Reduce 10dB RAM absorbing filler in the range of 2-20GHz

RAM fillers are usually particles made of "loss materials" (i.e. high dielectric constant loss components), or particles coated with "loss materials". Carbon is a good "loss material" because electrical loss is proportional to conductivity, and carbon's conductivity is between that of metals and insulators. The magnetic absorption layer requires the use of materials with average dielectric constant but high magnetic permeability (characterizing magnetic energy storage capacity), usually carbonyl iron (pure powdered metal) or iron oxide (also known as ferrite). These materials can be mixed into rubber or dispersed into coating materials, while ferrite is usually sintered into some kind of patch material.

The larger the dielectric constant, magnetic permeability, and loss components of a material, the more electromagnetic energy it can absorb. However, when electromagnetic waves propagate to the boundary between two media, the energy is reflected instead of entering the other medium. The amount of reflected energy depends on the impedance of the two media, which is the square root of the ratio of the magnetic permeability and dielectric constant of each material. The greater the impedance change when crossing the boundary, the more energy is reflected and the less energy is absorbed. Therefore, RAM design must comprehensively consider absorption rate and surface reflectivity to maximize the absorption of electromagnetic waves.

The electromagnetic properties of materials also vary with frequency. In the high-frequency radar frequency band, the impedance of any magnetic material cannot approach that of air (because when electromagnetic waves reach the surface of an aircraft, the aircraft surface is the boundary, and the media on both sides are skin material and air), so strong surface reflections are inevitably generated. However, if the surfaceAbsorbing materialWhen the thickness is 1/4 wavelength, the electromagnetic waves reflected by the metal substrate will produce a coherent cancellation effect with the surface reflection. Due to the high magnetic permeability of magnetic RAM, a smaller material thickness is required. A commercial "resonant absorber" with a resonant frequency of 1-18GHz and a thickness of 0.1-0.5cm can achieve an absorption performance of 20dB (99%). The inherent scope of this technology is not large and belongs to narrowband. It has significant absorption effects within 15% of the resonant frequency point.

Considering the limited bandwidth, heavy weight, and high cost, dielectric absorbers are the preferred choice for high frequency bandsBroadband absorbing materialDue to the lack of magnetic properties in dielectrics, their impedance differs greatly from that of air. However, by using layered materials - where carbon particles become increasingly concentrated in each layer of material - it is possible to achieve a gradual decrease in impedance while increasing dielectric constant, conductivity, and dielectric loss. By adjusting the design of layered materials, maximum cancellation can also be achieved. This impedance gradient dielectric absorber can reduce reflection by 20dB and its bandwidth can easily cover high-frequency regions. However, the thickness of the layered material needs to reach a certain value to achieve absorption in the low frequency band -2.5cm is required in the X-band (8-12 GHz) and 11.4cm is required in the 500MHz band.

Another method is to apply physical gradients. These "geometric transition" absorbers use uniform material tips perpendicular to the wave, with the most common being conical absorbers in anechoic chambers (used for RCS testing). At high frequencies, waves reflect back and forth in these structures, but each reflection results in energy loss. If the wavelength is large enough for the structure, the effect exhibited by the wave seems to be passing through a material with gradually changing properties. This type of absorber can reduce reflection by 60dB, but to be effective at 30MHz, the structural thickness needs to be 4.57m.

Contrary to common sense, some magnetic materials are more effective at low frequencies because their energy storage capacity, i.e. magnetic permeability, increases. In the range of 30M to 1000MHz, certain ferrites exhibit extremely high electromagnetic wave compression effects, with impedance close to that of air. A commercial ferrite tile with a thickness of 0.64cm and an area density of 34.18kg/m2 can reduce the reflection in the very high frequency (VHF) band by more than 20dB and the emission in the ultra-high frequency (UHF) band by 10dB.

So far, we have been discussing how to reduce specular reflection, but in fact, RAM is also very effective in reducing surface wave radiation. These electromagnetic waves are emitted by the current generated on the conductive surface when the radar irradiates the target. When these surface waves move along the surface, they emit traveling waves, usually with an emission angle close to the incident angle; When surface waves encounter discontinuous surfaces, such as reaching the edge of a body, or encountering surface gaps, structural steps, or material changes, edge waves will be excited. The energy of edge waves is more concentrated, approaching mirror reflection. The surface current passes not along the thickness direction of the material but along the length direction, and the role of RAM is like a waveguide, capturing and absorbing the current. A magnetic RAM with a thickness of only 0.076cm can effectively suppress surface current.

Of course, the above-mentioned technologies can be combined and applied. A layered magnetic material with a thickness of 0.76cm can reduce by 10dB in the range of 2-20GHz. By using a physical gradient dielectric layer as the front material and a magnetic material as the back, a hybrid RAM can be formed to reduce radar reflections from the VHF band to the Ku band.