Patent Description:
A conventional millimeter-wave radar for automotive application includes an antenna configured to receive and emit electromagnetic waves, a drive circuit configured to drive the antenna, and an electronic circuit including a power source.

The size (dimensions) of the antenna configured to receive and emit electromagnetic waves depends on the type of the antenna, but in many cases, among antennas of the same type, an antenna at a higher frequency has a smaller size because electromagnetic waves at a higher frequency have a shorter wavelength. As for the electronic circuit, progress has been made in integration and refinement along with the progress of the semiconductor technology, and downsizing has exponentially proceeded, which is not limited to this field.

However, in a conventional low-frequency radar product or an electronic circuit mounted product in an era when the semiconductor technology was yet to develop, an antenna and an electronic circuit have large sizes (dimensions), and thus are housed separately or independently disposed in the same housing in many cases.

Along with downsizing of an antenna and an electronic circuit as electric components of a millimeter-wave radar, a radar cover capable of housing these antenna and electronic circuit in the same housing has been disclosed (refer to Patent Literature <NUM>, for example).

As described above, since the antenna and the electronic circuit can be housed in the same housing or mounted close to each other, it is difficult to physically divide the antenna and the electronic circuit, and the boundary between the antenna and the electronic circuit is becoming unclear.

In designing of a housing in which these components of a millimeter-wave radar are housed, the electromagnetic wave propagation characteristic of the housing needs to be considered to effectively utilize electromagnetic waves (millimeter waves) and reduce unnecessary radiation of electromagnetic waves, which is requested for an electronic device.

When the antenna and the electronic circuit are separately housed or disposed independently from each other as in conventional cases, the housing can be designed only for each of the antenna and the electronic circuit with taken into consideration two viewpoints at effective utilization of the millimeter-wave radar and reduction of unnecessary radiation of electromagnetic waves.

However, in a recent millimeter-wave radar on which an antenna and an electronic circuit are mounted extremely close to each other due to the progress downsizing of the antenna and the electronic circuit, it is difficult to design a housing that simultaneously satisfies effective utilization of electromagnetic waves (millimeter waves) and reduction of unnecessary radiation of electromagnetic waves, which is requested for an electronic device.

Specifically, a different electromagnetic wave transmission property or screening property for the housing material is requested for each part of the millimeter-wave radar, and thus designing specifications of the housing material are different for each part. <FIG> lists requested designing specifications in a divided manner for parts A and B and frequency bands I and II.

Parts are divided into two in terms of transmission and screening of millimeter waves used by the millimeter-wave radar: a part A (part corresponding to a radome of the radar) at which electromagnetic waves (millimeter waves) are received and emitted; and the remaining part B in which the millimeter-wave radar is housed except for the part A.

Frequencies for which designing is considered are functionally are divided into two: a frequency band I (<NUM>) of millimeter wave band used as electromagnetic waves; and a wide frequency band for which reduction of unnecessary radiation of electromagnetic waves and entry of electromagnetic waves from the outside are considered, in particular, a frequency band II (equal to or lower than <NUM> approximately) of an electromagnetic compatibility (EMC) region (noise control region) of <NUM> or lower. The frequency band I of millimeter wave band is not limited to <NUM> but may be optionally set in the range of <NUM> to <NUM>.

The transmissivity T of electromagnetic waves for the housing material when the parts A and B, the frequency band I (<NUM>), and the frequency band II (equal to or lower than <NUM> approximately) are considered needs to be set for each of the parts A and B, the frequency band I (<NUM>), and the frequency band II (equal to or lower than <NUM> approximately).

For example, in a region AI of the part A and the frequency band I (equal to or lower than <NUM> approximately), millimeter electromagnetic waves need to be transmitted to achieve the radar function of the millimeter-wave radar, and thus the transmissivity T of the housing material for the electromagnetic waves is desirably "<NUM>". The state in which the transmissivity T is "<NUM>" is the state of perfect transmission in which the housing material transmits electromagnetic waves.

In particular, in radar usage, electromagnetic waves attenuate proportionally to the square of the distance between an antenna of the millimeter-wave radar and an object in theory. Thus, in terms of the round-trip distance between emission from the antenna and returning after reflection at the object, the electromagnetic waves attenuate proportionally to the fourth power of the distance, and the transmissivity T of the housing material largely affects the performance (detection sensitivity, certainty, and accuracy) of a product.

For example, in a region BI of the part B and the frequency band I (<NUM>), transmission of millimeter electromagnetic waves does not need to be functionally allowed, but entry of millimeter waves needs to be prevented to avoid interference and cross talk due to electromagnetic waves from another external instrument. In other words, the transmissivity T of the housing material is desirably "<NUM>" to screen millimeter electromagnetic waves. The state in which the transmissivity T is "<NUM>" is the state of perfect screening in which the housing material transmits no electromagnetic waves.

In a region All of the part A and the frequency band II (equal to or lower than <NUM> approximately) and a region BII of the part B and the frequency band II (equal to or lower than <NUM> approximately), the transmissivity T of the housing material is desirably "<NUM>" to screen, without transmission, electromagnetic waves in the frequency band II (equal to or lower than <NUM> approximately) of the EMC region for reduction of unnecessary radiation of electromagnetic waves.

Patent Literature <NUM> discloses a millimeter-wave radar cover housing a millimeter-wave radar including an antenna and an electronic circuit configured to drive the antenna, the millimeter-wave radar cover comprising: a first part provided in front of the millimeter-wave radar to protect the millimeter-wave radar and transmit millimeter electromagnetic waves emitted from the antenna; and a second part including a housing space in which the antenna and the electronic circuit except for the first part are housed, wherein the first part is made of a stacked structural body obtained by stacking at least one layer of a first constituent material having a negative permittivity in the frequency band of the millimeter waves and a second constituent material having a positive permittivity in the frequency band of the millimeter waves.

Non-Patent Literature <NUM> discloses a stacked structural body obtained by stacking at least one layer of a first constituent material having a negative permittivity in the frequency band of the millimeter waves and a second constituent material having a positive permittivity in the frequency band of the millimeter waves.

At the part A of such a millimeter-wave radar housing having the above-described configuration, the transmissivity T of the housing material is desirably "<NUM>" in the frequency band I (<NUM>) and "<NUM>" in the frequency band II (equal to or lower than <NUM> approximately), and completely opposite characteristics are requested for the frequency band I (<NUM>) and the frequency band II (equal to or lower than <NUM> approximately).

However, no housing material simultaneously satisfies such opposite characteristics. Thus, conventionally, with taken into consideration a wavelength reduction effect due to the relative permittivity of the housing material, designing has been made in priority of the radar function by using a housing having a thickness of integral multiples of the half wavelength of electromagnetic waves to be used, to set "<NUM>" to the transmissivity T of the housing material for electromagnetic waves in the frequency band I (<NUM>), but with less consideration on setting "<NUM>" to the transmissivity T of the housing material for electromagnetic waves in the frequency band II (equal to or lower than <NUM> approximately).

Thus, with a housing of the conventional material, it is possible to effectively utilize electromagnetic waves of a millimeter-wave radar, but it has been difficult to achieve sufficient reduction of unnecessary radiation of electromagnetic waves, which is requested for an electronic device.

The present invention is intended to solve the above-described problem and provide a millimeter-wave radar cover capable of effectively utilizing electromagnetic waves of a millimeter-wave radar, sufficiently reducing unnecessary radiation of electromagnetic waves, and preventing entering of unnecessary electromagnetic waves from the outside.

To achieve the above-described intention, the present invention provides a millimeter-wave radar cover housing a millimeter-wave radar including an antenna and an electronic circuit configured to drive the antenna, the millimeter-wave radar cover being characterized by including: a first part provided in front of the millimeter-wave radar to protect the millimeter-wave radar and transmit millimeter electromagnetic waves emitted from the antenna; and a second part including a housing space in which the antenna and the electronic circuit except for the first part are housed, wherein the first part is made of a stacked structural body obtained by stacking at least one layer of a first constituent material having a negative permittivity in the frequency band of the millimeter waves and a second constituent material having a positive permittivity in the frequency band of the millimeter waves, and the stacked structural body is curved in a direction centered at an emission source of the millimeter electromagnetic waves and departing from the emission source so that the millimeter electromagnetic waves are perpendicular to the curved stacked structural body when incident in any direction from the emission source.

In the millimeter-wave radar cover according to the present invention, it is preferable that the stacked structural body has an effective transmissivity close to one only when the millimeter electromagnetic waves are perpendicularly incident on the stacked structural body.

In the millimeter-wave radar cover according to the present invention, it is preferable that the stacked structural body is integrated with a dielectric lens having a predetermined permittivity for refracting the millimeter electromagnetic waves incident from the emission source.

In the millimeter-wave radar cover according to the present invention, it is preferable that the first constituent material is formed by geometrically disposing a conductive material and the stacked structural body has an effective transmissivity close to one for the frequency band of the millimeter waves when the first constituent material is stacked with the second constituent material.

In the millimeter-wave radar cover according to the present invention, it is preferable that the stacked structural body has an effective transmissivity close to zero for the frequency band of an electromagnetic compatibility (EMC) region lower than the frequency band of the millimeter waves.

The present invention can achieve a millimeter-wave radar cover capable of effectively utilizing electromagnetic waves of a millimeter-wave radar, sufficiently reducing unnecessary radiation of electromagnetic waves, and preventing entering of unnecessary electromagnetic waves from the outside.

The following specifically describes an embodiment of the present invention with reference to the accompanying drawings. In the description, for the purpose of illustration, the direction of arrow "a" points to the front surface side of a millimeter-wave radar cover <NUM> in <FIG>, and the direction of arrow "b" points to the back surface side thereof.

As illustrated in <FIG>, the millimeter-wave radar cover <NUM> is a housing that houses: an antenna <NUM> as emission source configured to receive and emit electromagnetic waves of, for example, <NUM> in the frequency band (<NUM> to <NUM>) of millimeter waves; and an electronic circuit <NUM> including, for example, a drive circuit configured to drive the antenna <NUM> and a power source, and protects these components from the outside.

The millimeter-wave radar cover <NUM> includes a first part A corresponding to a radome disposed in front of the antenna <NUM> configured to receive and emit electromagnetic waves, and a second part B corresponding to an accommodating part having a bottomed rectangular tubular shape and including an accommodating space in which the antenna <NUM> and the electronic circuit <NUM> except for the first part A are housed.

As illustrated in <FIG>, the first part A of the millimeter-wave radar cover <NUM> is a three-layer stacked structural body <NUM> obtained by stacking a first constituent material <NUM> and second constituent materials <NUM> sandwiching the first constituent material <NUM> therebetween on the front surface side (the direction of arrow "a") and the back surface side (the direction of arrow "b").

The second part B of the millimeter-wave radar cover <NUM> is formed of a shield material made of metal such as iron or a composite material obtained by providing, for example, resin with metal plating, and reduces unnecessary radiation of electromagnetic waves from the electronic circuit <NUM> and prevents interference and cross talk due to an external electronic device. Thus, the transmissivity T of the second part B is "<NUM>" in any of a frequency band I (<NUM>) of millimeter waves and a frequency band II (equal to or lower than <NUM> approximately) of an EMC region.

As described above, the first part A needs to allow passing of millimeter waves in the frequency band I (<NUM>) of a millimeter wave band to achieve the radar function of a millimeter-wave radar, and thus the transmissivity T of the stacked structural body <NUM> for electromagnetic waves at the first part A is desirably "<NUM>". The transmissivity T of the stacked structural body <NUM> at the second part B is desirably "<NUM>" to screen electromagnetic waves in the frequency band II (equal to or lower than <NUM> approximately) of the EMC region (noise control region) without transmission, thereby reducing unnecessary radiation of electromagnetic waves.

Such completely opposite characteristics that the transmissivity T at the first part A is "<NUM>" in the frequency band I (<NUM>) of millimeter waves and "<NUM>" in the frequency band II (equal to or lower than <NUM> approximately) of the EMC region are requested as described above. No material satisfies the opposite requests, and thus, the stacked structural body <NUM> as an artificial material is used in the present invention.

As illustrated in <FIG>, the stacked structural body <NUM> includes the first constituent material <NUM> and the second constituent materials <NUM> as a housing that simultaneously satisfies the effective transmissivity T of "<NUM>" in the frequency band I (<NUM>) of millimeter waves and the effective transmissivity T of "<NUM>" in the frequency band II (equal to or lower than <NUM> approximately) of the EMC region. However, the transmissivity T of "<NUM>" or "<NUM>" is a value in theory, and values thereof that can be actually achieved and evaluated are defined as follows: the transmissivity T of "<NUM>" is a transmissivity T equal to or higher than <NUM> (-<NUM> dB) and close to one; and the transmissivity T of "<NUM>" is a transmissivity T equal to or lower than <NUM> (-<NUM> dB) and close to zero.

As illustrated in <FIG>, the stacked structural body <NUM> in this case has a three-layer sandwich structure in which the first constituent material <NUM> is sandwiched between the second constituent material <NUM> disposed on the front surface side (the direction of arrow "a") and the second constituent material <NUM> disposed on the back surface side (the direction of arrow "b") and is integrally formed by, for example, an adhesive. However, the stacked structural body <NUM> is not limited thereto, but may have a double-layer structure as long as at least one second constituent material <NUM> and at least one first constituent material <NUM> are stacked, or may have a multiple-layer stacked structural body in which a plurality of second constituent materials <NUM> and a plurality of first constituent materials <NUM> are stacked in a total of four or more layers.

As illustrated in <FIG>, the first constituent material <NUM> is a conductive material of electronic conduction, such as copper (metal) having a rectangular shape as a whole, and formed in a lattice shape like a screen door by using, for example, copper metal wires. The first constituent material <NUM> is not limited to a rectangular shape but may have any other kind of shape such as circular shape or an ellipse shape in accordance with the shape of the radar. The conductive material of the first constituent material <NUM> is not limited to metal such as copper, but may be carbon, conductive macromolecule, conductive polymer, or the like, or a material provided with conductivity by mixing each of these materials (metal, carbon, conductive macromolecule, conductive polymer, or the like) into resin, rubber, elastomer, or the like.

Specifically, in the first constituent material <NUM>, for example, the size of the lattice and the number thereof are determined by a thickness t of each frame forming the lattice, a width d of the frame, and an array interval a of the frame. The array interval a is the distance between inner ends of adjacent frames forming the lattice, but not limited thereto. The array interval a may be the intercentral distance between the centers of the frames.

Each second constituent material <NUM> is formed of resin (such as polyimide, polytetrafluoroethylene, or polyethylene) or dielectric such as rubber having material strength and resistance necessary as a housing of the millimeter-wave radar cover <NUM>. Similarly to the first constituent material <NUM>, the second constituent material <NUM> has a rectangular shape as a whole, and has a size same as that of the first constituent material <NUM> or a size slightly larger than that of the first constituent material <NUM> to avoid protrusion of the first constituent material <NUM>.

As illustrated in <FIG>, the stacked structural body <NUM> is disposed in a free space, which is a situation in which reflection of electromagnetic waves radiated from the antenna <NUM> occurs at two interfaces of a surface 10a of the stacked structural body <NUM> on the front surface side (the direction of arrow "a") and an internal surface 10b of the stacked structural body <NUM>.

In such a situation, the transmissivity T of the stacked structural body <NUM> disposed in the free space holds a relation with the reflectance Γ, which is determined by the wave impedances η of different materials such as air and the stacked structural body <NUM> as indicated by Expression (<NUM>) below. Specifically, the transmissivity T of the stacked structural body <NUM> is determined by a wave impedance η1 of the free space (air) and the equivalent wave impedance η2 of the stacked structural body <NUM>. The reflectance Γ is given by (η2 - η1)/(η2 + η1).

Expression (<NUM>) indicates that the transmissivity T = "<NUM>" can be obtained by equalizing the wave impedance η1 of the incident side material (air) and the wave impedance η2 of the radiation side material (stacked structural body <NUM>). This oppositely means that the transmissivity T ≈ "<NUM>" can be obtained when the wave impedance η2 of the radiation side material (stacked structural body <NUM>) is smaller than the wave impedance η1.

Each wave impedance η is determined by the permittivity and permeability of the material and given by Expression (<NUM>) below.

Therefore, the wave impedance η1 of the incident side material (air) is given by Expression (<NUM>) below, and the wave impedance η2 of the radiation side material (stacked structural body <NUM>) is given by Expression (<NUM>) below. <MAT> <MAT>.

Since the wave impedances η1 and η2 are given by Expressions (<NUM>) and (<NUM>) in this manner, the transmissivity T is determined by the relative permeability µr1 and the relative permittivity εr1 of the free space (air) and the equivalent relative permeability µr2 and the equivalent relative permittivity εr2 of the stacked structural body <NUM>.

In Expression (<NUM>), when the relative permeability µr1 and the relative permittivity εr1 of the free space (air) are both taken to be substantially "<NUM>" and the equivalent relative permeability µr2 and the equivalent relative permittivity εr2 of the stacked structural body <NUM> have equal values, the wave impedance η1 of the air and the equivalent wave impedance η2 of the stacked structural body <NUM> have equal values, and accordingly, the transmissivity T = <NUM> can be achieved.

When the ratio of the equivalent relative permeability µr2 and the equivalent relative permittivity εr2 of the stacked structural body <NUM> decreases, in other words, when the relative permittivity εr2 as the denominator has a negative value and the absolute value thereof increases, the wave impedance η2 approaches to "<NUM>", and accordingly, the transmissivity T = <NUM> can be achieved.

However, the relative permeability µr of a non-magnetic body has a value substantially equal to one, whereas the relative permittivity εr of polytetrafluoroethylene, which is lowest as an industrial material of a typically used non-magnetic body, is two. Thus, the non-magnetic body needs to be mixed with a magnetic material to increase the relative permeability µr so that the relative permeability µr is equivalent to the relative permittivity εr. However, the magnetic material leads to a large loss of electromagnetic waves and is not suitable for the mixture nor single use.

Thus, in the present invention, the equivalent relative permeability µr2 and the equivalent relative permittivity εr2 of the stacked structural body <NUM> can set to be equal to each other by forming the stacked structural body <NUM> as a stack of the first constituent material <NUM> made of an artificial material having a negative relative permittivity εr in the frequency band I (<NUM>) of millimeter waves and the second constituent materials <NUM> each having a normal positive relative permittivity εr in the frequency band I (<NUM>) of millimeter waves.

The stacked structural body <NUM> needs to be formed such that the equivalent relative permittivity εr of the stacked structural body <NUM> is equal to the equivalent relative permeability µr = <NUM> in the frequency band I (<NUM>) of millimeter waves and the equivalent relative permittivity εr of the stacked structural body <NUM> is negative and has a large absolute value in the frequency band II (equal to or lower than <NUM> approximately) of the EMC region (noise control region).

The first constituent material <NUM> forming the stacked structural body <NUM> is made of, for example a conductive material of electronic conduction typically made of metal. However, the first constituent material <NUM> does not necessarily need to be made of metal but may be made of a conductive material of electronic conduction, not ion conduction nor hole conduction. Examples of the conductive material other than metal include carbon, conductive macromolecule, conductive polymer, or the like, or a material provided with conductivity by mixing each of these materials into resin, rubber, elastomer, or the like.

The relative permittivity εr of the conductive material used for the first constituent material <NUM> is described based on a Drude model as an electronic conduction model, and has a positive value at the frequency f (f ≥ fp) equal to or higher than a plasma frequency fp or a negative value at the frequency f (f < fp) lower than the plasma frequency fp as illustrated in <FIG>. In the Drude model, the relative permittivity εr of metal is given by the electron mass, electric charge, and the number of conduction electrons, and the plasma frequency fp is a frequency at which the relative permittivity εr is zero.

In this case, the plasma frequency fp of the metallic conductive material used for the first constituent material <NUM> is typically in a frequency band of a light region, and thus, as illustrated in <FIG>, the plasma frequency fp is set to be near the regions of microwaves, millimeter waves, and terahertz waves so that the frequency band I (<NUM>) of millimeter waves is slightly lower than the plasma frequency fp and the relative permittivity εr has a negative value smaller than zero. In this case, in the frequency band II (equal to or lower than <NUM> approximately) of the EMC region (noise control region), the relative permittivity εr is set to have a negative value equal to or smaller than -<NUM>.

The plasma frequency fp in the frequency band of the light region is set to be near the frequency band I (<NUM>) of the millimeter wave region as follows: the number of conduction electrons in a conductive material is restricted (decreased) to set the plasma frequency fp of the conductive material made of metal in the light region to be near the millimeter wave region.

Specifically, the number of conduction electrons can be restricted by decreasing the physical dimension and area of the conductive material to reduce the number of conduction electrons in the entire conductive material. Specifically, the restriction can be achieved by forming the first constituent material <NUM> in a lattice shape as illustrated in <FIG> to geometrically dispose the conductive material. In other words, the number of electrons can be physically restricted by reducing the area of the first constituent material <NUM>.

The first constituent material <NUM> does not necessarily need to be a lattice made of metal (hereinafter also referred to as a "metal lattice"). For example, the first constituent material <NUM> can be obtained by printing a copper foil pattern on the surface of a polyimide film and then forming the printed film into a lattice shape by etching. The material of the first constituent material <NUM> and the manufacturing method thereof may be any material and any manufacturing method with which the entire number of conduction electrons can be restricted to obtain a desired relative permittivity εr.

<FIG> illustrates a calculation result of the relative permittivity εr of the first constituent material <NUM> when each frame forming the lattice has a thickness t of <NUM>, a frame width d of <NUM>, and an array interval a of <NUM>. When the number of electrons is restricted by forming the first constituent material <NUM> in a lattice shape to reduce the entire area, as illustrated in <FIG>, the equivalent relative permittivity εr of the stacked structural body <NUM> can be made equal to the equivalent relative permeability µr = <NUM> and the plasma frequency fp in the frequency band of the light region can be set to be near the frequency band I (<NUM>) of the millimeter wave region. As a result, the relative permittivity εr of the first constituent material <NUM> in the frequency band I (<NUM>) depends on the permittivity ε of each second constituent material <NUM> stacked thereon, but is set to have a designed value of the relative permittivity εr at smaller than zero and greater than -<NUM>, and preferably have a negative value equal to or smaller than -<NUM>, which is between -<NUM> and -<NUM> inclusive approximately. The relative permittivity εr in the frequency band II (equal to or lower than <NUM> approximately) depends on the permittivity ε of the stacked second constituent material <NUM>, but is set to have a negative value equal to or smaller than -<NUM>.

However, the thickness t, the frame width d, and the array interval a of each frame in the lattice of the first constituent material <NUM> can be set as appropriate in accordance with a desired relative permittivity εr, and an optional shape such as a circular shape or a triangular shape may be selected. The disposition pattern of the lattice does not need to be uniform, but the density such as variance of the lattice may be optionally set.

Each second constituent material <NUM> is a dielectric having a normal positive relative permittivity εr in the frequency band I (<NUM>) of millimeter waves. The second constituent material <NUM> only needs to have material strength, workability, and various kinds of durability that are necessary for a radar cover, but desirably has a small electric loss to further improve performance. Specifically, the imaginary part ε" when the permittivity ε of the material of the second constituent material <NUM> in the frequency band I (<NUM>) is expressed in a complex permittivity is preferably small, and is, for example, preferably <NUM> or smaller, more preferably <NUM> or smaller.

In this manner, the stacked structural body <NUM> is formed by stacking at least one layer of the first constituent material <NUM> having a negative relative permittivity εr (equal to -<NUM> or smaller) in the frequency band I (<NUM>) of millimeter waves, and at least one layer of the second constituent material <NUM> having a positive relative permittivity εr in the frequency band I (<NUM>) of millimeter waves.

Accordingly, as illustrated in <FIG>, the equivalent relative permittivity εr of the stacked structural body <NUM> having a three-layer structure including the two second constituent materials <NUM> and the single first constituent material <NUM> can be made equal to the equivalent permeability µr of the stacked structural body <NUM>, which is equal to one, in the frequency band I (<NUM>) of millimeter waves, and the equivalent relative permittivity εr of the stacked structural body <NUM> can be made negatively large (-<NUM> or smaller) in the frequency band II (equal to or lower than <NUM> approximately) of the EMC region.

Specifically, it is desirable to set the equivalent relative permittivity εr of the stacked structural body <NUM> in the frequency band I (<NUM>) to be substantially one, specifically, <NUM> to <NUM>, more preferably, <NUM> and set the equivalent relative permittivity εr of the stacked structural body <NUM> in the frequency band II (equal to or lower than <NUM> approximately) to be -<NUM> or smaller, preferably, -<NUM> or smaller.

Accordingly, the equivalent relative permeability µr and the relative permittivity εr of the stacked structural body <NUM> become equal to each other, and the wave impedance η1 of the air and the equivalent wave impedance η2 of the stacked structural body <NUM> become equal to each other. As a result, as illustrated in <FIG>, the stacked structural body <NUM> can obtain the effective transmissivity T = <NUM> in the frequency band I (<NUM>).

Simultaneously, as the ratio of the equivalent relative permeability µr and the relative permittivity εr of the stacked structural body <NUM> decreases when the relative permittivity εr2 as the denominator has a negative value with a large absolute value, the wave impedance η2 approaches to "<NUM>", thereby achieving the effective transmissivity T = <NUM> in the frequency band II (equal to or lower than <NUM> approximately).

In the millimeter-wave radar cover <NUM> with the above-described configuration, the stacked structural body <NUM> obtained by stacking at least one layer of the first constituent material <NUM> having a negative relative permittivity in the frequency band I (<NUM>) of millimeter waves and at least one layer of the second constituent material <NUM> having a positive relative permittivity in the frequency band I (<NUM>) of millimeter waves is used as a radome at the first part A.

The stacked structural body <NUM> protects the antenna <NUM> and the electronic circuit <NUM> inside through the second constituent materials <NUM> and achieves the transmissivity T = <NUM> in the frequency band I (<NUM>) of millimeter waves, and the transmissivity T ≈ <NUM> in the frequency band II (equal to or lower than <NUM> approximately) of the EMC region.

Accordingly, the millimeter-wave radar cover <NUM> can transmit millimeter electromagnetic waves from the antenna <NUM> without electric attenuation due to the stacked structural body <NUM>, and receive reflected waves thereof without electric attenuation due to the stacked structural body <NUM>. Simultaneously, the millimeter-wave radar cover <NUM> can, through the stacked structural body <NUM>, reduce unnecessary radiation of electromagnetic waves in the EMC region and prevent interference and cross talk due to electromagnetic waves from another external instrument.

In a specific configuration of the stacked structural body <NUM>, for example, the first constituent material <NUM> was made of a metal lattice of copper, each second constituent material <NUM> was made of polyimide, the first constituent material <NUM> and the second constituent material <NUM> each had a size of <NUM>×<NUM>, and one of the second constituent materials <NUM>, the first constituent material <NUM>, and the other second constituent material <NUM> were stacked in the stated order to achieve a three-layer stacked structure.

The first constituent material <NUM> was produced by etching a copper foil having a thickness t of <NUM> into a lattice shape having a line width d of <NUM>, an array interval a of <NUM> in the longitudinal direction, and an array interval a of <NUM> in the lateral direction. In this case, the plasma frequency fp of the relative permittivity εr of the first constituent material <NUM> in the frequency band of the light region can be set to be near the frequency band I (<NUM>) of the millimeter wave region. Each second constituent material <NUM> was made of polyimide having a complex relative permittivity of <NUM> - j0. <NUM> in the frequency band I (<NUM>) of millimeter waves, and had a thickness of <NUM>.

The one second constituent material <NUM>, the first constituent material <NUM>, and the other second constituent material <NUM> were sequentially stacked and subjected to press molding under a pressure of <NUM> tons by using an adhesive to form the stacked structural body <NUM> having a three-layer stacked structure. As a result, the thickness of the formed stacked structural body <NUM> was <NUM>. In this state, the first constituent material <NUM> was crushed between the two second constituent materials <NUM> with no air between the three layers.

In this case, for example, the relative permittivity εr and the relative permeability µr of each of the first constituent material <NUM> and the second constituent materials <NUM> of the stacked structural body <NUM> having a three-layer stacked structure including the adhesive were adjusted as appropriate to achieve the transmissivity T = <NUM> in the frequency band I (<NUM>) of millimeter waves and the transmissivity T ≈ <NUM> in the frequency band II (equal to or lower than <NUM> approximately) of the EMC region.

A result of measurement of the equivalent relative permittivity εr and the equivalent relative permeability µr of the stacked structural body <NUM> is as illustrated in <FIG>. In addition, as illustrated in <FIG>, the transmissivity T of the stacked structural body <NUM> achieves the transmissivity T = <NUM> in the frequency band I (<NUM>) of millimeter waves and the transmissivity T ≈ <NUM> in the frequency band II (equal to or lower than <NUM> approximately) of the EMC region.

With such a millimeter-wave radar cover <NUM>, it is possible to emit millimeter waves through the stacked structural body <NUM> having a three-layer structure and detect the moving speed, distance, size, and the like of a physical body in the vicinity, in particular, on the front side (object such as a front vehicle or a human) based on the intensity of reflected waves of the millimeter waves, the time difference between the reflected waves, and the like.

In practice, when a physical body (object) on the front side is to be detected, any object on the front side is scanned with millimeter electromagnetic waves in the horizontal direction by a physical (mechanical scanning) or electrical (for example, digital beam forming or array antenna) method. The scanning method is not particularly limited, but the direction of emission may be changed by physically moving the antenna <NUM>, or the direction of emission may be changed by the electrical method without physically moving the antenna <NUM>.

Accordingly, millimeter electromagnetic waves emitted in a desired direction are reflected by an object in the desired direction and then can be received by the antenna <NUM>, and thus it is possible to detect a physical body (object) existing in a certain range on the front side in the horizontal direction.

However, there can be incoming millimeter waves from directions other than the desired direction when reflected waves from the physical body (object) are received. Examples of the incoming millimeter waves include what is called multipath waves, the propagation paths of which are different from each other due to the shape of the physical body (object) and a nearby reflective object (such as a guardrail, another vehicle, or the ground), and emitted waves and reflected waves from a millimeter-wave radar other than a millimeter-wave radar of the own vehicle and a millimeter-wave radar for automotive application.

In a typical millimeter-wave radar, the beam width of the antenna <NUM> is narrowed to increase the resolution in the horizontal direction, and thus it is unlikely to receive millimeter electromagnetic waves from directions other than a desired direction by the antenna <NUM>, but entering of incoming millimeter electromagnetic waves into the millimeter-wave radar increases background noise and affects the detection sensitivity.

Thus, as illustrated in <FIG>, the millimeter-wave radar uses a curved stacked structural body 10W having a configuration for preventing millimeter electromagnetic waves from directions other than a desired direction from being incident on the millimeter-wave radar upon emission of millimeter electromagnetic waves in the desired direction when a physical body (object) in the horizontal direction is to be detected.

The curved stacked structural body 10W has a basic structure identical to that of the stacked structural body <NUM> described above, and is a three-layer structure product obtained by stacking the first constituent material <NUM> and the second constituent materials <NUM> sandwiching the first constituent material <NUM> therebetween on the front surface side (the direction of arrow "a") and the back surface side (the direction of arrow "b"). A large difference between the curved stacked structural body 10W and the stacked structural body <NUM> is that the curved stacked structural body 10W is entirely curved in a convex shape in an external direction departing toward the front surface side (the direction of arrow "a") from the antenna <NUM> as the emission source of the millimeter-wave radar.

The curved stacked structural body 10W is characterized by having such oblique incidence transmissivity characteristics that the transmissivity T is high when millimeter electromagnetic waves are incident on the stacked structural body <NUM> in a perpendicular direction (hereinafter referred to as "perpendicular incidence") and the transmissivity T is low when millimeter electromagnetic waves are incident at an angle other than that of the perpendicular incidence.

Specifically, the curved stacked structural body 10W is centered at the antenna <NUM> as the emission source of millimeter electromagnetic waves and curved in a convex shape in a direction departing from the antenna <NUM> (to the outside of the radar) so that electromagnetic waves are perpendicular to the curved stacked structural body 10W when incident in any direction from the antenna <NUM>.

In this case, it is ideal that the curved stacked structural body 10W is curved at the curvature of a semicircular shape having a radius r0 = r1 at any part centered at the antenna <NUM>. With such an ideal curved stacked structural body 10W, millimeter electromagnetic waves emitted from the antenna <NUM> can be perpendicularly incident at any part of the curved stacked structural body 10W, and also millimeter electromagnetic waves perpendicularly incident on the curved stacked structural body 10W from the outside can be received by the antenna <NUM>.

However, since millimeter reflected waves and incoming multipath reflected waves from directions other than the direction of perpendicular incidence are incident on the curved stacked structural body 10W not perpendicularly but at some angles, the transmissivity T of the curved stacked structural body 10W is low for such waves.

As illustrated in <FIG>, the transmissivity T is one when millimeter electromagnetic waves incident on the curved stacked structural body 10W at an incident angle of <NUM>°, in other words, perpendicularly, and the transmissivity T is smaller than one when the incident angle is smaller or larger than <NUM>°. The state of the incident angle of <NUM>° is the state in which millimeter electromagnetic waves are incident in the state of a normal (at an angle along the normal) perpendicularly intersecting the tangent line of each second constituent material <NUM>. When the curved stacked structural body 10W has a semicircular shape of the radius r0 = r1, millimeter electromagnetic waves are perpendicularly incident on the curved stacked structural body 10W even when the millimeter electromagnetic waves are emitted from the antenna <NUM> while being tilted at an angle α.

However, although the incident angle of <NUM>° corresponds to a normal along which millimeter electromagnetic waves perpendicularly intersect the tangent line of each second constituent material <NUM>, it is regarded in a precise sense that the incident angle of millimeter electromagnetic waves is perpendicular to the second constituent materials <NUM> even when tilted to the tangent line in the range of ±<NUM>°.

In this case, the curved stacked structural body 10W does not necessarily need to have a semicircular shape having a radius r0 = r1 but may be entirely thinned in, for example, an aspherical shape (elliptical shape, in this case) having a radius r0 < r1 and tilted to the tangent line in the range of ±<NUM>° at most so that the degree of convex protrusion is reduced.

The curved stacked structural body 10W having such a structure can transmit millimeter electromagnetic waves emitted in a desired direction from the antenna <NUM> and allow the millimeter electromagnetic waves to reach an object. However, the curved stacked structural body 10W does not transmit millimeter electromagnetic waves from directions other than the desired direction nor allow the millimeter electromagnetic waves to reach the antenna <NUM>, and thus background noise does not increase, and the detection sensitivity is not affected.

Accordingly, the millimeter-wave radar can prevent entering of millimeter electromagnetic waves in a direction in which the object does not exist by the curved stacked structural body 10W at emission for detecting a physical body (object) in the horizontal direction while the direction of emission is scanned, for example, in a wide range of ±<NUM>° approximately with respect to the antenna <NUM>.

As illustrated in <FIG>, millimeter electromagnetic waves from the antenna <NUM> are transmitted when the millimeter electromagnetic waves are perpendicularly incident on the second constituent materials <NUM> of the curved stacked structural body 10W, in other words, when the millimeter electromagnetic waves are incident in a direction perpendicular to each of the second constituent material <NUM>, the first constituent material <NUM>, and the tangent line of the second constituent material <NUM>, but are not transmitted in other cases.

In the drawing, "transmitted (O)" means that the transmissivity T is equal to or higher than <NUM>, and "not transmitted (x)" means that the transmissivity T is lower than <NUM>. However, the value of the transmissivity T is not limited thereto but may be optionally set with taken into account usage and the detection accuracy on the measurement side.

Specifically, as illustrated in <FIG>, the transmissivity T is substantially equal to one when the incident angle of millimeter electromagnetic waves on the curved stacked structural body 10W is <NUM>°, but the transmissivity T is equal to <NUM> or is lower than <NUM> when the incident angle of millimeter electromagnetic waves is shifted by <NUM>° or <NUM>°, respectively.

The curved stacked structural body 10W made of a three-layer structural body having such oblique incidence transmissivity characteristics can be achieved by appropriately setting the thickness and permittivity of the dielectric in the second constituent materials <NUM> and the thickness t, the line width d, and the array interval a in the longitudinal and lateral directions of the metal lattice in the first constituent material <NUM>.

In particular, the thickness t of the metal lattice of the first constituent material <NUM> is important to achieve desired oblique incidence transmissivity characteristics and may be <NUM> to <NUM>, more preferably <NUM> to <NUM>, and other dimensions may be designed values as needed.

In addition, it is possible to use a curved stacked structural body 100W of a dielectric lens-integrated type (hereinafter also referred to as "dielectric lens-integrated curved stacked structural body") having a configuration with which millimeter electromagnetic waves from directions other than a desired direction are not incident on the millimeter-wave radar at emission of millimeter electromagnetic waves in the desired direction as illustrated in <FIG>, similarly to the curved stacked structural body 10W.

The dielectric lens-integrated curved stacked structural body 100W is a structural body in which a dielectric lens <NUM> made of a convex lens having a predetermined permittivity for refracting millimeter electromagnetic waves incident from the antenna <NUM> is disposed between the corresponding second constituent material <NUM> of the curved stacked structural body 10W and the antenna <NUM> and integrated with the dielectric lens <NUM>.

With the dielectric lens-integrated curved stacked structural body 100W integrated with the dielectric lens <NUM>, millimeter electromagnetic waves are refracted inward due to the permittivity of the dielectric lens <NUM>. Thus, the dielectric lens-integrated curved stacked structural body 100W allows perpendicular incidence of millimeter electromagnetic waves emitted from the antenna <NUM> at a wide angle corresponding to the refraction and can be entirely thinned like an aspherical shape (elliptical shape, in this case) having a radius r10 < r11.

As illustrated in <FIG>, the transmissivity T is one when millimeter electromagnetic waves are incident on the dielectric lens-integrated curved stacked structural body 100W at the incident angle of <NUM>°, in other words, perpendicularly, and the transmissivity T is lower than one when the incident angle is smaller or larger than <NUM>°.

In addition, since millimeter electromagnetic waves are refracted inward due to the permittivity of the dielectric lens <NUM> in the dielectric lens-integrated curved stacked structural body 100W, millimeter electromagnetic waves are perpendicularly incident on the dielectric lens-integrated curved stacked structural body 100W even when the millimeter electromagnetic waves are emitted from the antenna <NUM> while being tilted at the angle α.

In this case, as illustrated in <FIG>, in the dielectric lens-integrated curved stacked structural body 100W, millimeter electromagnetic waves incident on the dielectric lens <NUM> at a predetermined angle θ1 from the antenna <NUM> at an inside Rin of the millimeter-wave radar cover <NUM> are refracted slightly inward by the dielectric lens <NUM> and tilted at an angle θ2, travel through the inside of the dielectric lens <NUM>, and then are emitted to an outside Rout through the curved stacked structural body 10W.

In addition, since the dielectric lens-integrated curved stacked structural body 100W integrally includes the dielectric lens <NUM>, the degree (curvature) of convex protrusion can be reduced to increase thinning as compared to a case in which only the curved stacked structural body 10W is provided.

Some preferable embodiments of the present invention are described above, but the present invention is not limited to the millimeter-wave radar cover <NUM> according to the above-described embodiments and includes all aspects included in the concept and claims of the present invention. In addition, configurations may be selectively combined as appropriate to achieve at least part of the above-described problem and effect. For example, the shape, material, disposition, size, and the like of each component in the above-described embodiments may be changed as appropriate depending on a specific use aspect of the present invention.

Although the case in which the dielectric lens-integrated curved stacked structural body 100W integrated with the dielectric lens <NUM> made of a convex lens is used is described in the above-described embodiments, the present invention is not limited thereto but, for example, a dielectric lens-integrated curved stacked structural body 200W integrated with a dielectric lens <NUM> made of a concave lens as illustrated in <FIG> may be used.

Specifically, as illustrated in <FIG>, the dielectric lens-integrated curved stacked structural body 200W is an integration structural body in which the dielectric lens <NUM> made of a concave lens having a predetermined permittivity for refracting millimeter electromagnetic waves incident from the antenna <NUM> through the curved stacked structural body 10W is stacked on the second constituent material <NUM> on the outer periphery side in the curved stacked structural body 10W. The dielectric lens <NUM> as the concave lens is curved at a curvature same as that of the second constituent material <NUM> of the curved stacked structural body 10W.

As illustrating in <FIG>, in the dielectric lens-integrated curved stacked structural body 200W, millimeter electromagnetic waves are incident from the antenna <NUM> in a direction perpendicular (at an angle θ1 = <NUM>°) to the tangent line of the second constituent material <NUM> of the curved stacked structural body 10W on the inner periphery side are refracted outward by an angle θ2 (θ2 > θ1) as passing through the dielectric lens <NUM> and emitted to the outside Rout. Thus, the dielectric lens-integrated curved stacked structural body 200W allows millimeter electromagnetic waves to be emitted at a wide angle because of the integration with the dielectric lens <NUM> made of a concave lens, and also can be entirely further thinned.

Claim 1:
A millimeter-wave radar cover (<NUM>) housing a millimeter-wave radar including an antenna (<NUM>) and an electronic circuit (<NUM>) configured to drive the antenna, the millimeter-wave radar cover comprising:
a first part (A) provided in front of the millimeter-wave radar to protect the millimeter-wave radar and transmit millimeter electromagnetic waves emitted from the antenna; and
a second part (B) including a housing space in which the antenna and the electronic circuit except for the first part are housed,
wherein
the first part is made of a stacked structural body (10W) obtained by stacking at least one layer of a first constituent material (<NUM>) having a negative permittivity in the frequency band of the millimeter waves and a second constituent material (<NUM>) having a positive permittivity in the frequency band of the millimeter waves,
characterized in that
the stacked structural body is centered at the antenna as an emission source (<NUM>) of the millimeter electromagnetic waves and curved in a direction departing from the antenna so that the millimeter electromagnetic waves are perpendicular to the curved stacked structural body when incident in any direction from the emission source.