Salisbury screen
Encyclopedia
The Salisbury screen, invented by American engineer Winfield Salisbury in 1952, was one of the first concepts in radar absorbent material
, later known as "stealth technology
", used to prevent enemy radar
detection of military vehicles. It was first applied to ship radar cross section
(RCS) reduction. There have been many design refinements to the concept since that time, motivated by the increasing interest in stealth
technology.
which is the metallic surface that needs to be concealed, a lossless dielectric
of a precise thickness (a quarter of the wavelength
of the radar wave to be absorbed), and a thin lossy screen.
To understand the cancellation of the waves requires an understanding of the concept of interference. When two electromagnetic waves that are coherent
and are traveling in the same space interact, they combine to form a single resultant wave. If the two waves are "in phase" so their peaks coincide, they add, and the output intensity is the sum of the two waves' intensities. However, if the two waves are a half-wavelength "out of phase", so the positive peaks of one wave coincide with the negative peaks of the other, the two waves subtract, and the difference is zero.
The thickness of the dielectric is made equal to a quarter of the wavelength (λ/4) of the expected radar wave. Since the second wave (in step 2.) travels the thickness of the dielectric twice (once 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.
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 least 2.5 cm. 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.
Radar absorbent material
Radar-absorbent material, or RAM, is a class of materials used in stealth technology to disguise a vehicle or structure from radar detection. A material's absorbency at a given frequency of radar wave depends upon its composition...
, later known as "stealth technology
Stealth technology
Stealth technology also termed LO technology is a sub-discipline of military tactics and passive electronic countermeasures, which cover a range of techniques used with personnel, aircraft, ships, submarines, and missiles, to make them less visible to radar, infrared, sonar and other detection...
", used to prevent enemy radar
Radar
Radar is an object-detection system which uses radio waves to determine the range, altitude, direction, or speed of objects. It can be used to detect aircraft, ships, spacecraft, guided missiles, motor vehicles, weather formations, and terrain. The radar dish or antenna transmits pulses of radio...
detection of military vehicles. It was first applied to ship radar cross section
Radar cross section
Radar cross section is a measure of how detectable an object is with a radar. A larger RCS indicates that an object is more easily detected.An object reflects a limited amount of radar energy...
(RCS) reduction. There have been many design refinements to the concept since that time, motivated by the increasing interest in stealth
Stealth aircraft
Stealth aircraft are aircraft that use stealth technology to avoid detection by employing a combination of features to interfere with radar as well as reduce visibility in the infrared, visual, audio, and radio frequency spectrum. Development of stealth technology likely began in Germany during...
technology.
How it works
Salisbury screens operate on the same principle as optical antireflection coatings used on the surface of camera lenses and glasses to prevent them from reflecting light. The easiest to understand Salisbury screen design consists of three layers: a ground planeGround plane
In electrical engineering, a ground plane is an electrically conductive surface.-Radio antenna theory :In telecommunication, a ground plane structure or relationship exists between the antenna and another object, where the only structure of the object is a structure which permits the antenna to...
which is the metallic surface that needs to be concealed, a lossless dielectric
Dielectric
A dielectric is an electrical insulator that can be polarized by an applied electric field. When a dielectric is placed in an electric field, electric charges do not flow through the material, as in a conductor, but only slightly shift from their average equilibrium positions causing dielectric...
of a precise thickness (a quarter of the wavelength
Wavelength
In physics, the wavelength of a sinusoidal wave is the spatial period of the wave—the distance over which the wave's shape repeats.It is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings, and is a...
of the radar wave to be absorbed), and a thin lossy screen.
- When the radar wave strikes the front surface of the dielectric, it is split into two waves of equal intensity.
- One wave is reflected from the lossy surface screen. The second wave passes into the dielectric layer, is reflected from the metal surface, and passes back out of the dielectric into the air.
- The extra distance the second wave travels causes it to be 180° out of phase with the first wave by the time it emerges from the dielectric surface
- When the second wave reaches the surface, the two waves combine and cancel each other out due to the phenomenon of interference. Therefore there is no wave energy reflected back to the radar receiver.
To understand the cancellation of the waves requires an understanding of the concept of interference. When two electromagnetic waves that are coherent
Coherence (physics)
In physics, coherence is a property of waves that enables stationary interference. More generally, coherence describes all properties of the correlation between physical quantities of a wave....
and are traveling in the same space interact, they combine to form a single resultant wave. If the two waves are "in phase" so their peaks coincide, they add, and the output intensity is the sum of the two waves' intensities. However, if the two waves are a half-wavelength "out of phase", so the positive peaks of one wave coincide with the negative peaks of the other, the two waves subtract, and the difference is zero.
The thickness of the dielectric is made equal to a quarter of the wavelength (λ/4) of the expected radar wave. Since the second wave (in step 2.) travels the thickness of the dielectric twice (once 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 the 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 least 2.5 cm. 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.