SE545259C2

SE545259C2 - A radar reflector

Info

Publication number: SE545259C2
Application number: SE2230124A
Authority: SE
Inventors: Johan Wettergren
Original assignee: Qamcom Group AB
Priority date: 2022-04-28
Filing date: 2022-04-28
Publication date: 2023-06-07
Also published as: SE2230124A1

Abstract

A radar reflector (200) arranged to reflect electromagnetic radiation in one or more frequency bands, the reflector (200) having a first layer (210) comprising a plurality of conductive beads (211) embedded in a dielectric material (212). A dimension of at least some of the conductive beads (211) corresponds to a Mie scattering resonance of a wavelength in the dielectric material (212) of the electromagnetic radiation in a first frequency band out of the one or more frequency bands.

Description

TITLE A RADAR REFLECTOR TECHNICAL FIELD The present disclosure relates to radar reflectors, especially radar reflectors used to improve safety conditions around vehicles equipped with radar transceivers.

BACKGROUND Radar transceivers are increasingly being used in vehicles to detect objects in the environment, such as stationary obstacles, other vehicles, and pedestrians. However, the radar signal reflected by an object is not always strong enough to ensure reliable detection by a radar transceiver, especially if the object is a pedestrian or a person on a smaller vehicle such as a bicycle or electric scooter. Some weather conditions, such as heavy rain or wet snow, can also adversely affect the radar transceivers.

A pedestrian who wishes to be more clearly visible to the human eye or to a camera can choose to wear an optical safety reflector, which is arranged to strongly reflect visible light. However, equipment arranged to reflect the radio waves used by radar transceivers is not so readily available.

WO2020240364 A1 discloses a wearable radar reflective article.

Still, there is a need for improved radar reflectors.

SUMMARY lt is an object of the present disclosure to provide an improved radar reflector, and particularly an improved wearable radar reflector.

This object is at least in part obtained by a radar reflector arranged to reflect electromagnetic radiation in one or more frequency bands. The reflector has a first layer comprising a plurality of conductive beads. The plurality of conductive beads is embedded in a dielectric material. A dimension of at least some of the conductive beads corresponds to a Mie scattering resonance of a wavelength in the dielectric material of the electromagnetic radiation in a first frequency band out of the one or more frequency bands.

Mie scattering occurs when electromagnetic radiation interacts with objects that have a spatial dimension, such as a perimeter, circumference, or diameter, similar to the wavelength of the electromagnetic radiation. ln particular, a l\/lie scattering resonance occurs when an object scatters electromagnetic radiation at some wavelength particularly strongly. This effectively results in a large radar cross section (RCS) compared to the object size. Thus, if the radar reflector comprises conductive beads with a dimension tuned to a l\/lie resonance of electromagnetic radiation in a frequency band, this has the advantage that the radar reflector will display a high RCS for radiation in that frequency band.

According to one example, a dimension of at least some of the plurality of conductive beads corresponds to a l\/lie scattering resonance of a wavelength in the dielectric material of the electromagnetic radiation in a second frequency band out of the one or more frequency bands. That is, some of the conductive beads may have a spatial dimension corresponding to a Mie scattering resonance for the first frequency band, and some of the conductive beads may have a spatial dimension corresponding to a Mie scattering resonance for the second frequency band. Advantageously, this leads to the radar reflector having a high RCS in two separate frequency bands. This is particularly advantageous if those two frequency bands correspond to frequency bands of operation for some radar system, such as an automotive radar system.

For electrically conductive objects such as conductive beads, the Mie scattering resonances can easily be found as a function of the ratio of a perimeter of the object to the wavelength of the electromagnetic radiation. Thus, the relevant dimension of the conductive beads is preferably the perimeter of the beads.

According to one example, at least one frequency band of the plurality of frequency bands comprises frequencies between 24 GHz and 26 GHz. This frequency band is frequently used for automotive radar, so if the radar reflector displays a high RCS for this frequency band it will be easier to detect with automotive radar, which is an advantage.

At least one frequency band may also comprise frequencies between 76 GHz and 81 GHz. This frequency band is also frequently used for automotive radar, so the radar reflector displaying a high RCS for this frequency band is an advantage. lt may be noted that the dimension of the conductive beads can be simultaneously tuned to the second Mie scattering resonance for the 76 to 81 GHz frequency band and the first l\/lie scattering resonance for the 24 to 26 GHz frequency band. This means that a radar reflector could be designed which displays a high RCS for both frequency bands, using only one size of conductive beads. This would be advantageous for radar reflectors used around automotive radars, e.g. in traffic situations.

According to another example, at least one frequency band may comprise frequencies between 8 GHz and 12 GHz and / or between 2 GHz and 4 GHz. These frequency bands, commonly referred to as the X band and S band respectively, are frequently used in marine radar. lf the radar reflector is intended for use with marine radars, tuning the dimension of at least some of the conductive beads to either or both frequency bands is thus an advantage.

The conductive beads may be at least partly formed from a metallic material and may comprise a metal or metal alloy. This has the advantage of ensuring that the conductive beads are sufficiently electrically conductive.

According to some aspects, the dielectric material may be a plastic material. Plastic materials have the advantage of being easy to manufacture and to form into a desired shape. Some plastic materials also have the advantage of a low price. The plastic material may for example comprise an acrylic material such as an acrylate polymer or an acrylic resin. The plastic material may also comprise polytetrafluoroethene (PTFE) ln addition to the first layer, the radar reflector may also comprise a second layer arranged to reflect electromagnetic radiation in a high-frequency band. The high- frequency band may for example comprise the visible spectrum. This has the advantage of combining an optical reflector with the radar reflector, making it simultaneously easier to detect using radar and easier to spot by eye or using an optical sensor such as a camera. Such a combined reflector would for example be useful as a safety reflector worn by pedestrians in traffic.

Optical reflectors can be made in several ways. According to one example, the second layer may comprise a plurality of transparent beads, the transparent beads being half coated in a reflective material. According to another example, the second layer may comprise a plurality of micro-prisms.

The object is also obtained at least in part by a variety of products comprising a radar reflector as previously described. Compared to other radar reflectors such as trihedral reflectors, the radar reflector described here can be smaller in size but with a similar RCS. lt can also be made in a larger variety of shapes, which is an advantage especially for wearable products.

One example of such a product is a safety reflector comprising a radar reflector as described herein. Conventionally, safety reflectors are wearable optical reflectors used e.g. by pedestrians and bicyclists to increase the probability that they can be seen by other road users, particularly in poor visibility conditions. A safety reflector comprising a radar reflector as described herein would also be easier to detect by radar, which is an advantage.

Another example is a high-visibility garment comprising a radar reflector as previously described. Like safety reflectors, high-visibility garments are worn to increase the probability of being spotted by other road users or, in the case of e.g. construction work, by operators of nearby equipment. Having a high-visibility garment comprising a radar reflector has the advantage of also increasing the likelihood of being detected by radar.

A third example is a hard hat arranged to reflect electromagnetic radiation in one or more frequency bands. The hard hat is made at least partly from a dielectric material, where a plurality of conductive beads is embedded in the dielectric material. A dimension of at least some of the conductive beads corresponds to a Mie scattering resonance of a wavelength in the dielectric material of the electromagnetic radiation in a first frequency band out of the one or more frequency bands.

Hard hats are primarily worn as protection at construction sites and in other similar environments. A hard hat as described above would have the advantage of rendering the wearer more easily detectable by radar.

Another example is a tool arranged to reflect electromagnetic radiation in one or more frequency bands. The tool comprises a housing made at least partly from a dielectric material, where a plurality of conductive beads is embedded in the dielectric material. A dimension of at least some of the conductive beads corresponds to a Mie scattering resonance of a wavelength in the dielectric material of the electromagnetic radiation in a first frequency band out of the one or more frequency bands. Advantageously, this tool can be more easily detected by vehicle radars if it is, for example, accidentally left on the ground.

There is also herein disclosed a method for producing a radar reflector arranged to reflect electromagnetic radiation in one or more frequency bands. The radar reflector has a first layer comprising a dielectric material. The method comprises obtaining a plurality of conductive beads. A dimension of at least some of the conductive beads is selected to correspond to a Mie scattering resonance of a wavelength in the dielectric material of the electromagnetic radiation in a first frequency band out of the one or more frequency bands. The method also comprises embedding the plurality of conductive beads in the dielectric material.

The method may also comprise applying a second layer to the reflector, the second layer being arranged to reflect electromagnetic radiation in a high-frequency band. The high-frequency band may preferably comprise the visible spectrum.

The methods disclosed herein are associated with the same advantages as discussed above in connection to the different apparatuses.

Generally, all terms used in the claims are to be interpreted according to their ordinary meaning in the technical field, unless explicitly defined otherwise herein. All references to "a/an/the element, apparatus, component, means, step, etc." are to be interpreted openly as referring to at least one instance of the element, apparatus, component, means, step, etc., unless explicitly stated otherwise. The steps of any method disclosed herein do not have to be performed in the exact order disclosed, unless explicitly stated. Further features of, and advantages with, the present invention will become apparent when studying the appended claims and the following description. The skilled person realizes that different features of the present invention may be combined to create embodiments other than those described in the following, without departing from the scope of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS The present disclosure will now be described in more detail with reference to the appended drawings, where: Figure 1 schematically illustrates a traffic scenario involving a vehicle and a pedestrian; Figure 2 schematically illustrates a radar reflector; Figure 3 schematically illustrates a radar cross section as a function of a size pafametef; Figure 4 is a drawing of a safety reflector; Figure 5 represents a high-visibility garment; Figure 6 represents a hard hat or helmet; and Figure 7 is a flow chart iliustrating methods.

DETAILED DESCRIPTION Aspects of the present disclosure will now be described more fully with reference to the accompanying drawings. The different devices and methods disclosed herein can, however, be realized in many different forms and should not be construed as being limited to the aspects set forth herein. Like numbers in the drawings refer to like elements throughout.

The terminology used herein is for describing aspects of the disclosure only and is not intended to limit the invention. As used herein, the singular forms "a", "an" and "the" are intended to include the plural forms as well, unless the context clearly indicates othen/vise.

Modern vehicles are often equipped with one or more radar transceivers. These automotive radars are used to detect objects in the environment around the vehicle, which may be moving objects such as other cars, bicycles, and pedestrians, and also stationary objects that may present an obstacle to the vehicle. Information from the radar transceiver regarding detected objects can for example be used for advanced driver assistance systems (ADAS), such as automatic braking, or even for autonomous drive (AD) applications.

Automotive radars transmit radar signals which are reflected by objects in the environment. The reflected signals can then be received by the radar, which extracts information about the position and velocity of the objects from the reflected signals. The intensity of a reflected signal from an object will depend on the strength of the transmitted signal and the distance to the object, but also on the object itself. Different objects reflect larger or smaller part of the radar signal depending on, for example, the shape, size, and material of the object.

The ability of an object to reflect a radar signal can be expressed through a quantity known as the radar cross section (RCS). The RCS of a radar target is defined as the hypothetical area that would be required to intercept the transmitted radar signal at the target such that if the total intercepted power was re-radiated isotropically, the power density actually observed at the receiver due to the target is produced.

Objects comprising electrically conductive materials such as metals generally have a higher RCS than an object of similar size comprising only electrically insulating materials. As an example, a traffic cone made from plastic will have a low RCS, while a metal pole of a similar size will have a high RCS. Larger motor vehicles such as cars or trucks generally have a high RCS, while smaller vehicles such as motorcycles or bicycles tend to have lower RCS. Pedestrians also have a low RCS compared to other road users. lt may be desirable for road users with low RCS, such as pedestrians or bicyclists, to render themselves more easily detectable by radars by increasing their RCS. This can be accomplished using radar reflectors, devices that are arranged to have a high RCS relative to their physical size.

Figure 1 shows a traffic scenario 100 involving a vehicle 110, equipped with an automotive radar 111, and a pedestrian 120 wearing a radar reflector 121. The automotive radar 111 transmits a radar signal 131 towards the pedestrian 120. The radar reflector 121 and the body of the pedestrian 120 reflect a part of the radar signal, leading to a reflected radar signal 132 returning towards the vehicle 110. Due to the radar reflector 121 a larger part of the transmitted radar signal 131 is reflected compared to if the pedestrian 120 had not worn a radar reflector. This makes it more likely that the pedestrian 120 is detected by the automotive radar Although Figure 1 shows an ordinary car, the vehicle could be any vehicle that is equipped with a radar. The vehicle could also be a heavy-duty vehicle such as a bus or truck, or construction equipment such as an excavator. ln addition to general traffic situations such as a pedestrian crossing a road, radar reflectors can also be used in situations where people work or move about in close proximity to construction equipment. As an example, consider a building site where heavy-duty vehicles and workers moving on foot are both present. lf the workers carry radar reflectors, they can be more easily detected by the heavy-duty vehicles and accidents can thereby be avoided. This may especially be the case under poor visibility conditions such as when there is a lot of dust in the air.

Radar reflectors are also used in other environments. One example of this is marine radar reflectors, which are typically affixed to masts or other protruding parts of smaller marine vessels in order to render them more easily detectable by marine radar.

The most common type of radar reflector is the corner reflector. A corner reflector comprises at least two surfaces that meet at an angle to form a corner. Both surfaces should comprise highly reflective material, for example a metal. A common form of corner reflector is a trihedral reflector, where the corner is formed by three surfaces joined together in a pyramid shape. Corner reflectors show a high RCS for radar signals incident at certain angles relative to the surfaces, but much lower RCS for radar signals incident at other angles. This may be undesirable for a radar reflector carried by a pedestrian as described above, as the pedestrian would wish to be detectable from all directions. Multiple corner reflectors could also be used to cover more directions, but such a device could become impractically large. A radar reflector that reflects equally well in all directions, is smaller than a corresponding trihedral reflector with the same RCS, and that can be given an arbitrary shape, would therefore be desirable.

Figure 2 shows a radar reflector 200 arranged to reflect electromagnetic radiation in one or more frequency bands. The reflector 200 has a first layer 210 comprising a plurality of conductive beads 211, where the plurality of conductive beads 211 are embedded in a dielectric material 212. A dimension of at least some of the conductive beads 211 corresponds to a l\/lie scattering resonance of a wavelength, in the dielectric material 212, of the electromagnetic radiation in a first frequency band out of the one or more frequency bands. ln this context, a conductive bead is an electrically conductive body of arbitrary shape, such as an iron granule, that is small relative to the total size of the radar reflector. Herein, that a material or object is electrically conductive is taken to mean that it has an electrical conductivity corresponding to that of a metal, or an electrical conductivity of above 1000 (Qm)'l.

A conductive bead 211 can be substantially spherical in shape, but it may also be elongated, elliptical, uneven, or of any other suitable shape. According to some aspects the relevant dimension of a conductive bead 21 1, which is selected to ensure that it corresponds to a l\/lie scattering resonance, is the perimeter of the conductive bead.

The perimeter of the bead is here considered to refer to the perimeter in the E-plane. That is, it refers to the perimeter in the plane defined by the direction of the incident radar wave and the electric field vector of the radar wave. lt may be noted that for beads that do not have a similar size along different directions, such as elongated or elliptical beads, the perimeter in the E-plane will depend on the orientation of the bead relative to the direction and polarization of the incoming radar wave. That is, the RCS of the bead can become dependent on the direction of the radar wave. ln such cases, the conductive beads 211 may be arranged within the dielectric material 212 in such a way that they are not all aligned in the same direction. For example, if the beads are elliptical in shape, the major axes of neighboring beads may not be parallel but may be oriented in random directions. This would enable the radar reflector 200 to have a similar RCS for radar signals incident on the reflector from different directions. However, using conductive beads that have a similar size along multiple directions or are substantially round, spherical, or nearly spherical in shape is advantageous for ensuring that a similar RCS is produced for radar signals from different directions.

Mie scattering is a phenomenon that occurs when electromagnetic radiation interacts with objects that have a spatial dimension, such as a width, length, diameter, or perimeter, that is similar to the wavelength of the electromagnetic radiation. Under such circumstances, the scattering of electromagnetic waves from an object varies rapidly with the spatial dimension of the object. The scattering also displays resonances at certain object sizes, where the object scatters the radiation much more strongly than othen/vise.

An example is shown in Figure 3, which displays the ratio of the RCS of an electrically conductive particle to the projected area of the particle, as a function of the ratio between the perimeter of the particle and the wavelength of the radiation. Here, the projected area is the area of a projection of the particle on a flat surface. For a spherical particle it is the area of a circle with the same diameter as the spherical particle. The RCS is shown to fluctuate around 1 , with values above 1 indicating a l\/lie resonance. The strongest resonance occurs when the perimeter of the particle is approximately equal to the wavelength. This peak is indicated as 301 in the figure. The second peak 302 and third peak 303 occur when the ratio of perimeter to wavelength is approximately 2.4 and 3.2, respectively.

Thus, if the conductive beads 211 have a dimension, such as a perimeter, that corresponds to a l\/lie scattering resonance of a wavelength of electromagnetic radiation in a particular frequency band, each of the conductive beads 211 will display an RCS larger than its projected area. lf the perimeter corresponds to the first l\/lie scattering resonance peak 301 the RCS may for example be between four and five times larger than the projected area, while for the second peak 302 and third peak 303 the RCS may be approximately twice as large as the projected area. The radar reflector 200, which comprises a plurality of such conductive beads 21 1, will therefore also display a large RCS in that frequency band.

Here, a frequency band is an interval of frequencies defined either as the interval between as minimum frequency and a maximum frequency, or as band of a certain bandwidth centered on a center frequency. ln this case, the bandwidth is the difference between the maximum frequency and the minimum frequency, and the center frequency is the arithmetic mean between the maximum frequency and the minimum frequency. The minimum and maximum frequency of a frequency band may be determined by legal provisions that dedicate specific intervals of frequencies to specific applications. For example, frequencies between 76 and 81 GHz may be dedicated to automotive radar and the interval of frequencies between 76 GHz and 81 GHz can therefore be treated as a frequency band.

The wavelength of electromagnetic radiation will change within the frequency band, decreasing from a maximum value at the minimum frequency to a minimum value at the maximum frequency. According to one example, that the dimension of the conductive beads 211 corresponds to a l\/lie scattering resonance of the wavelength in the frequency band can be taken to mean that the dimension of the conductive beads 211 is selected based on the wavelength at the center frequency, so that a local maximum value for the RCS is obtained at the center frequency. With reference again to Figure 3, this may mean that the perimeter of the conductive beads 211 is selected to be substantially equal to the wavelength of the electromagnetic radiation at the center frequency, yielding an RCS corresponding to the maximum of the first peak 301 for the center frequency. The perimeter could also be selected to be substantially 2.4 times the wavelength or 3.2 times the wavelength, placing the center frequency at l\/lie scattering resonances corresponding to the second peak 302 and third peak 303, respectively.

According to another example, the width in terms of wavelength of a Mie scattering resonance, e.g. the first peak 301 in Figure 3, may be compared to the minimum and maximum wavelength of radiation in the frequency band and the dimension of the conductive beads 211 be adjusted so that both the minimum and maximum wavelength give an RCS larger than the projected area of the conductive beadsThat is, the dimension may be selected so that the range of wavelengths between the minimum and maximum wavelength fit within the width of the l\/lie scattering resonance peak. The dimension of the conductive beads can also be selected to give as high an RCS as possible over as much of the frequency band as possible, i.e., to maximize the RCS in the frequency band. ln some cases, there may be multiple frequency bands in which it is desirable for the radar reflector 200 to have a high RCS. For example, more than one frequency band may be used for automotive radars. ln this case, it may be necessary to use conductive beads 211 of multiple sizes, so that some conductive beads 211 have a dimension corresponding to a l\/lie scattering resonance for a first frequency band, some conductive bands 211 have a dimension corresponding to a second frequency band, etc. That is, a dimension of at least some of the plurality of conductive beads 211 may correspond to a l\/lie scattering resonance of a wavelength in the dielectric material 212 of the electromagnetic radiation in a second frequency band out of the one or more frequency bands.

According to some aspects, the conductive beads 211 may be at least partly formed from a metal material. The conductive beads 211 may thus comprise a metal or metal alloy such as steel, aluminum, copper, or brass. The conductive beads 211 may according to one example be solid metal or metal alloy beads such as solid steel beads. According to another example, the conductive beads 211 may comprise an outer layer or shell of a metal or metal alloy. The shell may be hollow or arranged around a core of some other material such as a plastic material. That is, the conductive beads 211 may comprise metallized plastic beads, such as beads formed from an acrylic resin and metallized with a layer of aluminum on the outer surface. Using metallized plastic beads may be cheaper than using solid metal beads. Metallized plastic beads also typically weigh less.

The conductive beads 211 may optionally comprise conductive materials that are not metals, such as conductive polymers or conductive carbon materials. ln the radar reflector 200, the conductive beads 211 are embedded in a dielectric material 212. A dielectric material is a material with very low electrical conductivity, or an electrical insulator. The dielectric material 212 in the radar reflector 200 may for example be a plastic material. A suitable plastic material could be selected based on factors such as low cost or ease of manufacturing. The plastic material may forexample comprise an acrylic material such as an acrylate polymer or an acrylic resin. lt may also comprise polytetrafluoroethene (PTFE). lt may be noted that for any dielectric material with a dielectric constant larger than that of air, the wavelength of electromagnetic radiation will be shorter in the dielectric material than in air. Since the conductive beads 211 are embedded in a dielectric material 212, it follows that the dimension or size of the conductive beads 211 should be selected based on the wavelength in the dielectric material rather than the wavelength in air. Thus, the dielectric constant of the dielectric material 212 can be accounted for in order to more accurately determine the size of the conductive beads As previously mentioned, the radar reflector 200 is arranged to reflect electromagnetic radiation in one or more frequency bands. These frequency bands are preferably selected based on what frequencies are used in relevant radar transceivers, such as automotive radars. Thus, at least one frequency band may comprise frequencies between 24 GHz and 26 GHz, as frequencies in this interval are often used for automotive radars. Likewise, at least one frequency band may comprise frequencies between 76 GHz and 81 GHz. This frequency interval is also commonly used for automotive radars.

As an example, consider the 24 GHz to 26 GHz frequency band. ln order for the conductive beads 211 to be tuned to the first l\/lie resonance peak 301 in Figure 3, the perimeter of the conductive beads could be selected to be approximately equal to the wavelength of radiation at the center frequency of the band. ln air, this wavelength is 12 mm. ln a dielectric material with a refractive index around 1.5, which is common in acrylic materials, the wavelength would be around 8 mm. Thus, a radar reflector 200 could be designed using an acrylic material as the dielectric material 212 and conductive beads 211 with a perimeter around 8 mm. Around 8 mm could meanmm plus or minus 2 mm, i.e. between 6 and 10 mm. lf the dielectric material instead had a refractive index of 2, the wavelength in the material would be 6 mm and the conductive beads 211 could be given a perimeter of between 4 and 8 mm. To tune the conductive beads 211 to the second Mie resonance peak 302 or the third resonance peak 303, the perimeter of the conductive beads 21 1 could be 2.4 or 3.2 times the wavelength, or around 14 mm or 19 mm respectively in a dielectric with a refractive index ofLikewise, for the 76 GHz to 81 GHz frequency band, the wavelength at the center frequency is 3.8 mm in air and 2.5 mm in a dielectric material with a refractive index of 1.5. Thus, in a radar reflector 200 using such a dielectric material, the conductive beads 211 should have a perimeter of 2.5 mm to correspond to the first Mie resonance peak 301, a perimeter of 6 mm to correspond to the second peak 302, and a perimeter of 8 mm to correspond the third peak lt should be noted that if a conductive bead 211 has a perimeter equal to around 2.4 times the wavelength of radiation in the 76 to 81 GHz band, the perimeter will at the same time be around 0.7 to 0.8 times the wavelength of radiation in the 24 to 26 GHz band. Thus, if the perimeter of the conductive beads 211 is selected to correspond to the Mie scattering resonance indicated by the second peak 302 in Figure 3 for the 76 to 81 GHz frequency band, it will automatically also correspond at least partly to the Mie resonance indicated by the first peak 301 for the 24 to 26 GHz frequency band. This is an advantage, as a radar reflector 200 comprising conductive beads 211 of one size can have a high RCS in two frequency bands used in automotive radar. Using only one size of beads facilitates manufacturing of the radar reflector 200. ln practice, this could for example mean using conductive beads 211 with a perimeter of 6 mm embedded in a dielectric material with a refractive index of around 1.

Other relevant frequency bands may e.g. be frequency bands used for marine radars, if the radar reflector 200 is intended for use as a marine radar reflector. Thus, at least one frequency band may comprise frequencies betvveen 8 GHz and 12 GHz and / or between 2 GHz and 4 GHz. These frequency bands, sometimes referred to as the X and S band respectively, are often used in marine radars. As an example, with a dielectric material that has a refractive index around 1.5, a perimeter of 20 mm for the conductive beads 211 would correspond to the first Mie resonance peak for the X band, while a perimeter of 67 mm would be needed for the S band.

According to some examples, the radar reflector 200 could be combined with reflectors arranged to reflect electromagnetic radiation outside the radio-frequency spectrum. ln particular, the radar reflector 200 may comprise a second layer 220 arranged to reflect electromagnetic radiation in a high-frequency band, that is, a frequency band above the radio-frequency part of the electromagnetic spectrum. This layer may for example cover all or part of the first layer 210 comprising the conductive beads 211 and dielectric materiallf the radar reflector 200 is used by a person wishing to become more easily detectable by automotive radars used in cars, trucks, or construction equipment, it would be an advantage to combine the radar reflector 200 with a conventional safety reflector. Herein, conventional safety reflectors are considered to be optical reflectors arranged to make people or objects more easily detectable in poor visibility conditions by reflecting visible light from sources such as vehicle headlights. Accordingly, the high-frequency band may comprise the visible spectrum, and the second layer 220 may thus comprise an optical reflector. The visible spectrum is the part of the electromagnetic spectrum visible to humans and is often considered to comprise frequencies betvveen 430 and 750 THz.

Several types of optical safety reflectors are known in the art. According to one example, the second layer 220 may comprise a plurality of transparent beads, the transparent beads being half coated in a reflective material. lncoming light will be refracted in the beads and subsequently reflected by the reflective coating, the combination of beads and coating thus serving as an optical reflector.

According to another example, the second layer 220 may comprise a plurality of micro-prisms. lncoming light will be refracted in the prisms such that it is reflected back in the direction it came from. The micro-prisms thus act as an optical reflector.

Radar reflectors 200 as described above can be incorporated in a number of products. As an example, Figure 4 shows a safety reflector400 comprising a radar reflector 200 as previously described. Optionally, the safety reflector 400 may comprise an optical reflector that is transparent to radar signals. The radar reflector 200 may then be sandwiched between two layers of optical reflector, providing a safety reflector 400 that increases probability of detection both by radar and optical means.

As another example, Figure 5 shows a high-visibility garment 500 comprising a radar reflector 200 as described above. High-visibility garments are often worn by people on construction sites, people performing infrastructure maintenance, and by ordinary pedestrians or bicyclists. A high-visibility garment comprising a radar reflector 200 would have the advantage of being more easily detectable by radar as well as being highly visible. ln addition to high-visibility clothing, other protective equipment is often worn on construction sites. One example of such equipment is a hard plastic helmet or hard hat. Figure 6 shows a hard hat 600 comprising a radar reflector 200 as previously described. Although the figure shows the radar reflector 200 as occupying only part of the hard-hat surface, it may also cover the entire hard hat The hard hat 600 may thus be arranged to reflect electromagnetic radiation in one or more frequency bands. The hard hat 600 is made at least partly from a dielectric material 212, and a plurality of conductive beads 211 is embedded in the dielectric material 212. A dimension of at least some of the conductive beads 211 corresponds to a l\/lie scattering resonance of a wavelength in the dielectric material 212 of the electromagnetic radiation in a first frequency band out of the one or more frequency bands. ln addition to wearable objects such as safety reflectors, garments, and protective gear, other objects that are hard to detect with radar could be equipped with radar reflectors 200. For example, traffic cones and other temporary road markers usually comprise plastic materials, with bright colors and added optical reflectors intended to ensure that they are noticeable to the human eye under varying lighting conditions. However, objects comprising mostly plastic materials generally have very low radar cross sections and are thus hard to detect by automotive radar. Adding a radar reflector 200 would thus significantly increase the chances of a traffic cone or other temporary road marker being detected by a radar transceiver. As for the hard hat described above, conductive beads 211 as shown in Figure 2 could optionally be mixed into the plastic forming the traffic cone to form the radar reflector On construction sites in particular, smaller tools may be accidentally left in positions where they may be run over by vehicles such as trucks. Thus, there is also herein disclosed a tool arranged to reflect electromagnetic radiation in one or more frequency bands. The tool comprises a housing made at least partly from a dielectric material 212, where a plurality of conductive beads 211 is embedded in the dielectric material 212. A dimension of at least some of the conductive beads 211 corresponds to a l\/lie scattering resonance of a wavelength in the dielectric material 212 of the electromagnetic radiation in a first frequency band out of the one or more frequency bands.

Such a tool would, advantageously, be easier to detect e.g. by vehicle radar.

Figure 7 is a flowchart illustrating a method for producing a radar reflector 200 arranged to reflect electromagnetic radiation in one or more frequency bands. The radar reflector 200 has a first layer 210 which comprises a dielectric material 212. The method comprises obtaining S1 a plurality of conductive beads 211. A dimension ofat least some of the conductive beads 211 is selected so as to correspond to a Mie scattering resonance of a wavelength in the dielectric material 212 of the electromagnetic radiation in a first frequency band out of the one or more frequency bands. The method also comprises embedding S2 the plurality of conductive beads 211 in the dielectric material As an example, the dielectric material could be melted or dissolved into a solution. The conductive beads 211 could then be mixed into the dielectric material and the dielectric material hardened by lowering the temperature or, if it is dissolved, evaporating the solvent. lf the dielectric material is a plastic material, it could be shaped through methods such as molding or casting.

According to some aspects, the method may also comprise applying S3 a second layer 220 to the reflector 200, the second layer 220 being arranged to reflect electromagnetic radiation in a high-frequency band. The high-frequency band may for example comprise the visible spectrum.

Claims

1. A radar reflector (200) arranged to reflect electromagnetic radiation in one or more frequency bands, the reflector (200) having a first layer (210) comprising a plurality of conductive beads (211), the plurality of conductive beads (211) being embedded in a dielectric material (212), wherein a dimension of at least some of the conductive beads (211) corresponds to a Mie scattering resonance of a wavelength in the dielectric material (212) of the electromagnetic radiation in a first frequency band out of the one or more frequency bands.

2. The radar reflector (200) according to claim 1, wherein the dimension of at least some of the plurality of conductive beads (211) corresponds to a l\/lie scattering resonance of a wavelength in the dielectric material (212) of the electromagnetic radiation in a second frequency band out of the one or more frequency bands.

3. The radar reflector (200) according to claim 1 or 2, wherein the dimension of at least some of the conductive beads (211) is a perimeter of at least some of the conductive beads (211).

4. The radar reflector (200) according to any previous claim, wherein at least one of the frequency band or bands comprises frequencies between 24 GHz and 26 GHz.

5. The radar reflector (200) according to any previous claim, wherein at least one of the frequency band or bands comprises frequencies between 76 GHz and 81 GHz.

6. The radar reflector (200) according to any previous claim, wherein at least one of the frequency band or bands comprises frequencies between 8 GHz and 12 GHz and / or between 2 GHz and 4 GHz.

7. The radar reflector (200) according to any previous claim, wherein the conductive beads (211) are at least partly formed from a metal material.

8. The radar reflector (200) according to any previous claim, wherein the dielectric material (212) is a plastic material.

9. The radar reflector (200) according to claim 8, wherein the plastic material comprises an acrylic material and/ or comprises polytetrafluoroethene.

10. The radar reflector (200) according to any previous claim, comprising a second layer (220) arranged to reflect electromagnetic radiation in a high-frequency band.

11. The radar reflector (200) according to claim 10, wherein the high-frequency band comprises the visible spectrum.

12. The radar reflector (200) according to claim 10 or 11, wherein the second layer (220) comprises a plurality of transparent beads, the transparent beads being half coated in a reflective material.

13. The radar reflector (200) according to claim 10 or 11, wherein the second layer (220) comprises a plurality of micro-prisms.

14. A safety reflector (121, 400) comprising a radar reflector (200) according to any of claims 1 to

15. A high-visibility garment (500) comprising a radar reflector (200) according to any of claims 1 to

16. A hard hat (600) arranged to reflect electromagnetic radiation in one or more frequency bands, the hard hat (600) being made at least partly from a dielectric material (212), where a plurality of conductive beads (211) is embedded in the dielectric material (212), wherein a dimension of at least some of the conductive beads (211) corresponds to a l\/lie scattering resonance of a wavelength in the dielectric material (212) of the electromagnetic radiation in a first frequency band out of the one or more frequency bands.

17. A tool arranged to reflect electromagnetic radiation in one or more frequency bands, the tool comprising a housing made at least partly from a dielectric material (212), where a plurality of conductive beads (211) is embedded in the dielectric material (212), wherein a dimension of at least some of the conductive beads (211) corresponds to a l\/lie scattering resonance of a wavelength in the dielectric material (212) of the electromagnetic radiation in a first frequency band out of the one or more frequency bands.

18. A method for producing a radar reflector (200) reflect electromagnetic radiation in one or more frequency bands, the radar reflector (200) arranged to having a first layer (210) comprising a dielectric material (212), the method comprising: obtaining (S1) a plurality of conductive beads (211), wherein a dimension of at least some of the conductive beads (211) corresponds to a l\/lie scattering resonance of a wavelength in the dielectric material (212) of the electromagnetic radiation in a first frequency band out of the one or more frequency bands, and embedding (S2) the plurality of conductive beads (21 1) in the dielectric material.

19. The method according to claim 18, comprising applying (S3) a second layer (220) to the reflector (200), the second layer (220) being arranged to reflect electromagnetic radiation in a high-frequency band.