**Abstract**

Absorbers are one of the key components in the realm of electromagnetic compatibility. Depending on the frequency range of interest, different types of absorbers can be utilized for this purpose. This chapter introduces the analysis and modeling of ferrite-based absorbers for low-frequency applications (below 1 GHz) and discusses the issues encountered in their installation, resulting in air gaps. Later, different kinds of pyramidal absorbers, commonly used in the broadband microwave frequency range (above 1 GHz), are presented, and analytical and numerical approaches for predicting their performance are reviewed. The combination of the ferrite tile and pyramidal dielectric absorbers is also provided. Then, some practical aspects of designing hybrid absorbers, including the influence of carbon loading and matching layer on their performance, are mentioned. Finally, the absorber operating frequency extension to the millimeter-wave spectrum using metamaterial structures or graphene material is presented.

**Keywords:** ferrite absorber, dielectric absorber, hybrid absorber, matching layer, air gaps, absorber modeling, millimeter-wave

### **1. Introduction**

The electromagnetic compatibility of the electronic devices is mainly considered in two ways, measuring radiated emission (RE) or radiated immunity (RI), where the test procedures are specified in different standards [1, 2]. The frequency spectrum of RE/RI tests starts from tens of kHz. Since the absorber technology cannot cover such low frequencies, the concentration of the absorber design goes around 70 MHz test frequencies [3]. Anechoic chambers (AC) in the form of fully or partially covered rooms with electromagnetic absorbers that simulate the open area test site (OATS) are the most common indoor facilities for the electromagnetic compatibility (EMC) tests, where the quality of the installed absorbers influences the precision of the tests [4]. Moreover, there are other types of shielded enclosures such as compact renege rooms, transverse electromagnetic (TEM) cells, and gigahertz transverse electromagnetic (GTEM) cells. Properly lining these test facilities with absorbing materials is a key factor for their expected operation [5–8].

When an electromagnetic wave illuminates an environment, it is reflected, transmitted, or absorbed. An efficient absorber can be realized by minimizing the contributions of the former two components [9]. For covering the test frequencies in different applications, broadband absorbers are necessary. Various approaches are proposed for bandwidth enhancement of the electromagnetic absorbers, multiresonance and multilayered structures being two important groups, realized by merging multiple closely spaced resonances [10, 11]. Broadband absorbers in the EMC field are commonly attained by tapered geometries, such as wedge or pyramidal configurations, or by parameter gradient flat structures to provide gradual impedance matching [12]. In all cases, the metallic backside prevents wave transmission [5]. Unit cell analysis, transmission-line model, homogenization method, finite difference time domain (FDTD) technique, and mode-matching technique are some of the approaches that have been used for the performance prediction of the absorbers [13–18]. In this chapter, the analysis and design of some important types of EMC absorbers are reviewed. The chapter also includes millimeter-wave absorbers for future EMC applications.
