Tennant, Alan University of Sheffield, Sheffield, United Kingdom.
Last reviewed:January 2020
- Basic theory
- Types of absorber
- Active RAM
- Related Primary Literature
- Additional Reading
The name given to a wide range of materials that absorb incident electromagnetic energy and convert that energy to other forms—typically heat. As a consequence, a radar-absorbing material (RAM) reduces the level of electromagnetic energy reflected, or scattered, from its surface and hence has traditionally been used in the military arena to reduce the magnitude of the radar cross section of various platforms, including air, land, and sea vehicles. More recently, RAM has been used in civil and commercial environments to reduce radar scattering from large buildings at airports, which can interfere with civil aviation radar systems. RAM is also being used to reduce radar scattering from large wind turbines, which can cause similar interference problems to both civil and military radar systems. Other less known uses of RAM include commercial products such as microwave food packaging that can help food to brown during the cooking process by absorbing microwave energy and re-radiating it as infrared radiation. RAM can be designed to operate at any frequency within the electromagnetic spectrum, but practical constraints generally limit its use to frequencies from approximately 100 MHz to 100 GHz, and the actual design approach varies greatly depending on the frequency of operation, the required reflectivity performance, and bandwidth. Although one of the main applications of RAM is to reduce the radar cross section (RCS) of an object, related techniques such as target shaping are generally more effective at reducing the bulk RCS of an object and RAM is usually applied as a complementary technique. Traditional RAM is a passive material and is designed to work over a fixed band of frequencies. However, a new class of active, or adaptive, RAM has been developed since the 1990s. This type of RAM is constructed from materials whose electromagnetic properties can be controlled by an external stimulus—typically a control voltage. Active RAM has the ability to change its properties to work over different frequency bands and produce different levels of reflectivity. A related active “absorber” is the phase-modulating surface or phase-switched screen (PSS). This device does not actually absorb incident electromagnetic energy, but modulates the energy scattered from its surface (in a process akin to that used in spread spectrum communications) to mimic the characteristics of a conventional absorber. See also: Electromagnetic radiation; Radar
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