Figure: Few EMC absorbers or RF absorbers shown

EMC absorbers are electromagnetic wave-absorbing materials designed to reduce or eliminate electromagnetic wave reflections or unwanted radiation in various applications by absorbing wave energy and converting it into heat energy. The other names of EMC absorbers are RF absorbers, microwave absorbers, Radar radar-absorbing material or RAM, EMI absorbers, magnetically radar-absorbing material or mag-RAM, EMI suppression materials, or surface wave absorbers. The absorbers are commonly used in anechoic chambers to create a free-space environment inside the chamber by stopping electromagnetic wave reflections through the absorption property. 

Absorbers are used in several applications, including RF anechoic chambers and shielding rooms, preventing TV ghost, radar absorption, protection against electromagnetic phenomena (e.g., lightning), nuclear electromagnetic pulse (EMP) shielding, stealth technology, preventing antenna side lobe,  and EMI suppression for IC, printed circuit boards, consumer electronics, notebook computers, communication equipment, and medical & automotive electronics. 

Figure: Anechoic chamber uses pyramidal absorbers (attached to walls, ceiling, and floor)

A typical absorber is made of a material matrix (base material) loaded with filler materials. The filler material can have either an electrical loss property (electrical type absorber) or a magnetic loss property (magnetic type absorber).  The electrical-type absorber is made of a material matrix loaded with dielectric lossy material (with complex permittivity) and is typically suitable for EM absorption at high frequencies (> 1 GHz). The magnetic-type absorber consists of a material matrix loaded with magnetic lossy material (with complex permeability) and is generally effective for EM absorption at frequencies up to 1 GHz.  The matrix material acts as a base material and decides the absorber's physical properties, such as temperature tolerance, weatherability, etc. Absorbers are designed to achieve excellent impedance matching with free space and high absorption capability, ensuring effective attenuation performance. 

Carbon-impregnated low-density polyurethane foam is an example of an electrical-type absorber. It attenuates incident electromagnetic energy by absorbing wave energy and converting it into heat through dielectric losses in the carbon material. These dielectric losses arise from mechanisms such as conduction loss, interfacial polarization, dipolar polarization, defect-induced polarization, and multi-reflection within the material. Ferrite-based absorbers, on the other hand, are examples of magnetic-type absorbers. They attenuate incident electromagnetic energy by absorbing wave energy and converting it into heat through magnetic losses, including eddy current and hysteresis losses in the ferrite material.

Absorbers are typically designed to withstand extreme weather and temperature conditions. They are available in various shapes, including flat sheets, pyramidal, convoluted pyramidal, wedge pyramidal, and hybrid pyramidal designs. Researchers focus on studying and fabricating different absorber materials and shapes to achieve better absorbers that exhibit properties such as (1) strong absorption, (2) minimized microwave reflection at the air-to-absorber interface, (3) broader bandwidth, (4) lightweight and low thickness,  (5) ignition resistance at very high temperatures, (6) frequency tunability, and (7) transparency and cost-effectiveness.

Ferrite Absorbers:

Ferrite absorbers are electromagnetic wave-absorbing materials made from ferrite compounds such as manganese-zinc (MnZn) or nickel-zinc (NiZn). Ferrite is a ceramic-like material composed of iron oxide (Fe₂O₃) combined with one or more divalent metal oxides. These absorbers function by absorbing incident electromagnetic wave energy and converting it into heat through magnetic losses. As a result, they help to minimize or eliminate electromagnetic wave reflections in anechoic chambers or shielded enclosures, ensuring reliable EMC testing. The ferrite absorbers are particularly effective at lower frequencies, typically between 30 MHz and 1 GHz.

The simplest type of ferrite absorbers is ferrite tiles, which provide absorption levels ranging from 10 dB to 25 dB within the 30 MHz to 1 GHz frequency range. These tiles are available in various sizes, from large (200 mm × 200 mm) to small (60 mm × 60 mm), with 100 mm × 100 mm (approximately 4" × 4") being the industry standard. Their thickness typically varies between 3.9 mm and 19 mm, with the most common options being 6.5 mm and 6.7 mm. Ferrite tiles can be mounted on the walls and ceiling of anechoic chambers or shielded rooms using screws. Some tiles are factory-glued to wood or steel panels. These tiles offer a compact, reliable, and effective solution to reduce or eliminate electromagnetic wave reflections in shielded enclosures.

Figure: White color Ferrite tiles installed in the walls and ceiling of the semi-anechoic chamber

Key benefits of ferrite tiles include being more rugged, less susceptible to fire, humidity, and chemicals, and occupying less space inside an anechoic chamber than pyramidal absorbers.

However, ferrite tiles also have notable drawbacks. Their weight poses a safety risk; a falling tile can cause serious injuries (additional reinforcement is required if they are installed on the ceiling). Another issue is the potential for performance degradation due to gaps between tiles. To ensure optimal performance, these gaps should be minimized. This can be achieved through precise machining of all six surfaces of each tile to ensure a tight fit during installation inside the chamber.

Applications of ferrite tiles/ferrite absorbers:

• Used in RF anechoic chambers and shielding rooms.
• Electromagnetic-wave reflection of buildings
• Prevents TV ghost
• Flexible ferrite tiles/sheets are also available, which are ideal for EMI suppression for IC.

Carbon-based absorbers:

Carbon-based absorbers are composed of carbon nanoparticles embedded in a polymer matrix. These absorbers are typically made by impregnating carbon materials (e.g., carbon black, carbon fibers, carbon nanotubes, graphite, and graphene-based materials) with polymers such as foam, rubber, and thermoplastics. Key properties of carbon materials include great thermal and electrical conductivity,  chemical inertness, mechanical, thermal, and chemical stability in a wide range of temperatures, and high resistance to common corrosive reagents.  

The carbon-based absorbers work by absorbing incident electromagnetic wave energy and converting it into heat through resistive and dielectric losses in the carbon material, thereby attenuating the wave energy.

Examples of carbon-based absorber materials include carbon-impregnated low-density polyurethane foam, carbon-loaded polystyrene, flexible foam impregnated with carbon black and ferrite, and carbon-mixed polypropylene. Carbon-impregnated low-density polyurethane foam and carbon-loaded polystyrene can be used to fabricate pyramidal broadband microwave foam absorbers. The pyramidal absorbers are available to support wide frequency bands of operation up to 500 GHz, and are commonly used in EMC anechoic chambers to reduce or eliminate electromagnetic wave reflections, ensuring accurate EMC measurement results.

Applications of carbon-based absorbers include electromagnetic compatibility (EMC) for electronic devices, anechoic chambers, radar absorption, protection against electromagnetic phenomena (e.g., lightning), nuclear electromagnetic pulse (EMP) shielding, and mitigation of human exposure to electromagnetic fields.

Pyramidal Absorbers:

 Pyramidal absorbers are shaped like pyramids and are designed to absorb electromagnetic waves in the high-frequency range, particularly in the RF and microwave spectrum. They are typically made of materials such as carbon-impregnated low-density Polyurethane foam, carbon-loaded polystyrene, carbon-loaded polyethylene, carbon-loaded polypropylene, or carbon ferrite-impregnated polyurethane foam. 

When an electromagnetic wave encounters the pyramidal absorber, it “sees” a gradual wave impedance transition from free space to the absorbing material. The front face of the absorber has an impedance close to 377 ohms (the impedance of free space) but gradually reduces to zero ohms at the back face. This gradual transition minimizes reflections. At the same time, the lossy properties of the absorber material convert wave energy into heat, attenuating the wave energy and reducing or eliminating electromagnetic wave reflections. The height of the pyramid is inversely proportional to the frequency of attenuation, i.e., smaller pyramids attenuate higher frequencies more efficiently. 

 Pyramidal absorbers are widely used in applications such as anechoic chambers, test boxes, microwave measuring facilities, radar cross-section (RCS) facilities, electronic warfare (EW) test ranges, antenna measurement, and wireless over-the-air (OTA) measurement systems. These absorbers are available in various sizes and colors and support wide frequency bands of operation, extending up to 500 GHz. They provide excellent performance at both normal and off-angle incidences and have high power-handling capabilities.

 Key benefits of pyramidal absorbers include lightweight design compared to ferrite tiles, ease of installation, and safety (as they do not pose a risk if they fall from the chamber wall or ceiling). 

However, a notable drawback is the reduction in chamber volume, as the pyramidal structures extend into the chamber space. This can limit the available testing area, especially in compact chambers. Despite this limitation, their effectiveness in minimizing electromagnetic reflections makes them indispensable for creating precise testing environments. These absorbers are available both in the conventional rigid pyramidal absorber and the hollow pyramidal absorber configurations.

 Hybrid absorbers:                                                                                           

Hybrid absorbers combine the benefits of ferrite and pyramidal absorber technologies, i.e., featuring a ferrite tile base with pyramids on top (see figure). This absorber design ensures a smooth transition from free-space impedance to the lossy ferrite tile base, offering high performance across a wide frequency range, including the lower band and frequencies above 1 GHz. The hybrid absorbers are used in EMC/EMI chambers, mixed-use test facilities, and pre-compliance & full-compliance testing. 

Figure: Hybrid absorber

While these absorbers take less space inside an anechoic chamber compared to pyramidal absorbers, they occupy slightly more space than ferrite tiles. Notable drawbacks include some reduction in chamber space, added weight (necessitating additional ceiling support), potential damage to the pyramid tips, and performance degradation if gaps between tiles are present.

Resistive absorbers:                                                                         

Resistive absorbers typically comprise a top metallic layer loaded with a resistive film, a lossy dielectric substrate in the middle, and a ground plane at the bottom. They are designed to achieve impedance matching with free space, ensuring optimal absorption of incident electromagnetic waves. These absorbers function by converting the energy of incident electromagnetic waves into heat through resistive losses in the resistive film and dielectric losses in the substrate material. The resistive losses occur due to surface currents induced by the incident waves. 

Resistive absorbers are ideal for attenuating electromagnetic radiation and are widely used in applications such as electromagnetic compatibility (EMC), printed circuit boards, and stealth technology.

Recent research focuses on metamaterial-based resistive absorbers that offer superior performance compared to conventional resistive sheet-based absorbers, such as Salisbury screen and Jaumann absorber configurations. Metamaterials are artificially engineered electromagnetic materials that provide broadband microwave absorption with stable angular performance, making them superior in modern applications.

Rubber-Based Absorbers

Rubber-based absorbers are made from natural or synthetic rubber materials embedded with ferrite or carbon particles. These absorbers can be made by mixing ferrite powder or carbon black filler into the rubber material. The ferrite based absorbers reduce or eliminate electromagnetic wave reflections in applications by absorbing incident wave energy and dissipating it as heat through magnetic losses.

The rubber absorbers are thin, flexible, and easy to cut and install, making them versatile for various applications. They are commonly available in flexible rubber sheets and are designed to attenuate electromagnetic waves in the GHz range. These absorbers are available with features such as easy to use, weatherproof, and washable with water. 

Applications of rubber-based absorbers include:

• Anechoic chambers to minimize electromagnetic reflections.
• Used in electronic devices for localized shielding (e.g., smartphone components). 
• Reducing reflections from ship masts and superstructures.
• Microwave (MW) communication equipment.
• Medical equipment to prevent EMI interference.
• Automotive and aerospace EMI suppression.
• Synthetic rubber based magnetic absorber prevents antenna side lobe

Magnetic Absorbers

Magnetic absorbers composed of magnetic materials such as ferrite and iron powder embedded in a flexible or rigid substrate. These materials possess high magnetic permeability and high magnetic loss, enabling them to attenuate electromagnetic radiation by converting electromagnetic energy into heat through magnetic losses.

Magnetic absorbers are available in both rigid epoxy forms and various elastomer forms, including silicone, nitrile, urethane, and neoprene. Examples of these absorbers include flat ferrite tiles, grid ferrite tiles, flexible ferrites, and silicone rubber-based sheets.

Compared to dielectric absorbers, magnetic absorbers are particularly effective at low-frequency ranges. They are also well-suited for specific applications, such as RFID systems operating at 13.56 MHz.

Magnetic absorber applications:

• Cavity Resonance Damping: Highly effective at damping cavity resonances by absorbing electromagnetic waves within enclosures or cavities.
• Near-Field Absorption: Used as near-field absorbers, placed near or directly on radiating elements to protect nearby circuits and components in electronic devices. 
• Microwave System Termination: A rigid magnetic absorber can serve as a load material in terminations for various microwave systems, such as circulators or couplers, to eliminate unwanted signals.
• Synthetic rubber based magnetic absorber prevents antenna side lobe.

Foam absorbers: 

Foam absorbers are made from open-cell or closed foam material, typically loaded with carbon particles. Examples include single-layer "Lossy" foam absorbers, multilayer foam absorbers, reticulated foam absorbers, convoluted foam absorbers, pyramidal foam absorbers, and polyethylene foam-based absorbers. 

Single-Layer "Lossy" Foam Absorber: 

A single-layer "lossy" foam absorber is a lightweight, low-cost uniform absorber sheet that can be made by dipping lightweight open-celled urethane foam into a resistive solution. It works by absorbing electromagnetic energy and converting it into heat, effectively reducing electromagnetic noise. This absorber material can be die-cut into custom shapes and is available with a pressure-sensitive adhesive for easy installation. It is typically supplied in the 3 to 10 GHz frequency range and is commonly used to reduce noise or cavity Q's in microwave components, including Amplifiers, Oscillators, Computer housings, and Wireless equipment.

Multilayer foam absorbers:

Multilayer foam absorbers consist of multiple open-cell foam sheets with stepped carbon loads, increasing progressively from the top layer to the bottom. Each layer is bonded together to form a cohesive structure. The top layer is very lightly doped with carbon to provide effective impedance matching with free space. The subsequent layers are progressively loaded with more carbon, increasing the material's insertion loss from the front to the back. This design provides electrically tapered reflection loss characteristics. These absorbers are lightweight and can be supplied with a bonded-on ground plane and pressure-sensitive adhesive. They are ideal for broadband applications, offering excellent performance across a wide frequency range from 500 MHz to 40 GHz with a reflection loss of -20 dB. 

Multilayer form absorber applications include crosstalk reduction, antenna side lobes reduction, shadowing parts for RCS Measurements, and used in anechoic chambers to reduce reflections from test equipment and other objects in the field of measurement. 

Figure: Foam absorbers

Reticulated Foam Absorbers:

Reticulated Foam Absorbers are lightweight, carbon-loaded sheets typically made with urethane foam as the base material.  These absorbers can be manufactured by spraying conductive coating that is graded through the thickness of the foam. This process creates an electrical grading, resulting in excellent broadband reflection loss performance, covering a frequency range from 1 to 20 GHz. These absorbers provide high levels of attenuation at normal and off-normal angles of incidence. 

Convoluted Foam Absorbers

Convoluted Foam Absorbers are made from urethane foam embedded with conductive carbon particles. The front surface of the absorber is shaped into a convoluted pattern, resembling an "egg crate," to achieve an effective impedance match with incident electromagnetic waves. This unique shape enhances reflection loss across a wide range of incident angles, making it highly effective at microwave frequencies, e.g., from 8 GHz to 80 GHz.

These absorbers are particularly suited for radar cross-section (RCS) and antenna measurement chambers operating in the microwave and millimeter-wave regions. They can be installed on metal surfaces inside test boxes, housings, radomes, network enclosures, or antennae. 

Note: The pyramidal foam absorbers are typically made from carbon-impregnated low-density polyurethane foam and are widely used in anechoic chambers. Polyethylene foam-based absorbers loaded with carbon are commonly installed on the ceilings of Electronic toll collection (ETC) toll gates and other facilities to effectively reduce electromagnetic interference.

Nano-Material Absorbers

Nano-material absorbers are advanced electromagnetic wave absorption materials developed using nanotechnology to deliver superior absorption performance. They consist of a polymer matrix filled with nanomaterial or nanocomposite material (which is a combination of two or more nanomaterials). 

These absorbers are typically manufactured by incorporating nanostructured materials, such as carbon nanofibers (CNFs), carbon nanotubes (CNTs), carbon nanospheres (CNSs), reduced graphene oxide (rGO), or magnetic nanoparticles (e.g., Fe₃O₄), into the polymer matrix. For example, nano-material absorbers can be created using a thermoplastic polyurethane (TPU) matrix filled with carbon nanotubes (CNTs). Carbon-based absorbers attenuate incident electromagnetic energy by converting it into heat through dielectric losses. 

Recently, nanocomposites have played an important role in developing microwave absorbers due to their ability to combine the advantages of individual nanomaterials. These materials offer excellent absorption, lightweight, thin profiles, and a wide operational frequency range. Research on nanocomposite materials for microwave absorbers is going on for designing thin, lightweight, transparent, and frequency-tunable absorbers to meet future challenges. Table 1 provides a few nanocomposite materials and their characteristic. 

Table 1: Nanocomposite materials for electromagnetic wave absorber [Referenced from an IEEE paper titled “ Green Nanocomposite-Based Metamaterial Electromagnetic Absorbers: Potential, Current Developments and Future Perspectives”].

Nano-material absorbers are ideal for applications requiring lightweight, high-performance solutions, such as EMI shielding for integrated circuits (ICs), EMI reduction in electronic devices (e.g., 5G and IoT), radiated EMI management in spacecraft, and stealth technology for military aircraft, naval ships, and other advanced systems.

Conductive Absorbers:

Conductive absorbers are thin films or sheets coated with conductive materials, such as metals or conductive polymers. They attenuate electromagnetic radiation by reflecting and absorbing wave energy, primarily through resistive losses. These absorbers are particularly well-suited for applications like shielding electronic housings, touchscreens, and flexible displays where transparency and flexibility are critical.