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Absorbing materials serve many functions in microwave engineering. Low density foams impregnated with lossy material are used to make the familiar radar-absorbing "pyramids" lining anechoic chambers. Sometimes when you have a nasty moding problem, the best solution is to coat the lid of a housing with an absorbing material. A lot of highly expensive defense equipment is coated with radar-absorbing coatings. We've had inquiries from people that want to make their Corvette stealthier to avoid police radar...
Before we go any further, the microwave industry often uses the adjective "absorptive" to describe absorbing materials. A lot of engineers mispronounce and misspell the word, like absorbive or absorbative. Get it straight, or get another job!
Hiding stuff from radar detection is an important design issue and absorbing materials are essential.
The radar cross-section of an object is a measure of how stealthy an object is with respect to radar detection - a smaller cross-section means more difficult to detect. We have brokent the topic of radar cross-section reduction into three pieces to illustrate the principles involved. Parts 1 and 2 define the terms and principles of the issue and part 3 discusses some of the solutions.
Part one reviews the fundamentals of electro-magnetic waves.
Part two is on radar cross-section physics.
Part three discusses radar absorbers and absorption mechanisms.