Radiation-absorbent material: Difference between revisions

One of the most effective types of RAM comprises arrays of [[pyramid]] shaped pieces, each of which is constructed from a suitably [[lossy material]]. To work effectively, all internal surfaces of the anechoic chamber must be entirely covered with RAM. Sections of RAM may be temporarily removed to install equipment but they must be replaced before performing any tests. To be sufficiently lossy, RAM can be neither a good [[electrical conductor]] nor a good [[electrical insulator]] as neither type actually absorbs any power. Typically pyramidal RAM will comprise a [[rubber]]ized [[foam]] material impregnated with controlled mixtures of [[carbon]] and [[iron]]. The length from base to tip of the pyramid structure is chosen based on the lowest expected frequency and the amount of absorption required. For low frequency damping, this distance is often {{cvt|60 |cm (24")|in}}, while high-frequency panels are as short as {{cvt|7.5–10 5|to|10|cm (3–4")|in|0}}. Panels of RAM are typically installed on the walls of an [[anechoic chamber|EMC test chamber]] with the tips pointing inward to the chamber. Pyramidal RAM attenuates signal by two effects: scattering and absorption. Scattering can occur both coherently, when reflected waves are in-phase but directed away from the receiver, or incoherently where waves are picked up by the receiver but are out of phase and thus have lower signal strength. This incoherent scattering also occurs within the foam structure, with the suspended carbon particles promoting destructive interference. Internal scattering can result in as much as 10&nbsp;dB of attenuation. Meanwhile, the pyramid shapes are cut at angles that maximize the number of bounces a wave makes within the structure. With each bounce, the wave loses energy to the foam material and thus exits with lower signal strength.<ref>E Knott, J Shaeffer, M Tulley, ''Radar Cross Section''. pp 528&ndash;531. {{ISBN|0-89006-618-3}}</ref> An alternative type of RAM comprises flat plates of [[Allotropes of iron|ferrite]] material, in the form of flat [[tile]]s fixed to all interior surfaces of the chamber. This type has a smaller effective frequency range than the pyramidal RAM and is designed to be fixed to good conductive surfaces. It is generally easier to fit and more durable than the pyramidal type RAM but is less effective at higher frequencies. Its performance might however be quite adequate if tests are limited to lower frequencies (ferrite plates have a damping curve that makes them most effective between 30&ndash;1000&nbsp;MHz). There is also a hybrid type, a ferrite in pyramidal shape. Containing the advantages of both technologies, the frequency range can be maximized while the pyramid remains small, (about {{cvt|10&nbsp;|cm)|in}}.<ref>[http://ieeexplore.ieee.org/xpl/freeabs_all.jsp?tp=&arnumber=875567&isnumber=18905 Fully compact anechoic chamber using the pyramidal ferrite absorber for immunity test]</ref>

Conversely the IJN saw great potential in anti-radar materials and the Second Naval Technical Institute began research on layered materials to absorb radar waves rather than paint. Rubber and plastic with carbon powder with varying ratios were layered to absorb and disperse radar waves. The results were promising against 3000 megacycle frequencies, but poor against 3cm3&nbsp;cm wave length radar. Work on the program was halted due to allied bombing raids, but research was continued post war by the American's to mild success.<ref name=":0" />

Foam absorber is used as lining of [[anechoic chamber]]s for electromagnetic radiation measurements.{{Citation needed|date=February 2010}} This material typically consists of a fireproofed urethane foam loaded with conductive carbon black [carbonyl iron spherical particles, and/or crystalline graphite particles] in mixtures between 0.05% and 0.1% (by weight in finished product), and cut into square pyramids with dimensions set specific to the wavelengths of interest. Further improvements can be made when the conductive particulates are layered in a density gradient, so the tip of the pyramid has the lowest percentage of particles and the base contains the highest density of particles. This presents a "soft" impedance change to incoming radar waves and further reduces reflection (echo). The length from base to tip, and width of the base of the pyramid structure is chosen based on the lowest expected frequency when a wide-band absorber is sought. For low-frequency damping in military applications, this distance is often {{cvt|60 |cm (24")|in}}, while high-frequency panels are as short as {{cvt|7.5–10 5|to|10|cm (3–4")|in|0}}. An example of a high-frequency application would be the police radar (speed-measuring radar K and Ka band), the pyramids would have a dimension around {{cvt|10 |cm (4")|in|0}} long and a 5x5 {{cvt|5|x|5|cm (2"x2")|in|0}} base. That pyramid would set on a 5x55&nbsp;cm x 5&nbsp;cm (2"x2") cubical base that is {{cvt|2,.5 |cm (1")|in|0}} high (total height of pyramid and base of about {{cvt|12,.5 |cm |in|disp=or 5"|0}}). The four edges of the pyramid are softly sweeping arcs giving the pyramid a slightly "bloated" look. This arc provides some additional scatter and prevents any sharp edge from creating a coherent reflection. {{Citation needed|date=June 2013}} Panels of RAM are installed with the tips of the pyramids pointing toward the radar source. These pyramids may also be hidden behind an outer nearly radar-transparent shell where aerodynamics are required. {{Citation needed|date=June 2013}} Pyramidal RAM attenuates signal by scattering and absorption. Scattering can occur both coherently, when reflected waves are in-phase but directed away from the receiver, or incoherently where waves may be reflected back to the receiver but are out of phase and thus have lower signal strength. A good example of coherent reflection is in the faceted shape of the F-117A stealth aircraft which presents angles to the radar source such that coherent waves are reflected away from the point of origin (usually the detection source). Incoherent scattering also occurs within the foam structure, with the suspended conductive particles promoting destructive interference. Internal scattering can result in as much as 10&nbsp;dB of attenuation. Meanwhile, the pyramid shapes are cut at angles that maximize the number of bounces a wave makes within the structure. With each bounce, the wave loses energy to the foam material and thus exits with lower signal strength.<ref>E Knott, J Shaeffer, M Tulley, Radar Cross Section. pp 528–531. {{ISBN|0-89006-618-3}}</ref> Other foam absorbers are available in flat sheets, using an increasing gradient of carbon loadings in different layers. Absorption within the foam material occurs when radar energy is converted to heat in the conductive particle. Therefore, in applications where high radar energies are involved, cooling fans are used to exhaust the heat generated. {{Citation needed|date=June 2013}}

Split-ring resonators (SRRs) in various test configurations have been shown to be extremely effective as radar absorbers. SRR technology can be used in conjunction with the technologies above to provide a cumulative absorption effect. SRR technology is particularly effective when used on faceted shapes that have perfectly flat surfaces that present no direct reflections back to the radar source (such as the F-117A). This technology uses photographic process to create a resist layer on a thin (about {{cvt|0,1778 .007|in|mm |disp=or 0,007"|order=flip}}) copper foil on a dielectric backing (thin circuit board material) etched into tuned resonator arrays, each individual resonator being in a "C" shape (or other shape—such as a square). Each SRR is electrically isolated and all dimensions are carefully specified to optimize absorption at a specific radar wavelength. Not being a closed loop "O", the opening in the "C" presents a gap of specific dimension which acts as a capacitor. At 35 GHz, the diameter of the "C" is near {{cvt|5 |mm|in}}. The resonator can be tuned to specific wavelengths and multiple SRRs can be stacked with insulating layers of specific thicknesses between them to provide a wide-band absorption of radar energy. When stacked, the smaller SRRs (high-frequency) in the range face the radar source first (like a stack of donuts that get progressively larger as one moves away from the radar source) stacks of three have been shown to be effective in providing wide-band attenuation. SRR technology acts very much in the same way that antireflective coatings operate at optical wavelengths{{dubious|date=November 2021}}. SRR technology provides the most effective radar attenuation of any technologies known previously and is one step closer to reaching complete invisibility (total stealth, "cloaking"). Work is also progressing in visual wavelengths, as well as infrared wavelengths (LIDAR-absorbing materials).{{Citation needed|date=June 2013}}