Salisbury screen

The Salisbury screen is maybe the first ever anti-radar or, to be more precise, anti-reflective concept; the so called RAM (radar absorbent material). It was first described in 1952 and was applied in ship radar cross section reduction (RCS). There have been many design refinements over the years especially because of the increasing interest for stealth planes, but the principles remain the same.

The most easy to understand salisbury screen design consists of a "ground plane" which is the metallic surface that needs to be concealed, a lossless dielectric of a given thickness (a quarter of the wavelength that will be absorbed) and a thin lossy screen.

The principle is this:

#The incident wave (which we will consider to be made up by parallel beams) is split into two (equal in intensity) waves that have the same wavelength (λ)
#The first wave is reflected by the exterior surface (the thin lossy screen) while the second beam travels through the dielectric, and it is reflected by the ground plane (which is the most inner layer of the salisbury screen)
#The reflected waves interfere and cancel each other’s electric fields (radar is an electromagnetic beam-microwave and IR)

To explain the phenomenon, we need to look at the interference theory. Two waves that are coherent interact, they combine to form a single output wave and if their peaks coincide, the output intensity is the sum of the two intensities. However, if the two waves are completely out of phase the two intensities cancel each other out (that only happens when the two waves are offset by one half a wavelength).

The second wave (in step 2.) travels twice (once from and once to the exterior thin lossy screen) the distance equal to one quarter a wavelength, for a total distance of one half a wavelength. Thus the two waves cancel each other.

The incidence angle the waves that are canceled do not come from the same exact incident wave. However, they are all similar thus they are coherent and interfere.

There are a few disadvantages inherent to this model (some of which have been solved). One would be the fact that salisbury screens work well only for a very narrow portion of the radar spectrum thus making it very vulnerable to multiple radar protected areas. Another problem is the thickness of the screen itself, the radar wavelengths are between 10 cm and 1 mm, thus for a longer wavelength, the thickness gets up to 2.5 centimeters which is quite difficult to cope with (e.g., in aerospace applications). Thus, research is being conducted for ultrathin Salisbury screens involving the Sievenpiper HIGP (high impedance ground plane) (source: Wiley Periodicals, Inc., Microwave Opt. Technol. Lett.), which shows remarkable improvements to the thickness of the screen.