going in and once coming out), the extra distance it travels is a half-wavelength (λ/2). So it is a half wavelength out of phase with the first wave when they combine, and the two waves cancel. Disadvantages There are significant disadvantages to the concept which have limited the use of Salisbury screens. One is that the simple Salisbury design above works well only at a single radar frequency, so the enemy needs only to change its frequency to defeat it. More complicated multilayer Salisbury designs can cover a band of frequencies, but only by increasing the thickness, and at best cover only a fraction of the radar spectrum. Another problem is the thickness of the screen itself. Radar wavelengths range between 10 cm and 1 mm, thus the thickness of the screen (a quarter-wavelength) must be at most 2.5 cm, and multilayer screens are much thicker. Adding this much bulk to the surfaces of aircraft can unacceptably degrade aerodynamic performance. Research is being conducted on ultra thin Salisbury screens using 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. References Materials Military technology Radar