Eccosorb Electromagnetic Principles and Applications

Introduction

It may be surprising to many that electromagnetic radiation absorbers were investigated as far back as the mid 1930's. The earliest record involves a simple resonant absorber that was designed for application to the back of a 2-GHz antenna in order to improve its front-to-back ratio.

Today, engineers working in VHF, UHF, microwave, and millimeter frequencies have access to a wide variety of commercially available materials. This variety is due to material characteristics that are appropriate for different application requirements. For example, some important requirements that we have encountered over the years are high physical strength, low weight, good weatherability, high resilience, good flexibility, minimum thickness, wide operational band width, low reflectivity, high power capability, extreme temperature capability, low outgassing, and low cost. Emerson and Cuming Microwave Products provides ECCOSORB® absorbers that have been optimized for each of the above conditions. ECCOSORB® is used in a wide variety of applications that span the frequency range from 30 MHz through 100 GHz. The following applications are among the most important:

To improve patterns of antennas.
To reduce undesirable reflections from objects and devices.
To cover the inside of test rooms (anechoic chambers) in order to achieve "free space" conditions for measurement of components and systems.
To reduce the RCS (radar cross section) of objects.
To achieve both "free space" termination and waveguide and coaxial termination (dummy loads).
To improve the shielding of enclosures and containers through use as gasket materials.

A. PRINCIPLES OF ABSORBER OPERATION

There are basically four groupings of the absorber types.

Those that are based on advantages from having complex magnetic permeability and complex dielectric constant essentially equal.
Those that are 'resonant" usually a quarter wavelength thick operating over a relatively narrow frequency range.
Those that are "broad band" characterized by a gradual tapering from the impedance of free space to that of the medium over appreciable thickness (in terms of wavelengths).
Those that are very thin (in terms of wave length) that are used to attenuate surface currents on metal structures .
Separate consideration of each type follows.

Km Materials
One of the early realizations from Maxwell's Equations concerning absorption was that a medium with complex permeability equal to its complex dielectric constant would have no interface reflection for radiation at normal incidence. This is because such a medium would have an impedance equal to that of free-space. The thickness of such a medium would have only to be adequate to attenuate the incident radiation to the desired level.

With appreciation of this understanding work was instigated in both Germany and Holland some fifty years ago toward finding materials having similar complex magnetic permeability and complex dielectric constant. Researchers found that a class of materials known as ferrites was significantly better than others in that regard. Ferrites are a form of sintered iron and other metallic oxides having a cubic crystal structure. While some ferrites did have similar Km and Ke over a narrow frequency range, it turned out that both the real and imaginary parts of Ke and Km varied so rapidly with frequency that their utilization as absorbers was limited. Subsequent development of a much wider range of ferrite compositions gave rise to absorber types which are commonly used today. Ferrite-based absorbers offer the remarkable feature of being able to provide reflection reductions of 10 to 25 dB in the 30 to 1000 MHz range with a thickness of only about a quarter of an inch. Since ferrite is inorganic, it cannot burn, has good outgassing properties, can be operated to elevated temperatures, and is capable of dissipating incident power by as much as 20 watts per square inch. Pure ferrite absorbers are employed primarily to create anechoic chambers and absorbing antenna caps for use at frequencies from 30 through 1000 MHz. Ferrites are also used as fillers in material systems designed for applications with low frequency requirements.

Resonant Absorbers
At microwave frequencies and nearly all UHF frequencies, Km is so much lower than Ke in most materials that the difference between the free space and material impedance precludes the employment of the previously described phenomena. There are, however, ways to get around this problem.

"Resonant" absorbers may be thought of as circumventing the problem of high interface reflection by canceling this reflection with another from the back surface. If the back surface is a conductor, it may be shown that these two vectors cancel at a frequency where the medium is essentially a quarter-wave (or odd multiples of a quarter wave) thick. By adjusting the thickness and complex magnetic permeability of the medium, a condition of low reflection is achieved at the resonant frequency for angles near normal incidence (or other angles, if desired). It is desirable for the medium to have magnetic permeability since band width is directly proportional to the Km of the material. The loss, then, is distributed throughout the quarter-wave thickness.

ECCOSORB® SF and SF-U are examples of a distributed loss material. They are in the form of thin, flexible, silicone or urethane sheets, having powdered iron pigmentation for greater band width. When mounted directly on a metal surface, the reflectivity is generally better than -20 dB for angles up to 30° from normal incidence over 15% band width around resonance. Versions of this material are available with resonant frequencies in the range of 1 through 18 GHz. Sheet thickness varies from 0.2 to 0. 04 inches (0.5 to 0.1 cm) over this range.

Another means of matching the impedance of free-space is to lump the loss in a resistive sheet with resistivity equal to 377 ohms per square. One can then construct an absorber known as the Salisbury Screen. This absorber can be understood best when viewed on the basis of transmission line theory. It may be recalled that a simple transmission line termination can be effected by placing a pure resistance across a shorted transmission line at a point a quarter wave in front of the short. There is complete absorption at that frequency when this resistance is equal to the characteristic impedance of the line. The free-space analogy involves placing a resistive sheet a quarter-wave length in front of a conductive surface. Here the resistivity of the sheet must equal 377 ohms per square to be equal to the impedance of free space. For those wanting to make their own Salisbury Screen type absorber, 377-ohm films are available in a resistive plastic sheet, ECCOSORB® VF-30.

Because resonant absorbers are much thinner than other types, they are preferred for most applications where wide operational band width is not a requirement. They are customarily chosen to improve antenna patterns by covering parts of antennas and to improve radar performance by covering nearby reflecting surfaces. Their physical flexibility is often useful in coating surfaces of complex shape. They are usually mounted directly on the metal that is causing the reflection. Where the surface is sharply curved or small in terms of wave lengths, some reduction in performance is to be expected. For cases where the material cannot be mounted directly on metal, it is necessary to provide a conducting rear surface (such as foil or silver paint) to achieve low reflection at the resonant frequency. The limited reflectivity characteristics of resonant absorbers, especially at wide angles of incidence, lead to the use of the thicker broad-band types in many applications.

Broad-Band Absorbers
In contrast to resonant absorbers, which use the back-face reflection to good advantage, broad-band types must have sufficient attenuation so that any energy reflected from the back face returns to leave the material as very low-level reflection. To get this degree of attenuation in practice requires appreciable thickness in terms of wave length. While it might be expected that a material of higher loss tangent and higher dielectric constant would provide adequate loss at reduced thickness, it is important to note that a medium with such properties would also have an impedance quite different from that of free space and, therefore, be higher in front-face reflection. Such front-face reflections can be significant in a lossy medium and may strongly deteriorate the performance of an absorber.

Therefore, to understand broadband absorber principles, one must understand the concept of tapered impedance. Broadband absorbers are designed by varying the impedance from that of free space at its incident surface to lower impedance, more lossy material at its rear surface. Incident energy "sees" free space as it enters the material and is attenuated as it propagates through a medium which becomes more lossy.

Instead of having a medium of uniform properties, we adjust the dielectric constant to be very low at the front surface and increase it to a relatively high number at the rear. Consequently, the front face reflection and the overall thickness, which is required to make the back face reflection negligible, are greatly reduced, since a very high impedance can be tolerated in the lossy material at the rear. Extremely low reflection levels can be achieved in this manner, resulting in performance more than 60 dB below incident energy levels under best conditions.

In practice, compromises and approximations can be tolerated in broadband absorbers by:

using discrete layers of stacked, homogeneous materials, each with a higher complex dielectric constant, thereby achieving, a stepped rather than continuous variation.
using parallel resistive sheets of decreasing surface resistivity separated by low loss dielectric material.
using pyramidal or geometric tapering to simulate a continuous change of dielectric constant.
Flat Broad-Band Absorbers

Typical of a broadband absorber made up of stacked layers is ECCOSORB® AN. They are thin broad-band absorbers only about l /4 thick at their low frequency performance limits. Reflectivity is better than -20 dB over the design range.

ECCOSORB® AN is available in a number of thickness’ as appropriate for operation from the common radar bands upward. The fact that it is flexible, lightweight, easily cut, and inexpensive makes it the ideal choice for many applications, such as in radar antenna nacelles, antenna and ACS target mounts, on reflecting objects for cross section reduction, and around antennas for improved side-lobe control . Since ECCOSORB® AN is an open cell foam it is not intended for environments without weather protection. For weather-proof applications, it is available sealed in impervious nylon-fabric containers, called ECCOSORB® AN-W.

ECCOSORB® HR is another example of a broadband absorber based on an open cell foam. Rather than stacking materials each with uniform impregnation, as with ECCOSORB® AN, ECCOSORB® HR has a tapered impregnation with a continuous variation in electrical properties. Absorption with a continuously varying loss may offer better average reflectivity depending on the operational frequency range.

An alternative to stacking discrete homogeneous materials with different electrical properties, as in the ECCOSORB® AN series, or a tapered impregnation for a continuous variation in electrical properties, as in the ECCOSORB® HR, is to formulate a homogeneous material which attenuates energy, while at the same time minimizes surface reflections. ECCOSORB® FGM and ECCOSORB® FGMU are examples of this type of material. Although reflectivity is a bit higher, due to the change in medium from that of free-space, the advantage of a homogeneous broad-band material is that it is very thin (30 mils) and flexible. ECCOSORB® FGM and ECCOSORB® FGMU can be used to line cavities and enclosures where multiple bounce interferes with the device operation.

These flat-faced broadband types are limited to a reflectivity near -20 dB. This limitation is imposed not by reflections from the rear interface or the impedance gradient but by the front-face reflections resulting from the relatively high dielectric constant of the material of construction.

Geometrically Shaped Broad-Band Absorbers

In contrast, achieving the impedance gradient by geometrical shaping of a medium of constant impedance provides the opportunity for a much more complete transition to take place from free space to the dissipative medium. Geometrical shaping means breaking the front surface into an aggregate of shaped pointed elements (such as cones or pyramids), where the axis of individual elements is oriented perpendicular to the plane of the absorber. A wave entering such a medium encounters a smoothly changing ratio of medium to the adjacent free space. This is similar to the case where the actual properties are changing smoothly. Where these elements are sharp, any interface reflection is negligible and only a reflection remains that is directly related to the gradient or impedance change per wave length of depth into the medium. Such alteration of the incident surface, however, requires large physical space. Therefore, pyramidal absorbers are utilized mainly in anechoic chambers.

Where this gradient changes smoothly and gradually over many wave lengths, e.g. 10l , reflection coefficients as low as 60 dB become available. Because over much of the frequency range of interest materials of such electrical thickness would have large physical thickness, an absorber utilizing physical shaping is generally based on foam materials to provide light weight.

A variety of practical absorbers exists that capitalize on the very low reflectivity levels that result from physical shaping. ECCOSORB® VHP and HPY are composed of sharp pyramidal tips. These absorbers are able to provide reflection coefficients into the -60 dB range at high frequencies. These materials represent the state of the art with regard to low-reflection capability. Both are in the form of carbon-loaded, flexible, foam pyramids. They differ in that the pyramids of ECCOSORB® VHP are homogenous whereas those of ECCOSORB® HPY are partially hollow. The hollow material offers advantage with regard to light weight, power dissipation. and fire retardance, whereas the solid version is able to achieve a given reflectivity level with slightly less thickness. Both are available in a range of thickness’ the extremes of which are 3 inches and 180 inches (7-6 cm and 457 cm) . The thicker versions have seen applications to as low as 26 MHz. With regard to the opposite end of the spectrum, measurements at 100 GHz have confirmed that reflection is low, at least, through that part of the frequency range. Both of these absorbers are used primarily for covering the inside of rooms and shielded enclosures to create anechoic chambers. Both are available in a new, highly fire-retardant version, identified as ECCOSORB® VHP-NRL and HPY-NRL. These are adding an important new measure of safety which is becoming increasingly important for the safety and compliance of facilities equipped with anechoic chambers.

Versions of the geometrically shaped absorber also exist for outdoor applications, such as to reduce ground reflections or reflections from nearby objects. Weather tolerance is achieved in the case of ECCOSORB® HPY-W by enveloping the absorber in a reinforced nylon fabric. ECCOSORB®SPY, the outdoor version of VHP, is made of an open cell, very porous foam, which allows water to pass through readily. Since such construction also passes air readily, these absorbers are customarily used over ventilation duct openings in anechoic chambers. Further, as a consequence of their open structure, they are able to dissipate more RF power than other types of the same shape. With forced-air cooling, this absorber can dissipate power levels as high as 15 watts per square inch of absorber surface.

Transmission Line Broad-Band Absorbers

While all of the previously described absorbers are for free-space use, there are counterpart materials which are comparably effective for absorption of energy traveling on transmission lines, such as wave guides, coaxial lines, open-wire lines, and strip lines. ECCOSORB® MF, for example, is a high-loss, high-permeability solid material that is available in rods and sheets for machining into sharp-pointed termination. A version of this absorber, known as ECCOSORB® CR, comes as a casting resin for molding into especially shaped parts.

All absorbers described to this point have been designed for minimum specular reflection, i.e. the condition where the angle of reflection is equal to the angle of incidence. Such absorbers are designed to operate according to the optics approximation where the wave geometry of the wave/medium interaction is the sole criterion. However, in reality, a wave/medium interaction is composed of main and side lobe reflections. Specular absorbers reduce primarily the reflection level of the main lobe rather than the side lobes of the pattern of incident energy and are good approximations where angles of incidence are not too great. Surface current absorbers, on the other hand, attenuate primarily the side lobe energy.

Surface Current Absorbers
Surface current absorbers attenuate the side lobe energy reducing the currents flowing along reflecting bodies. These currents give rise to reflections when they encounter a discontinuity, such as a gaps, a step, a sharp change of curvature, or an edge. Such reflections can be important in establishing the absolute reflection levels from large, irradiated targets of complex shape. The absorbers operate by reducing the amplitude of currents before they encounter such discontinuities.

Surface current absorbers are in the form of thin sheets, having high magnetic permeability. ECCOSORB® FDS and ECCOSORB® GDS are typical with 0.030 inch (0.08 cm) thickness. A castable ferrite version is ECCOSORB® CFS-8480.

Non-magnetic surface current absorbers are also available in the form of carbon-impregnated foam sheets. These materials function differently in the sense that the conversion of electromagnetic energy to heat is accomplished by the excitation of the carbon filler rather than by the magnetic re-orientation of iron. An example of a carbon-impregnated material is the ECCOSORB®LS series, which comes with varying degrees of carbon loading. The advantage of this material is its low density, high loss and lower cost.

B. APPLICATIONS

Anechoic Chambers
An important application of absorbers is in construction of anechoic chambers. Using today's state-of-the-art absorbers and chamber designs, "free-space" with regard to amplitude and phase uniformity can be simulated to a very high degree. It is not uncommon at high microwave frequencies to be able to create a chamber test volume (quiet zone) in which the level of reflections from all regions is greater than 60 dB below the level of the incident signal. Chambers are customarily used to frequencies as low as 30 MHz and as high as 100 GHz. The lower frequency limit requires pyramidal absorbers, having thickness’ as great as 15 feet (4 6 m). Most chambers use a mix of absorber thickness’ and types in various regions in order to achieve optimum performance and low cost. Chambers range in size up to 52 feet by 52 feet (15.8 m by 15.8 m) in cross section and to 175 feet (53.3 m) in length. It is interesting that there are some sixty existing anechoic chambers with length greater than 60 feet, which speaks well of their usefulness in terms of dollars invested. Chambers have been built for testing of objects as large as aircraft and tanks.

While many chambers are in the shape of rectangular rooms, many others are in the shape of a horn and are known as tapered chambers. This shape effectively avoids specular reflections from the side walls, floor, and ceiling, which, in rectangular chambers, represent the primary limitation to effective performance at lower frequencies. Many chambers are built in shielded enclosures, where isolations of up to 120 dB are effective in preventing the radiation of high power into or from the chamber.

The most significant advance with regard to chambers in recent years relates to the flammability of the microwave absorbers. As the result of several fires that were very costly, the U.S. Naval Research Laboratory developed a set of standards for absorber quality, which greatly decreased the possibility of fire. These tests establish limits with regard to case of ignition and volume of noxious gasses given off during combustion. More specifically, they establish that chamber absorbers:

Will not ignite when probed with 220 volts AC.
Will not ignite when exposed to a 2000° C flame for 30 seconds.
Will not ignite when a cartridge heater at 600° C is inserted into the material for 10 minutes.
That concentrations of CO, HCL, and HCN not exceed certain limits for specified test conditions.
Chambers are employed primarily for the following measurement purposes:

Both primary and secondary antenna patterns
Both monostatic and bistatic radar-cross-section patterns.
Radome bore-sight error.
Complete systems, such as satellites, radar, aircraft, computers, missiles, vehicles, and electronic devices. These systems are measured in chambers with regard to such characteristics as comparability, susceptibility, vulnerability, system sensitivity, effective radiated power, tracking ability, and boresight accuracy.
Chambers provide a standard, reproducible environment for the measurement of a wide variety of electrical and electronic devices to establish that they meet requirements concerning spurious, harmonic, and noise emissions. A sampling of such devices that are measured in chambers are microwave-ovens, communication equipment, typewriters, motor generators, lights, computers, relays, television sets, etc.

Reduction of Radar Cross Section
Probably the first application of absorbers was during World War II for the purpose of achieving radar camouflage of submarine snorkel and periscope. In the early 1940’s, effective signal reductions with both resonant and broad band types were demonstrated.

Since that time, the information on such applications has disappeared in view of the significance to the military. There has been references, however, in the open literature to application of absorbers to parts of aircraft such as jet engine inlets.

Improvement of Antenna Patterns
Today, in the face of rapidly increasing electromagnetic pollution and proliferation, it becomes increasingly important that both transmitting and receiving antennas have low side lobe levels and narrow beam widths in order to reduce interference. Absorbers make a valuable contribution to this respect to a class of antennas known as tunnel antennas. These are antennas constructed with a forward extending lip or tunnel, which is covered on the inner surface with absorber. Where such tunnels have a length in the order of twice the antenna aperture size, appreciable narrowing of the main lobe can be achieved with only small gain reductions. Such a tunnel can reduce side lobe drastically. Even relatively short extensions of only a few wave lengths in extent can provide side lobe reductions. While this technique has been used most commonly with parabolic reflector antennas, a variety of antenna types, including planar arrays and horns have been so adapted in the case of horns, the absorber generally is continued forward at the flare angle of the horn rather than parallel to the direction of propagation. Absorber has also been mounted directly on the inner surface of horns for this purpose.

Tunnel antennas may be thought of as operating to provide a smooth amplitude taper transition across the aperture to zero at the edge. This is conducive to reduction of diffraction around the edge into the side lobe region.

Absorbers are useful in a number of other areas with regard to antennas. They are used to cover feeds, feed-mounting struts, transmission lines, and the rear surfaces of antennas. They are customarily used in cavity-backed spiral antennas to prevent radiation of a circularly-polarized signal of the opposite sense. They are used to cover surfaces or objects adjacent to antennas to prevent antenna pattern degradation.

Improvement in Radar Performance
Not all radar can be located in the clear, such as the nose of an aircraft, at the top of the highest mast of a ship, or at the top of a tower. Many must be located in less than optimum environments, where, during antenna motion, nearby objects or surfaces are illuminated by the main beam, which introduces significant errors into radar data. Covering these objects or surfaces with absorber is effective in minimizing or eliminating this problem. Where radar energy strikes these surfaces at angles up to perhaps +-30° from normal incidence, resonant absorbers tuned to the radar frequency are adequate. If the geometry is such that specular reflections take place at wide angles of incidence, thicker broad band absorbers are more appropriate, due to their much better reflection properties at wide angles.

On shipboard, these sources of reflection are typically the side if the tower holding the radar antenna nearby bulkheads, and other antennas and equipment. With aircraft, for example, a forward-looking radar, mounted beneath the body, may encounter reflections from the lip of the bay in which it is located or from positions of the aircraft surface. This latter problem has been handled by covering such surfaces with special resonant absorbers, designed for optimum performance at the particular and polarization imposed by the operating conditions.

Almost all aircraft radar, both military and commercial, use absorbers on the metal surfaces behind and around their antennas to avoid problems associated with changing reflection conditions during the antenna scan. Since these surfaces are usually of complex shape, absorbers for these regions are often made to fit the particular shapes involved.

Just as there are some conditions for testing systems and equipment where it is advantageous to replace the antenna with a transmission line termination or dummy load, there are other conditions that make it necessary or advantageous to test with the antennas in the system. In test involving the antennas themselves, it is desirable that stray radiated energy be absorbed rather than being allowed to escape and cause interference. The customary solution to the problem is to cover the antenna with a metallic enclosure, which itself is covered on its inner surfaces with absorber. Such devices are called antenna caps.

Antenna caps are useful in measuring antenna properties, such as VSWR, power handling capability, and function as part of a complete system (such as a radar satellite) in a non-radiating mode. In some cases, they are designed as shielded enclosures signal attention of up to 100 dB. In some cases, they include a probe antenna, which is used to sample radiation from the system or transmit signals to it. Probes are often included where calibrations are to be run or measurements are to be made of radiated power or system sensitivity. In some cases, a movable probe is included where tracking is involved.

Antenna caps are custom designed to the conditions required. Variations with regard to such factors as geometry, frequency, VSWR, and power dissipation, must be taken into account. VSWR may be as low as 0.01 over wide frequency ranges. When designed for the job, antenna caps can absorb the full power output of radar. In some cases, they become rather sophisticated pieces of equipment, e.g. in the case of a cap for an aircraft nose radar. Such a cap may: be of large size, have wheels so that it may moved to and from the aircraft, have a jack as means of raised and adjusted to the right elevation, have an RF seal to the aircraft skin behind the radar in order to achieve high shielding effectiveness, have associated electronic equipment, include means for removing the absorbed power.

Microwave Devices
With the proliferation of telecommunications technology, the consequent push for electromagnetic compatibility and a general trend toward higher frequencies, electronics design is confronting challenging constraints. Absorbers are now being called on in a variety of applications in the commercial marketplace. Absorbers are currently widely used for lowering the quality factor of cavities where microwave circuitry is operating, reducing the resonance in electromagnetic cavities and general problem solving where the characterization of electromagnetic phenomenon in enclosures is extremely difficult. Furthermore, the competitiveness of the design cycle in the commercial telecommunications marketplace makes it extremely expensive and time consuming to fully characterize how device operation can be degraded by internal resonance. With ECCOSORB® as an important tool, microwave and electronics engineers can determine design requirements more quickly and economically and thereby gain that vital competitive edge. Emerson and Cuming Microwave Products is ideally poised with experience and know-how to respond to the exciting challenges confronted in the modern telecommunications marketplace.


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Excerpts from William H. Emerson, "Electromagnetic Wave Absorbers, Useful Tools for Engineers."