Over the past few years, there has been an indispensable increase in the demand for electromagnetic wave absorbers due to the problem of ‘wave pollution’ being aggravated by the fast-growing wireless industry. With the rapid advancement of the electronic gadget industry, countless electronic devices have become part and parcel of our lives. Almost all electronic devices such as computers, mobile phones, digital circuits, etc., are capable of emitting harmful electromagnetic wave radiations, which can even affect biological tissues and hence become a cause of deadly ‘cancer’ like diseases. This generates the dire need of focusing careful attention on the development of electromagnetic wave interference (EMI) shields which can prevent mankind from its adverse ramifications.
What is Electromagnetic Wave Absorber?
Electromagnetic wave absorbers are the materials that prevent the reflection or transmission phenomenon of electromagnetic radiations that are incident upon them. Instead, these materials absorb those electromagnetic radiations, which further depends upon many factors that determine the level of absorption they exhibit. The absorption percentage varies for different frequencies, thicknesses and type of material.
Types of Electromagnetic Wave Absorbers
Electromagnetic wave absorbers are mainly of two types:
Resonant absorbers: The resonant absorbers are those which depend upon frequency due to the desired resonance of these materials at a particular wavelength. These absorbers ultimately rely upon the material's properties interacting with the incident radiations in a resonant way at a specific frequency. For instance, Salisbury screen, Jaumann absorber, crossed grating absorbers, etc., are resonant absorbers.
Broadband absorbers: The broadband absorbers do not depend on a particular frequency, so they can work effectively across a broad spectrum. These absorbers generally rely on the materials whose properties are independent of frequency and, therefore, can absorb radiations over a large bandwidth.
The typical electromagnetic spectrum is divided over a number of frequency bands and the electronic devices and phenomena operating in those frequency bands. These days, huge attention of researchers is focused on optimizing microwave absorbers as most of the mobile communication devices and the advent of 5G technology are causing major harm in the form of hazardous microwaves in this region which typically extends from 1 GHz to 100 GHz. Due to the fact that microwaves are a subpart of the entire electromagnetic spectrum, so microwave absorbing materials (MAMs) are interchangeably called as electromagnetic (EM) wave absorbers or EMI shielders/protectors.
Working of Electromagnetic Absorbers
Due to the speedy technological advancements in electromagnetic radiation emitting devices like radars, microwave ovens, computers, and medical equipment, etc., a lot of EM wave pollution is being produced, which is becoming a threat to mankind. This is equally hazardous to the electronic devices themselves and has harmful effects on them. Due to this reason, EMI shielding devices are in great demand these days. The phenomenon of shielding basically occurs through two mechanisms:
Absorption of electromagnetic radiations (EMR) by particular particles
Reflection of EMR in a particular direction
However, the first one is generally preferred as it leads to proper attenuation of EMR. The figure below represents the working mechanism of microwave or electromagnetic wave absorbers through the process of absorption, transmission and reflection.
For a material to be a good absorber, there should be zero or negligible transmission and reflection. For achieving less transmission and reflection percentage, a material should have strong dielectric properties, permanent magnetic properties like high saturation magnetization, large magneto-crystalline anisotropy, high permeability, high resistivity, high Curie temperature and low coercivity, etc. Properties such as high dielectric losses, high magnetic losses, and thermal stability are the desired attributes for EMI suppression applications. Although a lot of research work has already been done in this field but the achievement of optimum standards for electromagnetic wave absorbers such as very low reflection loss, wide absorption bandwidth, small matching thickness, and perfect impedance matching, is still a challenging task.
Measurement of Absorption
At high frequencies, reflection and transmission are easy to measure, so scattering parameters (S-parameters) are generally measured in order to determine the absorption capacity of a material. These are measured with the help of different analysers such as vector network analyser (VNA). The VNA measures the frequency response of a component or a network of components. Rather than voltage and current, the analyser measures the power of incoming and returning high-speed signals because the power of signals can be measured more accurately at high frequencies. The processor of VNA then calculates the return loss and insertion loss of the material. The analysed results are available in different formats such as real/imaginary; magnitude/phase; smith chart etc. The range of frequency for which the electromagnetic properties are tested and number of points across that frequency band become inputs to the VNA. The VNA measures the vector response of a high-speed signal to a network by applying a continuous wave at one frequency at a time.
Different Techniques for Microwave Absorption Analysis:
Open circuit technique
In this technique, when an electromagnetic wave is incident on the sample, the wave is either reflected, or absorbed, or transmitted. The material is not blocked by any device so it is named as open-circuit technique.
Here, Incident power = Reflected power + Transmitted power + Absorbed power
The absorption percentage is calculated from experimentally obtained S-parameters from VNA as per the relation:
Short circuit technique
In this technique, the values of reflection loss are simulated at different thicknesses of the sample using experimentally obtained complex electromagnetic parameters. Here, it is assumed that a metal plate is placed on the back side of the material, so, this technique is also called as back-metal plate technique. When an electromagnetic wave is incident on the sample, the wave is either reflected or absorbed because transmission is blocked by the metal plate. Therefore, incident power is the sum of reflected and absorbed power only.
Therefore, Incident power = Reflected power + Absorbed power
The absorption percentage is calculated as:
In this technique, reflection loss minima just correspond to absorption maxima so these terms can be used interchangeably.
Impedance Matching
For perfect absorption, the condition of impedance matching should be satisfied. Reflection loss is calculated as per the relation:
For perfect absorption, there should be no reflection, i.e., Zn → 1. It means that the input impedance of the wave (Zi) should be equal to the characteristic impedance (Zo). Zo is equal to 377 Ω with no imaginary part, therefore, the real part of Zi should be close to 377 Ω and the imaginary part should be close to zero. The values of Zn are calculated and matched with the RL spectra. If the RL dips correspond to the values of Zn which are very close to unity, the condition of impedance matching is said to be fulfilled.
Applications of Electromagnetic Wave Absorbers
Electromagnetic wave (EMW) absorbers have many applications varying for different frequency bands of the electromagnetic spectrum. Some of them are mentioned below:
Wireless Communications: EMW absorbers play an important role in absorbing the harmful radiations emitted in wearable devices, RFID systems, 4G/5G communications, satellite communications, cell phones, Wi-Fi devices, etc., to increase their safe use for mankind.
Microwave Components: EMW absorbers are used in various microwave components such as filters, emitters, amplifiers, and switches, etc., for proper functionality. With the help of absorbers, these components are able to handle extremely high peak power levels.
Energy Harvesting: EMW absorbers are used in solar cells and help to enhance the solar energy collection due to their high absorptivity. The cells are designed in such a way that the surface impedance is matched to free space which results in perfect absorption and hence, maximum energy harvesting.
Sensors: EMW absorbers are used in various types of sensors such as multi-frequency sensors, healthcare sensors, thermal sensors, etc., for different application areas such as cancer detection, glucose level measurements, blood diseases diagnostics, etc.
Consumer and Industrial electronics: EMW absorbers are widely used in the field of Electronics for mitigation of Electromagnetic interference (EMI), enhancement of electromagnetic compatibility of the devices and development of technology related to the EMI shielding.
Defense and Aeronautics: EMW absorbers are largely used as radar absorbing materials in stealth technology such as hidden engines. These are used to lessen the intensity of the reflected signals from the aircraft coverings so that the enemies cannot easily detect defence aircrafts. The absorbers are formed in the form of thin layer absorbing paints and the aircrafts are coated with these paints.
Imaging: Absorbers are needed in the field of imaging such as phase imaging, medical imaging and thermal imaging as different types of electromagnetic waves are employed in imaging purposes.
For diminishing the mutual coupling of closely placed antennas.
For mitigating the harmful effects of cavity resonance in enclosed microwave integrated circuits (MMIC).
As protective shields in microwave ovens to prevent any leakage from the oven to the outer environment.
Future Scope and Conclusion
To recapitulate, according to the facts aforementioned above, one can reach to the conclusion that electromagnetic wave absorbers are being researched for a long time. However, due to the incessant increase in the use of wireless communications and various other upcoming technologies, there will be large occurrences of electromagnetic radiations so the concept of electromagnetic wave absorbers will grow significantly in the near future and their utilization will be enhanced in novel directions. There are many great options for the development of EM wave absorbers such as wide bandwidth absorbers instead of frequency selective absorbers. The concept of a unity absorber with zero reflection can be realized in the future with the use of metamaterials that derive their optimum usage from their geometry rather than their electromagnetic properties.
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