Magnetic vs. Dielectric Absorber Selection
RF and microwave absorbing materials (RF absorber and microwave absorber) are optimized for electrical performance. The electric or magnetic properties of these materials are altered to allow absorption of RF and microwave energy. Common dielectric materials – such as foams, plastics, and elastomers have no magnetic properties, which gives them a permeability of 1. High dielectric loss materials, such as carbon, graphite, and fibers are used to modify the dielectric properties of these base materials. In magnetically loaded materials, fillers such as ferrites, iron, and unique alloys, increase the permeability of the base materials. RF absorbers and microwave absorbers generally are either considered magnetic resonant frequency absorbers, or graded dielectric materials.
Magnetic resonant frequency absorbing materials can be designed to absorb energy either a single discrete frequency or at multiple frequencies. By controlling the critical magnetic and dielectric loading and thickness of each of the layers, multiple frequencies can be designed for. Magnetic resonant frequency absorbers operate via phase cancellation. The incoming wave incident upon the absorbing material is partially reflected and partially transmitted. The transmitted portion undergoes multiple internal reflections to give rise to a series of emergent waves. At the design frequency, the sum of the emergent waves is equal in amplitude to (by 180 degrees out of phase with) the initial reflection portion. In theory, zero reflection takes place at the design frequency. In practice, absorption of >30dB may be achieved.
Distributed magnetic fillers suspended in a flexible elastomeric matrix is the most common form of resonant frequency absorbing material. By increasing the permeability and permittivity of the elastomer, the refractive index is also increased, and thus allowing for a thinner material. Magnetic resonant frequency absorbers are typically sold in thickness ranging from 0.020″ to 0.175″, depending on the frequency which they are optimized for. Based on the principle of phase cancellation, the lower the frequency to be attenuated, the thicker the material must be.
In contrast, graded dielectric absorbers achieve absorption through a gradual tapering of impedance from that of free space to that of a lossy state. Little initial reflection results in a material with an adequate transition from free space to lossy, resulting in high levels of attenuation. The absorbing medium is most commonly a conductive carbon coating, which is coated onto a polyurethane based foam substrate. High levels of reflection loss, in many cases better than 20dB, can be achieved in materials which are <1/3 wavelength thick. The most common type of graded dielectric absorber is the Reticulated Foam Absorbing material. The open celled nature of reticulated foam, enables MAST Technologies to apply a tapered carbon coating through the thickness of the foam, minimizing any initial reflections. Another method for achieving the graded impedance, is by physically tapering the coatings, as is achieved with Convoluted Foam Absorbers. Graded dielectric absorbers are typically sold in thicknesses ranging from 0.125″ to 5.0″ thick.
How to chose?
Designers of RF absorbing and microwave absorbing (RF absorbers, microwave absorbers) materials must consider electrical, physical, and application parameters when determining which type of absorber to specify. Engineers must account for desired effect (reflection loss, insertion loss, cavity resonance reduction, or surface wave attenuation), frequency band, coverage area, environment, and so on.
Generally, Magnetic Resonant Frequency Absorbers offer better environmental resistance than the more broadband Graded Dielectric Absorbers. Elastomers have been used in rf and microwave absorbers in some of the harshest environments for over 50 years. A variety of elastomers are available for design into a specific environment. See Technical Bulletin 102 for more details on which elastomer to choose. Additionally, when Resonant Frequency Absorbers are applied with adhesive to an electrically conductive substrate, the chance of surface or traveling waves being present will be minimized. By comparison, Graded Dielectric Absorbers are typically used in a protected environment. Although the polyurethane foams used are robust solutions, when subjected to extreme environments, the foam may breakdown over time.
Both the elastomeric type and foam based materials are most commonly sold as die-cut parts with a pressure sensitive adhesive (PSA) backing. Inexpensive steel ruled dies enable MAST Technologies or other converters to cut the sheet material down into specific geometries. Parts can either be die-cut, where the part is cut free from the sheet of material, or kiss cut, where the part may be supplied in a pad making presentation to the production line more seamless. Kiss cutting typically allows for simpler removal of the PSA liner paper.
For more information or a more detailed explanation of how to choose RF absorbers, RF absorbing materials, microwave absorbers, and microwave absorbing material please contact Andrew Sundsmo at 858-452-1700, or firstname.lastname@example.org.