RF Absorbers

RF Absorbers

RF absorbers, also referred to as radar absorbers or microwave absorbers, protect equipment from radio frequency interference (RFI) by absorbing the magnetic energy before it can reach the equipment. Constructed from materials of high magnetic permeability, RF absorbers are available in a wide variety of designs including pyramidal, flat, wedge and hexagonal, each of which have advantages for differing applications. For instance, pyramidal RF absorbers are best suited for use in test chambers such as free space or anechoic chambers in industrial and military testing. Conversely, flat RF absorbers are useful for antenna test facilities and EMC measurements in the electronics industry. Wedge RF absorbers are also advantageous for free space chambers, particularly when pyramidal RF absorbers would provide too much backscatter for efficient testing. Lastly, hexagonal RF absorbers, also referred to as honeycomb, are best suited for aerospace parts due to their high strength-to-weight ratio. In addition to the various shapes of RF absorbers, there are many different types including corner block, hybrid, convoluted, walkway and ferrite. Different materials that are used to fabricate RF absorbers include low density polyethylene (LDPE) foam, urethane foam, polystyrene, graded dielectric foam, ferrite or a variety of composite materials. In addition to the more common foam RF absorbers, there are some rubber sheet and sprayable absorbers.

Since there are many different types of RF absorbers, there are numerous fabrication and design methods as well; however, the most common type is fabricated from foam. Foam RF absorbers begin fabrication through foam cutting, which takes the size desired for the RF absorber from a much larger foam sheet in addition to creating pyramidal or wedge shapes. Next, the foam is impregnated with lossy dielectrics, which are materials used for the dissipation of electrical energy. Then the impregnated foam is placed within a drying oven, in which the time it takes to cure depends upon the size and weight of the foam. Lastly, the cured foam undergoes treatment in order to retain conductive carbon and fire retardant chemicals and allows the RF absorber to reach optimal electrical performance. The RF absorber is typically spray-coated with a light blue paint at the end of the process. Some of the additional RF absorber fabrication processes include laminated foam and sprayable absorbers. Laminated foam RF absorbers are fabricated in a slightly different manner; after impregnation they undergo lamination with flexible adhesives and latex. Multiple layers are bonded together with a contact adhesive and a reflective surface is used as a backing. Sprayable RF Absorbers are commonly made from one of three materials: epoxy casting resin, silicone casting resin and UHF silicone casting resin. Sprayable RF absorbers must be brushed or painted onto the surface of the device they are intended to protect.