Theory behind Microwave Absorbers
It’s important for engineers to know the theory behind microwave absorber sheets so that they can choose the correct product for their particular projects. Using the proper material will prevent interference issues that could damage components or cause signal degradation.
The Science of Microwave Absorbers
Microwave absorbers are made of two main components, a filler material that does the absorption, and a material matrix to hold the filler. The filler controls how and what frequencies the material will absorb, while the matrix provides other benefits such as flexibility, weather resistance, and temperature resistance. Choosing the right microwave absorber sheet depends on both of these components matching your project’s needs.
Different absorption materials are measured by a combination of electrical permittivity and magnetic permeability. Microwave absorbers are filled with dielectric materials. As a microwave strikes these materials, the wave becomes attenuated and lose energy due to heat conversion. The amount of energy lost depends on the frequency of the wave and the dielectric constant of the material. Absorption materials can affect permittivity and permeability in different proportions over different frequency ranges. If an engineer can calculate the permittivity and permeability of an RF absorber over a frequency range, it is possible to completely determine the performance of the material.
Reflectivity in Microwave Absorber Sheets
Another factor in absorption effectiveness is reflection. While there are materials that do allow some of the energy of a microwave to pass through completely, others have a metal backing to contain microwave radiation. This is where thickness comes into play. While not entirely accurate, it can be useful to think of microwaves bouncing off both the front and back sides of the material. Both receive a reduction in frequency due to the absorption material.
However, when a wave strikes the metal backing of the microwave absorber sheet it becomes reflected, turning the phase 180 degrees. If the front-reflecting and the rear-reflecting waves are now equal in magnitude, the waves will cancel each other out. This uses the principle of RF interference to reduce the energy of microwaves.
In order for this to occur, the material needs to be ¼ the wavelength of the frequency it is meant to absorb. Depending on the application, a simple narrowband reflective material can work, such as in an antenna that is operating on a single frequency. However, for broader ranges of frequency materials with different reflectivity can be stacked together to form broadband reflective materials. These tend to be much thicker.
Microwave absorbers can also use an impedance gradient to achieve this same effect. By using a gradient, the microwave doesn’t get a chance to create a large reflection that needs cancellation. A flat sheet could be made up of layers with greater impedance as the wave travels through it. This is called a parameter gradient.
Another form is a physical gradient. In these, the material is homogenous but is shaped in such a way that that the wave encounters more material the further it travels through. Pyramids are common shapes for this type of gradient. The wave encounters the tip and begins absorption, encountering a larger surface area the deeper it passes. This type of material is frequently used in anechoic chambers where no reflection is wanted at all.