Since the advent of the Radar in the 1930s the race was on to develop materials that could disguise structures so as not to be detected by the radar and surprise the enemy. These materials are known as Radar Absorbing Materials or RAM. They are designed to absorb the Radio Frequency (RF) emitted by the radar, so that the incoming signal does not reflect back to the radar and anything covered with Radar Absorbing Materials is not “seen” by the radar.

As soon as 1936 the first patent (FR Patent 802728) was granted to Machinerieen who was using carbon black. By the second World War the germans were already applying carbonyl iron powder (“Wesch”) for submarines, and in 1952 Salisbury published his renown patent (US 2599944). From then, onwards, many different Radar Absorbing Materials have been developed and are still being developed. Radar technology keeps evolving and so do their counterfeiting materials, and with the increasing number of applications of RF a need has arisen for, not only defense applications, but also, civil applications; the reduction of electromagnetic (EM) radiation is very useful to avoid EM clutter in weather and navigation radars, for structures such as wind turbines or airport structures. Moreover, electronic systems give rise to electromagnetic interferences (EMI), interferences in radars and low efficiency in systems due to their coupling, so the isolation of antennae and apparatuses having a high EM radiation output or of those apparatuses that can be affected by EM radiation, such as imaging equipment for medical applications, is also a wide field of application requiring the reduction of electromagnetic radiation.

The Industry and the Academia are pushing the boundaries of what was possible in the search of the ideal Radar Absorbing Material, looking for thinner and lighter materials, more durable and easily applicable, cheaper and with increasing frequency coverages. The range of materials on offer is large, however, depending on the physical principle they use, they can be grouped into two main categories:

• Destructive interference: the incident radiation is cancelled out by antiphase reflection, placing a reflective surface at a quarter wavelength from the surface of the material.
• Lossy materials: the material posses a large absorption (high imaginary part of either permittivity or permeability) that effectively absorbs the incident radiation.

Furthermore, surface shaping can be used to help either group in order to increase their effectiveness.

Both have a tendency to be thick, because of the thickness needed for antiresonance and absorption to take place. Lossy materials tend to be heavier, because of the amount of particles needed for the loss to be large enough. Durability and easiness of application depend heavily on the substrate used, and destructive interference based materials tend to be narrow band, although careful layering can produce higher bandwidths. The choice of RAM is highly dependent on the application and a cost/effectiveness study, where weight, thickness, durability and ease of use are key factors that must be taken into account.

Traditionally Radar Absorbing Materials are made by dispersing electromagnetic particles ranging from metallic particles to carbon nanotubes in convenient matrices. Some of the material commercially available are carbon impregnated polyurethane foam, carbon-loaded silicone and neoprene, iron-loaded urethane, evaporated metal alloys and fibrous carbon paints, knitted materials, fabrics, coated honeycomb, conducting polymers, glass and carbon fiber composites. Now there are also materials made out of tiny circuits that resonate, and thus, absorb the incoming energy.