In this lesson we will discuss microwave absorber sheets. Various types of microwave absorber sheets have been developed by different companies, and this article introduces microwave absorber sheets supplied by Murata Manufacturing.
<Microwave absorber sheets based on two principles>
Microwave absorber sheets absorb microwaves using two different principles.
The left side of Figure 2 shows types that absorb microwave energy using the magnetic loss of the materials in the same manner as the ferrite cores introduced in earlier articles. These types have a structure consisting of a magnetic powder (sendust or ferrite) mixed into a sheet made of silicon or other rubber material. Microwaves entering these sheets generate a magnetic field within the magnetic materials, and part of the energy is lost or absorbed when that magnetic field is returned to microwave energy. For this reason, the affected frequencies differ according to the magnetic materials used, but these types are effective for a wide frequency range of several hundred MHz to several GHz.
The type shown on the right side of Figure 2 absorbs microwaves using a different principle. These sheets reflect microwaves from both the front and back surfaces of the sheet as shown in Figure 3. The two reflected waves overlap with each other when they leave the absorber, but a phase difference occurs due to the difference in the distances traveled by each of the waves. At a specific frequency (the frequency with a wavelength equal to four times the absorber thickness), the phases of the microwaves reflected by the front and back surfaces are offset by 180°, with the result that the two waves cancel each other out and the microwave is eliminated. In actual use, the amplitudes of the reflected waves are not exactly the same, so some of the microwave energy remains, but a large proportion is eliminated. As a result, this type of microwave absorber greatly absorbs microwaves of a specific frequency, but does not effectively absorb other frequencies. In addition, due to the relationship between the wavelength and the sheet thickness, these types are effective for the frequency band from several GHz to several tens of GHz.
<Methods of using microwave absorber sheets>
Like ferrite cores, microwave absorber sheets of the type that absorb by magnetic loss are used as EMI countermeasures. These types are generally shaped like sheets, so while they are occasionally wrapped around cables, they are more often attached to boards or inside set cabinets (Figure 4). Users can prepare their own adhesive materials when attaching these sheets, but these sheets are generally purchased with seal-type adhesive pre-applied to the rear of the sheet. In addition, some sheets use materials with high thermal conductivity, so effects as heat radiating materials can also be expected.
On the other hand, types that absorb by using the phase difference of the reflected waves are effective against limited frequencies, so they are used to absorb microwaves of specific frequencies. Figure 5 shows an example of use to prevent microwave interference within a CS converter. In addition, this type of absorber sheet is also used in ETC gate antennas to prevent microwaves from jumping to adjacent lanes.
Written by: Y.M., Murata Manufacturing Co., Ltd.