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Microwave Absorber and Its Types

The unwanted emission of electromagnetic waves, especially in high frequency, is very difficult to control. This can affect the functionalities of the surrounding electrical devices. This is when microwave absorber is installed in the circuit to suppress radiation caused by cavity resonance and reduce the effects of radiated signals outside a circuit by minimizing the oscillation. Apart from that, this material can even reduce the reflection transmitted from an object. Absorbers are used in a wide range of applications and in different physical forms. Some of the absorbers can withstand weather and extreme temperatures and yet reduce interference between circuit components.

Absorber Theory

There is a filler inside the material matrix of the absorbers. The constituents of the filler perform the absorbing function. As far as the matrix material is concerned, it is chosen on the basis of its physical properties like temperature resistance, weatherability, and so on. The absorbers are characterized by their electric permittivity and magnetic permeability. Hence, it is important to have detailed knowledge of permittivity and permeability to study the performance of the absorber. Since both these components are functions of frequency, they can vary significantly over even a small frequency range.

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Types of Absorbers

  • Free Space – These types of absorbers are available in two forms – reflectivity absorbers and insertion loss absorbers. The function of the former is to reduce the reflection level and the latter to reduce the signal travelling from one point to another.
  • Reflectivity-narrowband – If the thickness of asingle layer homogeneous material is equal to ¼ wavelength, there will be a resonance because theincoming waves are partially reflected at the time of transmission through the front surface of the material. Next, this wave moves to the back of the absorber undergoing a total reflection and then propagating back through the front face of the absorber. If the wave reflected off the front face is equal in magnitude and same phase as 108 degrees with the wave reflected off the back face, there will be a cancellation of waves, with no reflection at all. Moreover, if the impedance of the wave at the surface of the material is equal to 377 ohms, there will be complete absorption by the material.Salisbury screen is considered as one of the earliest absorbers.
  • Reflectivity-broadband – This can be classified in the following categories:

Multilayer – Some of the absorbers show broadband reflectivity performance because of the stacked-in multiple discrete layers. These enable the 377 ohm input impedance work at the higher range of frequencies. As far as the design of this absorber material is concerned, it is very similar to a quarter wave transformer.

Impedance Gradient – This is yet another type of broadband absorbers that use impedance gradient, and measurement at the front is close to 377 ohms, which reduce to zero ohms at its back face. The chances of reflection are low because there is no abrupt transition layer. Usually, these types of absorbers are found in the shape of a pyramid and offer the best performance due to the outstanding reflectivity. These are used in anechoic chambers.

Another type of absorbers of this nature uses a parameter gradient, which has the capability to work on wide brands. The material used is a flat sheet with varying electrical parameters.

Jaumann Absorber – This is an extension of the Salisbury screen conceptto multiple layers. This is light in weight and resistant to water. As resistive sheets are separated by low loss dielectrics, the performance of the broadband is enhanced. The resistivity of the sheets in a Jaumann absorber decreases from front to back.

All these types of microwave absorbers attenuate the energy in electromagnetic waves and help the devices to perform without any interference. Contact with an electromagnetic shielding solutions provider to know the option that’s best for your need.

Written by Alice Walker