, can be used. To cover both of the above ranges, hybrid absorbers constructed by combining the ferrite tiles and pyramidal absorbers are a solution. In the higher microwave/THz frequencies, metamaterial absorbers can be used. In the following subsections, a detailed discussion of each category is presented. Note that the required absorber's reflectivity is dedicated by the frequency range and its application. Specifically, the military standard requires 6 dB normal incidence reflectivity for 50–250 MHz frequencies and 10 dB normal incidence reflectivity above 250 MHz. Moreover, for immunity tests, 18 dB normal incidence reflectivity for 80–1000 MHz spectrum is essential. For emission tests in 3-meter chambers, 18 dB normal incidence reflectivity for 30–1000 MHz region and 12 dB at 45° for 30–1000 MHz spectrum fulfill the requirements, while in 10-meter chambers, 20 dB normal incidence reflectivity for 30–1000 MHz spectrum and 15 dB at 45° for 30–1000 MHz range are required [19]. Typical reflectivity of absorbers in different frequency bands can be found in the data sheets provided by the manufacturers, from which an appropriate absorber model can be specified. **Table 1** is given as an example [20]. Finally, depending on the operating frequency range, NRI arch, waveguide, coaxial line, time-domain method, and free-space approach can be used for the performance evaluation of the absorbers [17, 21]. The test procedures are specified in IEEE Standard 1128 [22]. The difference between measured reflection coefficient (S11) with and without the absorber is the reflectivity in the former method. In the latter approach, reflectivity is attained by comparing the received power with that using the perfect electric conductor (PEC) plate [20, 23].
