HITEK supplies and converts Microwave absorbing materials and controlled dielectrics from Emerson & Cuming Microwave. If you have not used absorbing materials before please read the section below and the associated links at the bottom prior to proceeding to the materials sections. You will find it and the links useful and informative.
For an RF/microwave absorber product you first need to determine whether your application is in free space or in an enclosed volume.
Free space absorbers are designed to absorb (or similarly, not reflect) propagating plane waves that might otherwise reflect of a conductive surface. They usually work in one of two ways, by resonant phase cancellation or through a gradual impedance change.
Single layer homogeneous resonant absorbers (ECCOSORB® SF, for example) are inherently narrow banded (-20 dB or better reflection but only over a +/-5% bandwidth) but offer excellent performance. The material thickness is critical as it must be a quarter wavelength (in the material) at the target frequency. Choose these materials based on the desired centre frequency. Bandwidth can be increased by using multiple resonant layers (ECCOSORB® AN). Several grades are available. Choose these materials based on your lowest frequency of operation.
Impedance transition absorbers act as a gradual impedance change from that of free space to the absorption material at the rear of the absorber. Some exhibit the impedance change based on geometry such as ECCOSORB® VHP or most others in the anechoic product line which are pyramidal in shape, while others are manufactured with an integral impedance gradient such as ECCOSORB® HR. The impedance transition absorbers tend to exhibit excellent broadband performance.
In an enclosed space, different considerations are used to choose an absorber. Here the primary issues are nonpropagating standing waves. A common problem is when a circuit board cover is used to protect a circuit, creating cavities, which might resonate.
These problems are very difficult to foresee beforehand and often absorbent material must be used to damp the resonances. Since no two cavities are identical in size, shape and frequency or mode of excitation, it is difficult to determine beforehand which absorber would be best.
Considerations in choosing a cavity resonance dampening absorber
Inserting a microwave absorbent material in a cavity exhibiting VSWR resonance will act to dampen the resonance in several different ways. Sometimes its presence in the cavity is merely shifting the VSWR peaks to different locations such that their effect on circuit performance is reduced. Sometimes the absorber will change the characteristics of the cavity such that its resonant frequency is outside the band of circuit operation hence that mode is not excited. More commonly, the absorber really is absorbing. From basic electromagnetics, at a conductive surface the tangential electric field is zero and the magnetic field is at a maximum. Hence an absorber with high magnetic loss is desired. Also from EM theory, a high permittivity-permeability material in a cavity will tend to ’suck’ much of the field energy into itself.
ECMP manufactures a wide range of thin elastomeric materials with high magnetic loss. An additional desirable feature of such absorbers is their low conductivity so that they will not short out circuits if they accidentally come into contact.
As a general rule, lower frequency cavity resonances would require a heavier loaded and thicker material for equivalent absorption. At higher frequencies, though the heavier, thicker material might work just fine, cost considerations should point you towards a lighter thinner material.