Electromagnetic interference (EMI) is a common electromagnetic pollution phenomenon in the work of electronic, telecommunications, communications, networks, radars and other systems. In the communication field, OTA (Over-The-Air) measurement objects are more developed to small mobile terminals. At the same time, the microwave anechoic chamber, the platform for OTA measurement, has higher and higher requirements for a pure electromagnetic environment. Absorbing material is a kind of material that can absorb electromagnetic waves and reduce the interference of clutter to the system, so it is widely used in various measurement darkrooms and other electromagnetic shielding fields.
There are many types of absorbing materials. According to their electromagnetic wave absorption mechanism, they can be divided into resistive loss type, dielectric loss type and magnetic loss type. The absorbing materials used in the darkroom mainly include absorbing foam and absorbing rubber. The former is hard or soft resistive loss type foam composite material, and the latter is mostly magnetic loss type rubber composite material. However, most foam absorbing materials are pyramid-shaped and large in size, and their absorbing performance is proportional to the height of the material, which limits the space used by the material. The common form of absorbing rubber is the absorbing patch. Due to the flat structure, the material generally has a narrow bandwidth. In order to meet the characteristics of "light weight, thin thickness, bandwidth and strong absorption" of absorbing materials, researchers have done a lot of work, including adopting a honeycomb pyramid structure, adjusting the thickness of absorbing materials or expanding the type and amount of absorbents Bandwidth and improve absorption strength. In addition, the use of resistors to prepare impedance graded structures and metamaterials can also improve the material's absorbing properties. These improvement measures have a certain degree of restriction from the technological and economic point of view.
Carbonyl iron is a commonly used magnetic loss absorbing agent, which is widely used in magnetorheological fluids, inductor cores and electromagnetic wave absorbing materials. The unique onion head structure of carbonyl iron makes it have high electromagnetic performance, and its electromagnetic parameters can be adjusted by processing technology to obtain better absorption effect. The carbonyl iron is dispersed in the rubber polymer matrix, and a small-scale impedance gradient absorber can be prepared through structural design. This structure combines the advantages of the geometric cone of the large foam material and the flexibility of the rubber flat material. At the same time, through simulation and optimization, the obtained wave absorbing material has the characteristics of small size, strong absorption, easy processing, and customization, which can be applied to Measurement environment with limited space.
The electromagnetic performance of the composite material was tested using a sheet with a thickness of 3mm. Figures 3 and 4 show the characteristic electromagnetic parameters of silica gel-carbonyl iron composites (silica gel S+carbonyl iron 1#) with different volume fractions of absorbers. As the volume fraction of the absorber increases from 32% to 40%, the real part and imaginary part of the material's complex permeability show an increasing trend, u'increases from 2.0 to 2.5, and u" increases from 1.0 to 1.5; As the frequency increases, the real part of the permeability gradually decreases, and the imaginary part does not change significantly in a narrower frequency band. Similarly, Figure 4a and Figure 4b show the graphs of the composite permittivity. When the volume fraction increases When it reaches 40%, the real part of the complex permittivity increases from 11 to 16.5, and because the material is of non-electric loss type, the imaginary part of the permittivity is basically 0. This trend of electromagnetic parameters changing with the volume parameters of the absorber is also affected by other Experimental research confirmed.
The miniaturization of mobile terminals (such as mobile phones, tablets, music equipment, etc.) has promoted the development of active testing in small darkrooms. This type of test application has the advantages of fast test speed, low construction cost, and high cost-effectiveness ratio. It is a small Application of rapid testing system. Through computer simulation to guide the design and preparation of the material, we have obtained a small pyramidal rubber absorbing material, which is small in size and strong in absorption, especially suitable for small or micro darkrooms. The material has the following characteristics:
(1) The dimensions of the geometrical pyramids have an impact on the wave-absorbing performance of the material. The height of the pyramid base has a greater impact on the low-frequency reflectivity, and the base height and the total height of the cone have a greater impact on the high-frequency reflectivity.
(2) For the given electromagnetic parameters, the optimal size of the optimized pyramid is L=2mm, W=6mm, H=10mm within the wireless communication frequency band.
(3) The wave absorbing performance of the rubber pyramid is better than the single-layer and double-layer flat structure of the same material.
(4) By adjusting the ratio of raw materials, absorbing materials of different frequency bands can be designed according to requirements.