Application of electromagnetic absorbers to radar retroreflective devices.

Abstract

A radar retroreflective (R3) device is provided that includes an electromagnetic absorber disposed over selected regions of the device to reduce specular reflections without substantially reducing the retroreflection of the device.

Description

[Background technology]

[0001] Vehicle-based radar systems are widely used to detect objects or landmarks. Passive retroreflectors, such as Van Atta arrays, are becoming increasingly accessible by providing retrodirectivity capabilities in wireless systems. Summary of the Invention

[0002] For example, it is desirable to optimize passive radar retroreflectors to reduce interference from radar signals of surrounding vehicles and unwanted reflected signals by vehicles or objects. The present disclosure provides radar retroreflector (R3) devices that include electromagnetic absorber material to reduce interference and enhance retrodirectivity.

[0003] In one aspect, the present disclosure describes a radar retroreflective (R3) device that includes a dielectric substrate including a first major surface and a second major surface opposite the first major surface, an antenna array of electromagnetic (EM) elements disposed on the first major surface of the dielectric substrate, the antenna array of electromagnetic elements being electrically interconnected to re-radiate incident EM waves back at a retroreflective angle substantially in the direction of arrival, and an electromagnetic absorber disposed on the first major surface of the dielectric substrate.

[0004] In another aspect, the present disclosure describes a method that includes disposing an antenna array of electromagnetic (EM) elements on a first major surface of a dielectric substrate, the antenna array being electrically interconnected to re-radiate an incident EM wave back toward a substantial direction of arrival at a retroreflection angle, and disposing an electromagnetic absorber on the first major surface of the dielectric substrate, the electromagnetic absorber at least partially surrounding the antenna array of EM elements and configured to reduce reflections of the incident EM wave without substantially reducing retroreflection of the incident EM wave.

[0005] Various unexpected results and advantages are obtained in the exemplary embodiments of the present disclosure. One such advantage of the exemplary embodiments of the present disclosure is that by applying an EM absorber to the R3 material / device, the associated specular reflection can be significantly reduced while maintaining the retroreflective performance. The embodiments described herein can improve the signal-to-noise ratio (SNR), allowing for clearer radar object detection.

[0006] The above is a summary of various aspects and advantages of exemplary embodiments of the present disclosure. The above "Summary" is not intended to describe each illustrated embodiment or every implementation of specific exemplary embodiments of the present disclosure. The following figures and "Description of the Preferred Embodiments" more particularly exemplify certain preferred embodiments that employ the principles disclosed herein. [Brief description of the drawings]

[0007] The present disclosure may be more fully understood from the following detailed description of various embodiments of the disclosure when considered in conjunction with the accompanying drawings, in which: [Figure 1] FIG. 2 is a schematic diagram illustrating reflections and retroreflections from a radar retroreflective (R3) device, according to one embodiment. [Figure 2A] FIG. 2 illustrates a top view of a radar retroreflective (R3) device, according to one embodiment. [Figure 2B] FIG. 2B is a cross-sectional view of a portion of the device of FIG. 2A. [Figure 3A] FIG. 2 is a side perspective view of the first embodiment. [Figure 3B] 1 is a plot of reflection and retroreflection versus angle for Example 1. [Figure 4A] FIG. 11 is a side perspective view of the second embodiment. [Figure 4B] 1 is a plot of reflection and retroreflection versus angle for Example 2. [Figure 5A] FIG. 11 is a side perspective view of the third embodiment. [Figure 5B] 1 is a plot of reflection and retroreflection versus angle for Example 3. [Figure 6A]FIG. 11 is a side perspective view of the fourth embodiment. [Figure 6B] 1 is a plot of reflection and retroreflection versus angle for Example 4.

[0008] In the drawings, like reference numbers refer to like elements. The above-identified drawings may not be drawn to scale and illustrate various embodiments of the present disclosure, although other embodiments are also contemplated, as noted in the Detailed Description. In all cases, the disclosure describes the disclosure disclosed herein by representing exemplary embodiments, and not by express limitation. It should be understood that numerous other modifications and embodiments may be devised by those skilled in the art that are within the scope and spirit of the present disclosure. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0009] FIG. 1 is a schematic diagram showing reflections and retroreflections from a radar retroreflective (R3) device, according to one embodiment. The radar retroreflective (R3) device 10 includes analog radio frequency (RF) components disposed on its major surface 11 configured to reflect an incident signal 2 from a source (not shown) toward the source, i.e., toward the substantial direction of arrival at a retroreflected signal 4 at a retroreflection angle 5. The acceptable retroreflection angle 5 may range, for example, from -40 degrees to 40 degrees, or from -60 degrees to 60 degrees. Retrodirectivity may be achieved by an antenna array of electromagnetic (EM) elements disposed on a dielectric substrate. One typical antenna array for achieving retrodirectivity is the Van Atta array, which may include an array of antennas connected in symmetric pairs by equal length transmission lines or transmission lines with a difference in length equal to a multiple of the guided wavelength. Exemplary structures of Van Atta arrays and methods for their preparation and use are described, for example, in "Van Atta Reflector Arrays", ED Sharp and MADiab, IRE Transactions on Antennas and Propagation, pp. 436-438, 1960, and in U.S. Pat. No. 2,908,002 (LC Van Atta).

[0010] As shown in FIG. 1, in addition to the desired retro-reflected signal 4, there is also an undesired reflected signal 4' from the device 10 when reflecting the incident signal 2 from the device 10. In many cases, the reflected signal 4' may be undesired because it may not be reflected back to the actual direction of arrival and may go undetected, causing interference. The present disclosure describes devices and methods for reducing interference, for example, from radar signals of surrounding vehicles and undesired reflected signals by vehicles or objects. In some embodiments, the antenna array re-radiates back incident EM waves in the frequency range of 20 GHz to 130 GHz. It should be understood that the radar retro-reflecting (R3) devices described herein may function for any desired radar frequency band, such as, for example, the 24 GHz frequency band, the 76-77 GHz frequency band, the 77-81 GHz frequency band, the 79 GHz frequency band, etc.

[0011] In various embodiments, an electromagnetic absorber is provided on selected regions of the R3 device to reduce specular reflection of the incident EM wave without substantially reducing retroreflection of the incident EM wave from the R3 device. While an electromagnetic absorber can reduce specular reflection through absorption, the electromagnetic absorber may have some side effects, such as reducing retroreflection, shifting the angle of retroreflection, etc. Some current embodiments provide a means to overcome the side effects. In some examples, the electromagnetic absorber may reduce retroreflection of the incident EM wave from the device by 10 dB or less, 5 dB or less, or 3 dB or less. In some examples, the electromagnetic absorber may reduce reflection of the incident EM wave from the device by 1.5 dB or more, 2 dB or more, 3 dB or more, or 4 dB or more. In some examples, the electromagnetic absorber may shift the retroreflection angle of the incident EM wave by 20 degrees or less, 15 degrees or less, 10 degrees or less, or 5 degrees or less.

[0012] Figure 2A is a top view of a radar retroreflective (R3) device 20, according to one embodiment. Figure 2B is a cross-sectional view of a portion of the device of Figure 2A. Device 20 includes a dielectric substrate 22 including a first major surface 221 and a second major surface 222 opposite first major surface 221. Dielectric substrate 22 may include any suitable dielectric material, such as, for example, polytetrafluoroethylene (PTFE) or a composite thereof.

[0013] An antenna array 24 of electromagnetic (EM) elements is disposed on or embedded within a first major surface 221 of the dielectric substrate 22. The EM elements can be any conductive pattern that can be either transparent or opaque. In some examples, the antenna array can include a transparent conductive pattern. A conductive layer 26 is disposed on a second major surface 222 of the dielectric substrate 22.

[0014] The electromagnetic elements 24 are electrically interconnected by transmission lines 242 to re-radiate the incident EM waves back at a retroreflection angle substantially in the direction of arrival. An exemplary antenna array includes a Van Atta reflector array. In the embodiment shown in FIG. 2A, the EM elements are connected to form three pairs of columns 24a, 24b, and 24c. Each pair is arranged to form a symmetrical pattern. It should be understood that the array of antennas may include any number of pairs and be arranged in any desired pattern, so long as the symmetrical pairs are connected by transmission lines of equal length or a difference in length equal to a multiple of the guided wavelength, i.e., the transmission lines do not impart additional phase difference to the incident waves.

[0015] The electromagnetic absorber is disposed on or embedded within one or more selected regions of the first major surface 221 of the dielectric substrate 22. As shown in FIG. 2A, the first major surface 221 of the dielectric substrate 22 may be divided into multiple types of regions 2a, 2b, 2c, and 2d, as described below. The peripheral region 2a of the dielectric substrate 22 substantially surrounds the antenna array 24. The peripheral region 2a may refer to at least a portion or all of the space from the vicinity of the antenna array 24 (indicated by the dashed box 241) to the edge 223 of the dielectric substrate 22. The antenna region 2b of the dielectric substrate 22 directly supports the antenna array of the electromagnetic (EM) elements 24 and the transmission lines 242 connecting the EM elements 24. The first gap region 2c is located between and separates adjacent rows of the EM elements 24. The second gap region 2d is located between and separates adjacent transmission lines 242. In some examples, the continuous area occupied by the antenna array on the substrate may refer to the sum of the antenna area 2b, the first gap area 2c, and the second gap area 2d. The outer edge area 2a may substantially surround the continuous area of ​​the antenna array. In some examples, the area ratio of 2a / (2b+2c+2d) may be in the range of, for example, 2:1 to 1:10, which may depend on the desired array of EM elements being used.

[0016] In some embodiments, the electromagnetic absorber may be disposed on or embedded within the peripheral region 2a of the dielectric substrate 22. The electromagnetic absorber is disposed a certain distance d away from the region 2b (e.g., the region inside the frame 241) that supports the antenna array 24. In some examples, the distance d may be in the range of λ / 10 to λ, where λ is the wavelength of the incident EM wave. In the embodiment shown in FIG. 2B, the EM absorber 202 is disposed on the peripheral region 2a of the dielectric substrate 22 adjacent to the edge 223 of the dielectric substrate 22.

[0017] In some embodiments, the electromagnetic absorber may be disposed on or embedded within the first gap region 2c located between and separating adjacent rows of EM elements 24. It should be understood that the electromagnetic absorber may not contact the EM elements 24 and the transmission line 242. In the embodiment shown in FIG. 2B, the EM absorber 204 is disposed on the first gap region 2c between adjacent EM elements 24.

[0018] In some embodiments, the electromagnetic absorber may be disposed on or embedded within the second gap region 2d that is located between and separates adjacent transmission lines 242. It should be understood that the electromagnetic absorber may not contact the transmission lines 242.

[0019] The electromagnetic absorbers described herein are disposed on or embedded within selected regions of a substrate surface and are configured to reduce specular reflection of incident EM waves from the substrate without substantially reducing retroreflection of the incident EM waves from the substrate. The electromagnetic absorbers have suitable EM properties to absorb incident EM waves and reduce specular reflection. Exemplary electromagnetic absorber materials are described, for example, in U.S. Patent Application Publication No. 2020 / 0053920 (Ghosh), U.S. Patent No. 9,704,613 (Ghosh, Roy, and Satarkar), and "Structural and high GHz frequency EMI (Electromagnetic Interference) properties of carbonyl iron and boron nitride hybrid composites," Mater. Res.Express 6, 106305 (2019).

[0020] Complex permittivity (ε r= ε'-jε") is an important parameter that can determine the microwave absorption properties of a composite material, where the real part of the complex permittivity (ε') represents the storage ability of electrical energy and the imaginary part of the permittivity (ε") describes the loss ability of electrical energy. The loss tangent value, tan δ = (ε" / ε'), is often used to quantify the loss value of a dielectric material. In general, to achieve the desired absorption performance, it is useful to keep the loss tangent (tan δ) value high while keeping the value of the real part of the permittivity ε' low to reduce reflection.

[0021] In some embodiments, suitable composite materials may have a dielectric loss tangent in the frequency band of interest, for example, in the range of about 0.05 to about 0.8, about 0.1 to about 0.8, about 0.2 to about 0.8, or about 0.25 to about 0.75. The high dielectric loss of the composite materials may be due to the high loading levels of the hybrid ceramic and conductive particles (e.g., CuO and carbon black particles). The preferred value of the dielectric permittivity is typically less than 10 in the frequency range of interest.

[0022] In some embodiments, suitable absorber composite materials may include one or more ceramic filler materials. Exemplary ceramic filler materials may include at least one of copper (II) oxide (CuO) or titanium (II) oxide (TiO) in a polymer matrix, with the filler loading in the composite being 50 to about 95% by weight. In some embodiments, the EMI composite material may include a high loading level of ceramic particles (e.g., CuO particles) dispersed in a suitable matrix material (e.g., polymer). In some embodiments, the polymer matrix material may include a cured polymer system, such as, for example, silicone, epoxy, cyclic olefin copolymer (COC), low density polyethylene (LDPE), high density polyethylene (HDPE), polystyrene (PS), polypropylene (PP), polyphenylene sulfide (PPS), polyimide (PI), syndiotactic polystyrene (SPS), polytetrafluoroethylene (PTFE), butyl rubber, acrylonitrile butadiene styrene (ABS), polycarbonate (PC), polyurethane, or combinations thereof.

[0023] In some embodiments, a suitable absorber composite may include a ceramic filler material and a conductive filler material. Exemplary conductive filler materials may include at least one of carbon black, carbon bubbles, carbon foam, graphene, carbon fibers, graphite, carbon nanotubes, metal particles, metal nanoparticles, metal alloy particles, metal nanowires, polyacrylonitrile fibers, or conductive coated particles, and the conductive filler loading in the composite is 0.1-3 wt %.

[0024] In some embodiments, the electromagnetic absorber may include thermally conductive and electromagnetically absorbing particles. Exemplary particles may include dual-layer core particles. Exemplary particles are described in U.S. Pat. No. 5,389,434 (Griswold et al.) and WO 2021 / 198849 A1 (Lu et al.). For example, the particle may include Al2O3 as a core, the middle layer is a thin conductive metal, and the outermost layer is an Al2O3 insulating layer. In some examples, the EM properties of the absorber may be controlled by controlling the particle loading volume in the composite. For example, at a particle loading volume of 45% by volume, the absorber may have a dielectric constant of ε' or greater than 8 and ε" or greater than 3.

[0025] The electromagnetic absorber may be formed on selected regions of the substrate in a single layer or multiple layers. It is to be understood that the electromagnetic absorber may be formed on the substrate surface or embedded within the substrate adjacent to the substrate surface. In some embodiments, the electromagnetic absorber in a single layer or multiple layers may have a thickness in a range of, for example, 0.01 mm to 10 mm, 0.02 mm to 5 mm, or 0.05 mm to 2 mm. It is to be understood that the thickness of the electromagnetic absorber may depend on the wavelength λ of the incident EM wave (e.g., an optimized thickness may be a quarter wavelength).

[0026] In some embodiments, an anti-reflective (AR) material may be provided on the electromagnetic absorber to further reduce reflections from the electromagnetic absorber. Referring again to FIG. 2B, an anti-reflective material 206 is provided on top of the electromagnetic absorber 204. A suitable anti-reflective composite may include, for example, a ceramic filler material such as copper (II) oxide (CuO) in a polymer matrix. In some examples, the AR composite may not include a conductive filler in its polymer matrix, while the absorber composite may contain a hybrid filler, for example, a mixture of ceramic CuO and a conductive filler in its polymer matrix.

[0027] The electromagnetic absorber and anti-reflective materials described herein may be applied to selected areas of a substrate surface by any suitable method or process. In one example, the electromagnetic absorber may include a hybrid filler, such as a silicone composite having 50-80 wt% CuO and 0.6-1 wt% carbon black, which may be prepared by processes such as mixing, curing, pressing, etc. In one example, the anti-reflective material may include a ceramic filler, such as a silicone composite having 20-80 wt% CuO, which may be prepared by processes such as mixing, curing, pressing, etc.

[0028] Various modifications and variations can be made to the exemplary embodiments of the present disclosure without departing from the spirit and scope of the present disclosure. Therefore, it should be understood that the embodiments of the present disclosure are not limited to the exemplary embodiments described below, but are governed by the limitations set forth in the claims and any equivalents thereof.

[0029] List of Exemplary Embodiments Exemplary embodiments are listed below. It should be understood that any one of the embodiments 1 to 10 and the embodiments 11 to 15 can be combined.

[0030] In embodiment 1, a dielectric substrate including a first main surface and a second main surface opposite the first main surface; an antenna array of electromagnetic (EM) elements disposed on a first major surface of a dielectric substrate, the antenna array of electromagnetic elements being electrically interconnected to re-radiate incident EM waves at a retroreflection angle back toward the substantial direction of arrival; and an electromagnetic absorber disposed on or embedded within a first major surface of the dielectric substrate.

[0031] Embodiment 2 is the device of embodiment 1, wherein at least a portion of the electromagnetic absorber is disposed on an outer edge of the dielectric substrate that at least partially surrounds the antenna array.

[0032] Example 3 is the device of example 1, wherein the electromagnetic absorber comprises one or more ceramic filler materials and, optionally, one or more conductive filler materials in a polymer matrix.

[0033] Embodiment 4 is the device of embodiment 1, wherein the electromagnetic absorber comprises thermally conductive particles each having an electrically conductive metal layer.

[0034] Embodiment 5 is the device of embodiment 1, further comprising an anti-reflective coating on the electromagnetic absorber.

[0035] Embodiment 6 is the device of embodiment 5, wherein the antireflective coating has a dielectric constant and a dielectric loss tangent relatively lower than the dielectric constant and the dielectric loss tangent of the electromagnetic absorber.

[0036] Example 7 is the device of example 1, wherein the electromagnetic absorber shifts the retroreflection angle of the incident EM wave by 10 degrees or less.

[0037] Example 8 is the device of example 1, wherein the electromagnetic absorber reduces retroreflection of an incident EM wave from the first major surface by no more than 3 dB.

[0038] Example 9 is the device of example 1, wherein the device reduces reflection of an incident EM wave from the first major surface by at least 3 dB.

[0039] Embodiment 10 is the device of embodiment 1, wherein the antenna array comprises a Van Atta reflector array.

[0040] Embodiment 11 is disposing an antenna array of electromagnetic (EM) elements on a first major surface of a dielectric substrate, the EM elements being electrically interconnected to re-radiate incident EM waves at a retroreflection angle back toward the substantial direction of arrival; disposing an electromagnetic absorber on the first major surface of the dielectric substrate, the electromagnetic absorber being configured to reduce reflection of an incident EM wave from the first major surface without substantially reducing retroreflection of the incident EM wave from the first major surface.

[0041] Example 12 is the method of example 11, wherein the electromagnetic absorber is disposed on or embedded within an outer edge of the first major surface, at least partially surrounding the antenna array.

[0042] Example 13 is the method of example 11, wherein the electromagnetic absorber reduces retroreflection of an incident EM wave from the first major surface by 3 dB or less.

[0043] Example 14 is the method of example 11, wherein reflection of the incident EM wave from the first major surface is reduced by at least 3 dB.

[0044] Example 15 is the method of example 11, wherein the antenna array re-radiates back the incident EM waves in the frequency range of 20 GHz to 130 GHz. EXAMPLES

[0045] These examples are for illustrative purposes only and are not meant to limit the scope of the appended claims.

[0046] A radar retroreflective (R3) material / device was designed and simulated using a commercial electromagnetic modeling tool, CST Microwave Studio from Dassault Systemes (Waltham, MA, USA). The R3 device was designed with the configuration shown in FIG. 2A. The R3 device has a dielectric substrate with a size of 31.6 mm×15.8 cm×0.127 mm. The dielectric substrate has a dielectric constant of 2.2. An antenna array of electromagnetic (EM) elements is provided on the substrate surface. The EM elements are arranged in six rows, each with eight elements. Each EM element has a size of 1.13 mm×1.263 mm. The transmission lines have a width of 0.3 mm. The transmission lines connecting adjacent EM elements in each row have a length of 1.371 mm. The gap between adjacent EM elements in adjacent rows is 1.17 mm.

[0047] To reduce the specular reflection of the R3 device while maintaining its retroreflection, an EM absorber was applied to selected areas on the R3 device. The EM absorber has a dielectric constant of 8 and a loss tangent (tan δ) of 0.25. The thickness of the EM absorber was swept from 0.1 mm to 0.5 mm. Four examples (Examples 1-4) were calculated at 77 GHz at different application areas on top of the R3 material / device.

[0048] Example 1 FIG. 3A is a side perspective view of Example 1. The EM absorber was applied to the entire surface except the area of ​​the antenna array (the EM elements and the transmission lines connecting the EM elements). Referring to FIG. 2A, the application area includes areas 2a, 2c, and 2d. FIG. 3B is a plot of reflection and retroreflection versus angle for Example 1. As shown in FIG. 3B, the reflection was significantly reduced by 16 dB with the 0.3 mm thick EM absorber, while the retroreflection was also reduced by 6 dB. The angle of retroreflection was also shifted by 5°-10° with the EM absorber between the arrays (e.g., on area 2c of FIG. 2A).

[0049] Example 2 FIG. 4A is a side perspective view of Example 2. The EM absorber was applied on the peripheral area and center gap of the array. Referring to FIG. 2A, the application area includes area 2a, the center gap in area 2c, and 2d. FIG. 4B is a plot of reflection and retroreflection versus angle for Example 2. As shown in FIG. 4B, the reflection was significantly reduced by 25 dB with the 0.3 mm thick EM absorber, while the retroreflection was reduced by only 4 dB. Compared to Example 1, there was less shift in the angle of retroreflection.

[0050] Example 3 FIG. 5A is a side perspective view of Example 3. The EM absorber was applied on the peripheral area surrounding the antenna array (the EM elements and the transmission lines connecting the EM elements). Referring to FIG. 2A, the application area includes area 2a and area 2d. FIG. 5B is a plot of reflection and retroreflection versus angle for Example 3. As shown in FIG. 5B, the reflection was significantly reduced by 20 dB with the 0.3 mm thick EM absorber, while the retroreflection was reduced by only 3 dB. Compared to Examples 1 and 2, there was less shift in the angle of retroreflection.

[0051] Example 4 FIG. 6A is a side perspective view of Example 4. The EM absorber was applied on the peripheral area surrounding the antenna array (the EM elements and the transmission lines connecting the EM elements). Referring to FIG. 2A, the application area includes only area 2a. FIG. 6B is a plot of reflection and retroreflection versus angle for Example 4. As shown in FIG. 6B, the reflection was significantly reduced by 8 dB with the 0.3 mm thick EM absorber, while the retroreflection was reduced by only 0.5 dB. Compared to Example 3, the retroreflection intensity is less affected by the EM absorber.

[0052] Unless otherwise indicated, all numbers expressing quantities or ingredients, property measurements, and the like used in the specification and embodiments are to be understood in all instances as being modified by the term "about". Thus, unless indicated to the contrary, the numerical parameters set forth in the foregoing specification and accompanying list of embodiments may vary depending upon the desired properties one of ordinary skill in the art would obtain utilizing the teachings of the present disclosure. At the very least, each numerical parameter should at least be construed in light of the number of reported significant digits and by applying ordinary rounding techniques, which is not intended to limit the application of the doctrine of equivalents to the scope of the embodiments described in the claims.

[0053] Throughout this specification, references to "one embodiment," "a particular embodiment," "one or more embodiments," or "an embodiment" mean that the specific features, structures, materials, or characteristics described in connection with that embodiment are included in at least one of the specific exemplary embodiments of the disclosure, regardless of whether the term "exemplary" is included before the term "embodiment." Thus, the appearance of phrases such as "in one or more embodiments," "in a particular embodiment," "in one embodiment," or "in an embodiment" in various places throughout this specification do not necessarily refer to the same specific exemplary embodiments of the disclosure. Moreover, specific features, structures, materials, or characteristics may be combined in any suitable manner in one or more embodiments. Although certain exemplary embodiments have been described in detail herein, it will be understood that those skilled in the art will be able to readily conceive modifications, variations, and equivalents of these embodiments upon reading the above description. Thus, it should be understood that this disclosure is not to be unduly limited to the exemplary embodiments described thus far. Moreover, various exemplary embodiments have been described. These and other embodiments are within the scope of the following claims.

Claims

1. a dielectric substrate including a first main surface and a second main surface opposite to the first main surface; an antenna array of electromagnetic (EM) elements disposed on the first major surface of the dielectric substrate, the antenna array of electromagnetic elements being electrically interconnected to re-radiate incident EM waves at a retro-reflection angle back substantially in the direction of arrival; an electromagnetic absorber disposed on or embedded within the first main surface of the dielectric substrate; 1. A radar retroreflective (R3) device comprising:

2. The device of claim 1 , wherein at least a portion of the electromagnetic absorber is disposed on an outer edge of the dielectric substrate that at least partially surrounds the antenna array.

3. The device of claim 1 , wherein the electromagnetic absorber comprises one or more ceramic filler materials and, optionally, one or more conductive filler materials in a polymer matrix.

4. The device of claim 1 , wherein the electromagnetic absorber comprises thermally conductive particles each having a conductive metal layer.

5. The device of claim 1 further comprising an anti-reflective coating on the electromagnetic absorber.

6. The device of claim 5 , wherein the anti-reflective coating has a dielectric constant and a dissipation factor that are relatively lower than the dielectric constant and dissipation factor of the electromagnetic absorber.

7. 10. The device of claim 1, wherein the electromagnetic absorber shifts the retroreflection angle of the incident EM wave by 10 degrees or less.

8. 10. The device of claim 1, wherein the electromagnetic absorber reduces retroreflection of the incident EM wave from the first major surface by no more than 3 dB.

9. 10. The device of claim 1, wherein the device reduces reflection of the incident EM wave from the first major surface by at least 3 dB.

10. The device of claim 1 , wherein the antenna array comprises a Van Atta reflector array.

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