Resonant absorbing materials use the principle of interference to reduce the reflection of electromagnetic waves, also known as A/4 absorbing materials, including Dallenbach layer, Salisbury screen and Jaumann layer. The impedance of this type of material does not match that of air, and the material has certain requirements for thickness, so it cannot completely absorb all the electromagnetic energy.
(1) Dallenbach layer absorbing material
The Dallenbach layer absorbing structure consists of a uniform lossy dielectric layer placed in front of the conductive plate, as shown in Figure 2-27. Studies have found that a single-layer Dallenbach structure cannot obtain a wide-band absorbing material. Therefore, people use a multi-layer structure to broaden its absorption frequency band. Mayer F. uses two or more absorbing layers to increase the absorption bandwidth. The first layer is the anoxic body layer at the interface between the absorbing material and the air, and the second layer is a short fiber layer containing metal. The Lagrangian method can be used to optimize multi-layer Dallenbach absorbers. This method has been used to design cone-shaped and A/4 absorbers.
(2) Salisbury screen absorbing material
The Salisbury screen is a resistive screen with suitable impedance placed at A/4 on the reflective surface of the metal back to form a resonant wave-absorbing structure, as shown in Figure 2-28. This structure can make the electromagnetic waves reflected from the metal backing and the impedance layer have opposite phases, so as to achieve "zero reflection". When the sheet resistance is equal to the impedance of the free space, the optimal Salisbury screen thickness d=1/Z0a where a is the conductivity of the sheet; d is the thickness of the resistive layer. The absorbing performance of the Salisbury screen is related to the thickness of the absorbing body and the dielectric constant of the dielectric layer. The increase in the thickness and dielectric constant of the absorber will increase the electrical length, which will shift the absorption peak to low frequencies and achieve strong low-frequency absorption. It can be seen from the structure of the Salisbury screen that the resistive film of the purely electrical lossy material should be placed at the maximum electric field, that is, A/4 away from the metal surface; while the magnetic lossy material should be placed at the maximum magnetic field, that is, the metal surface is the best. The electrical loss screen must be placed at A/4 above the metal surface, which results in a thicker absorbing structure, so this structure can only be applied to occasions that do not limit the size of the material. In addition, the Salisbury screen cannot achieve "zero reflection" for electromagnetic waves outside the resonant frequency, and the bandwidth of the absorbing frequency is relatively narrow. Conductive polymers can also be used as absorbing materials to design and prepare Salisbury screens, and use the optical transmission matrix method to study its absorbing properties.
(3) Jaumann layers of absorbing materials
On the basis of the Salisbury screen, the amount of time to increase the number of air-enhanced screen sheets and isolation layers is used to improve the absorption and propagation direction and the bandwidth, thus developing a new absorbing material, called Jaumann absorbing material, as shown in Figure 2- 29 shown. Each layer of material with different electromagnetic properties can be shown in Figure 2-29. Jaumann layer can be optimized and combined with various algorithms to obtain a wave-absorbing material with appropriate impedance and better performance. Through analysis and research on impedance matching characteristics, it is found that the tapered combination of the dielectric layer can meet the impedance matching conditions. At the same time, the introduction of reactive factors such as capacitive reactance and inductive reactance can better improve its thickness/bandwidth ratio. It satisfies the characteristics of continuous change in the impedance of the dielectric layer while increasing the loss, and then realizes the resonance absorption of the structure absorbing material in a wider frequency band. The internal structure of the multilayer Jaumann layer absorbing material is separated by a low-loss dielectric. A 6-layer Jaumann layer absorbing material has a reflectivity of -30dB at 7~15GHz. J.R, Nortier, etc. used the equivalent transmission line theory to study the 7-layer resistor Jaumann type wave absorber, and the results show that the structure has an ideal absorption bandwidth.