You’ve probably heard a bit about microwave absorbers and how they are used to reduce or absorb the energy that is present in a microwave. But what are they exactly? And how do they work? Go ahead, read on.
Simply put, microwave absorbers are special materials, often elastomer or rubber based, which are designed to offer a user-friendly approach to the reduction of unwanted electromagnetic radiation from electronic equipment. They also work well to minimize cavity to cavity cross-coupling, and microwave cavity resonances. When comprised of a silicone elastomer matrix with ferrous filler material, microwave absorbers provide RF absorption performance over a broadband frequency range from 500 MHz to 18 GHz.
Dielectric types, in which the absorbing filler acts on the electric field
Magnetic absorbers, in which the absorbing fillers act on the magnetic field
The microwave absorber itself is considered a dielectric medium, which is an electrical insulator that can be polarized by an applied electric field. When such a material is placed in an electric field, electric charges do not flow through the material as they do in a conductor, but instead, the charges shift equilibrium positions causing dielectric polarization. This creates an internal electric field.
An EMI microwave absorber is filled with dielectric ferromagnetic materials. As a microwave strikes these materials, the wave becomes attenuated and loses energy. The energy loss is due to a conversion from EMI energy to heat energy via phase cancellation.
The amount of attenuation of the microwave is dependent on the frequency and the electrical permittivit, (dielectric constant) and magnetic permeability of the material. The amount attenuation varies by frequency.
There are two general classes of microwave absorbing materials, and they have to do with the frequency range that the products can effectively attenuate.
Broad band: these microwave absorbers, such as Parker Chomerics CHO-MUTETM, operate over a large frequency spectrum, but generally have only low to moderate attenuation.
Narrow band or tuned: these microwave absorbers operate over a much smaller frequency spectrum, however in that range, their attenuation is better than Broad Band absorbers.
There are two general scenarios for microwave absorbing materials:
Cavity resonance is in the near field, very close to the EMI field source. This occurs when signals are trapped / contained within a metal enclosure or compartment. Not only is energy absorbed but induced currents from the field itself are impeded by the absorber.
Near field absorption can take place right on the radiating element. In order to avoid shorting, the absorber must have a very high resistance. Absorber thickness plays heavily in this condition.
At the end of the day, there are many theoretical factors that will determine how well a particular absorber will attenuate in an application.
However the typical approach to an absorber solution is to narrow down the selection of a product and a thickness, and then evaluate these samples in the customer’s specific application through trial and success. Ultimately, it really only matters if the product works for the customer in their application and not what theory says.