Radio wave absorber and antenna device

Abstract

To reduce mutual coupling between elements of antennas used in wireless base stations and terminals and improve radiation efficiency using a simple method.SOLUTION: The present invention includes a dielectric substrate and first and second antenna elements, and by providing a radio wave absorber between the first and second antenna elements, mutual coupling between the elements is reduced and radiation efficiency is improved.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to an antenna device having a radio wave absorber for improving isolation between antenna elements. [Background technology]

[0002] In recent years, array antennas used in wireless base stations and terminals have begun to use patch array antennas, in which multiple radiating elements are formed with conductive patches on a printed circuit board made of dielectric material, etc. Patch array antennas are thin and can control their directivity by matching the radio waves of adjacent radiating elements, so they have high-performance antenna characteristics and can be made at low cost.

[0003] The center distance d between the radiating elements of an array antenna (radiating element center distance) is generally set to d = λ / 2, where λ is the wavelength in air. If d > λ / 2, this is undesirable because grating lobes (energy radiation in unwanted directions) occur. Also, if d < λ / 2, mutual coupling between elements occurs, which changes the antenna characteristics, and is therefore undesirable.

[0004] In an actual array antenna, even if the center distance between radiating elements (radiating element center distance) d is λ / 2, the distance between the end faces of adjacent radiating elements (effective element spacing) ' is significantly smaller than the radiating element center distance d (d' << λ / 2). This causes the problem of very large inter-element coupling (increased coupling rate) between the radiating elements. This is because, in reality, radiating elements have a finite size, so the effective element spacing d' is significantly smaller than the radiating element center distance d.

[0005] Furthermore, due to wiring and other components, there are cases where an antenna must be designed with d<λ / 2 or less.

[0006] Furthermore, in order to increase the element area of ​​the radiating element, increase the individual gain, and reduce transmission loss (feeder loss), printed circuit boards with low dielectric constants and low dielectric dissipation factors are sometimes selected. In such a structure, the center distance of the radiating elements of the array antenna becomes even narrower, creating a vicious cycle in which the mutual coupling between elements increases further. The increase in mutual coupling between elements results in a drastic decrease in the antenna radiation efficiency. In this case, the radiation efficiency of each radiating element, rather than the entire array antenna (array factor), decreases, and in some cases the radiation efficiency may be halved.

[0007] In order to prevent such mutual coupling between elements, for example, a technique has been disclosed in which an isolation structure having electrical connections between elements is provided (Patent Document 1). [Prior art documents] [Patent documents]

[0008] [Patent Document 1] WO2018 / 235593 Summary of the Invention [Problem to be solved by the invention]

[0009] Isolation structures with electrical connections, such as those in the prior art, can have complicated circuits, and because they include metal layers, they require design consideration of the effects of reflection on the surface.

[0010] An object of the present invention is to improve radiation efficiency by a simple method. [Means for solving the problem]

[0011] The present invention has a dielectric substrate, first and second antenna elements, and a radio wave absorber between the first and second antenna elements, thereby reducing mutual coupling between the elements and improving radiation efficiency. Specifically, this is achieved by the following means.

[0012] [1] An antenna device having a dielectric substrate, first and second antenna elements, and a radio wave absorber between the first and second antenna elements. [2] The antenna device according to [1], wherein the radio wave absorber contains magnetic powder and a binder and selectively absorbs specific frequencies. [3] The antenna device according to [1] and [2], wherein the height of the radio wave absorber is 30 mm or less. [4] The antenna device according to any one of [1] to [3], wherein the radio wave absorber has a lattice shape and is placed in contact with a dielectric substrate. [5] The antenna device according to any one of claims [1] to [4], wherein the magnetic powder contains powder of magnetoplumbite-type hexagonal ferrite. [6] The antenna device according to any one of [1] to [5], wherein the dielectric substrate has a relative permittivity of 1 to 40. [7] A radio wave absorber used in the antenna device according to any one of [1] to [6]. [Effects of the Invention]

[0013] According to the present disclosure, in an antenna device, radiation efficiency can be improved with a simple configuration. [Brief explanation of the drawings]

[0014] [Figure 1] FIG. 1 is a perspective view showing a configuration of an antenna device according to an embodiment. [Figure 2] (a) is a plan view of the array antenna in Figure 1, and (b) is a cross-sectional view of the array antenna in Figure 1. [Figure 3] Schematic diagrams showing other configuration examples of the isolation structure, where (a) is a plan view of the array antenna and (b) is a cross-sectional view of the array antenna. [Figure 4] Schematic diagrams showing other configuration examples of the isolation structure, where (a) is a plan view of the array antenna and (b) is a cross-sectional view of the array antenna. DETAILED DESCRIPTION OF THE INVENTION

[0015] The present invention will be described in detail below. However, the present invention is not limited to the following embodiments and can be practiced with appropriate modifications within the scope of the object of the present invention.

[0016] Fig. 1 is a perspective view showing a schematic configuration of an antenna device according to an embodiment, Fig. 2(a) is a plan view of the antenna device of Fig. 1, and Fig. 2(b) is a cross-sectional view of the antenna device of Fig. 1.

[0017] 1 and 2 includes a dielectric substrate 1, first and second antenna elements 2 and 3 provided on the upper surface (corresponding to the first surface) of the dielectric substrate 1, a ground conductor 4 provided on the lower surface (corresponding to the second surface) of the dielectric substrate 1, and a radio wave absorber (isolation structure) 5 provided between the first and second antenna elements 2 and 3. Here, the first antenna element 2 is a transmitting antenna, and the second antenna element 3 is a receiving antenna.

[0018] In Fig. 1, the direction in which the first and second antenna elements 2, 3 are aligned is the X direction (corresponding to the first direction), the direction perpendicular to the X direction in a plan view is the Y direction (corresponding to the second direction), and the direction perpendicular to the substrate surface is the Z direction. Fig. 2(b) shows a cross section along the X direction that passes through the first and second antenna elements 2, 3 and the radio wave absorber (isolation structure) 5. The planar shape of the antenna elements 2, 3 is approximately square. However, the planar shape of the antenna elements 2, 3 is not limited to this.

[0019] The antenna elements 2, 3 may be metallic, such as copper, silver, or gold, or may be constructed of dimensionally variable materials, including magnetic materials.

[0020] The radio wave absorber (isolation structure) 5 has the function of improving the isolation of the antenna device. That is, the radio wave absorber (isolation structure) 5 acts as an electrical wall against radio wave signals propagating through the dielectric substrate 1. In particular, the radio wave absorber of the present invention has the characteristic of strongly absorbing specific radio waves, and can efficiently absorb radio waves.

[0021] [Radio wave absorber] The radio wave absorber of the present disclosure has a radio wave absorbing layer containing magnetic powder and a binder, and does not contain metal, and the filling rate of the magnetic powder in the radio wave absorbing layer is 35 volume % or less, and when the filling rate of the magnetic powder in the radio wave absorbing layer is P volume % and the thickness of the radio wave absorbing layer is Q mm, the radio wave absorber satisfies the relationship 0.65≦(P / 100)×Q. The radio wave absorber of the present disclosure has a radio wave absorbing layer containing magnetic powder and a binder, and the filling rate of the magnetic powder in the radio wave absorbing layer is 35% by volume or less. When the filling rate of the magnetic powder in the radio wave absorbing layer is P% by volume and the thickness of the radio wave absorbing layer is Q mm, the relationship 0.65≦(P / 100)×Q is satisfied. Therefore, despite not having a metal layer, the transmission attenuation and return loss in the millimeter wave band are both 10 dB or more. A radio wave absorber having a transmission loss and a return loss of 10 dB or more can absorb at least 90% of radio waves.

[0022] ~ Configuration of radio wave absorber ~ The radio wave absorber of the present disclosure preferably has a radio wave absorbing layer and does not have a metal layer. The radio wave absorber of the present disclosure may have layers other than the radio wave absorbing layer (so-called other layers) as needed, within the scope that does not impair the effect of the radio wave absorber of the present disclosure. Examples of other layers include a protective layer, an adhesive layer, and a release layer. Furthermore, the radio wave absorber of the present disclosure may have a metal-free reflective layer as another layer within the range that does not impair the effect of the radio wave absorber of the present disclosure, but an embodiment that does not have a metal-free reflective layer is preferable.

[0023] ~Shape of radio wave absorber~ The radio wave absorber of the present disclosure may have a planar shape or a three-dimensional shape. The planar shape is not particularly limited, and examples thereof include a sheet shape, a film shape, and the like. The three-dimensional shape may be a lattice shape, a tube shape (cylindrical shape, square tube shape, etc.), a horn shape, a box shape (however, one of the faces is open), or the like.

[0024] [Radio wave absorbing layer] The radio wave absorbing layer includes magnetic powder and a binder. The components contained in the radio wave absorbing layer will be described in detail later.

[0025] The filling rate of the magnetic powder in the radio wave absorbing layer is 35% by volume or less, preferably 8% by volume or more and 35% by volume or less, more preferably 15% by volume or more and 35% by volume or less, even more preferably 20% by volume or more and 35% by volume or less, and particularly preferably 25% by volume or more and 35% by volume or less. When the filling rate of the magnetic powder in the radio wave absorbing layer is 35% by volume or less, the radio wave absorber can achieve a return loss of 10 dB or more even if it does not contain metal.

[0026] The filling rate of the magnetic powder in the radio wave absorbing layer is a value measured and calculated by the following method using a scanning electron microscope (SEM). The electromagnetic wave absorbing layer is cut into pieces measuring 5 mm x 5 mm. The cut electromagnetic wave absorbing layer is attached to a stage, and then a cross section is processed in the thickness direction using a focused ion beam (FIB). The processed electromagnetic wave absorbing layer is set on the stage so that its cross section is at the top, and then a cross-sectional SEM image with a field of view of 30 μm x 40 μm is obtained using a field emission scanning electron microscope (FE-SEM) at a voltage of 15 kV and an observation magnification of 3,000 times. The obtained cross-sectional SEM image is binarized to determine the proportion of magnetic powder, and the magnetic powder filling rate is calculated. The above operation is carried out five times by changing the cutting position of the electromagnetic wave absorbing layer, and the arithmetic mean value of the calculated values ​​is taken as the packing rate of the magnetic powder in the electromagnetic wave absorbing layer. The arithmetic mean value is rounded off to one decimal place.

[0027] As the focused ion beam (FIB) device, for example, a high-performance focused ion beam (FIB) device (product name: MI4050) manufactured by Hitachi, Ltd. can be suitably used. However, the focused ion beam (FIB) device is not limited to this. As a field emission scanning electron microscope (FE-SEM), for example, a field emission scanning electron microscope (product name: SU-8220) manufactured by Hitachi, Ltd. can be suitably used. However, the field emission scanning electron microscope (FE-SEM) is not limited to this.

[0028] When the filling rate of the magnetic powder in the radio wave absorbing layer is P volume % and the thickness of the radio wave absorbing layer is Q mm, the radio wave absorbing layer satisfies the relationship 0.65≦(P / 100)×Q≦Q, preferably satisfies the relationship 0.65≦(P / 100)×Q≦5.0, more preferably satisfies the relationship 0.65≦(P / 100)×Q≦3.5, even more preferably satisfies the relationship 0.65≦(P / 100)×Q≦1.75, and particularly preferably satisfies the relationship 0.65≦(P / 100)×Q≦1.0. If the relationship 0.65≦(P / 100)×Q is satisfied, the radio wave absorber can achieve a transmission attenuation of 10 dB or more.

[0029] The thickness of the radio wave absorbing layer is not particularly limited as long as it satisfies the above-mentioned relationship "0.65≦(P / 100)×Q". The thickness of the radio wave absorbing layer is, for example, preferably 30 mm or less, more preferably 10 mm or less, and even more preferably 5 mm or less, from the viewpoint of freedom in installation location. The lower limit of the thickness of the radio wave absorbing layer is not particularly limited, but is preferably 2 mm or more from the viewpoint of mechanical properties, for example.

[0030] The thickness of the electromagnetic wave absorbing layer is a value measured using a digital length measuring device, and specifically, is the arithmetic mean value of measurements taken at nine arbitrarily selected points. As the digital length measuring machine, for example, a digital length measuring machine (product name: Litematic (registered trademark) VL-50A) manufactured by Mitutoyo Corporation can be suitably used. However, the digital length measuring machine is not limited to this.

[0031] As an embodiment that satisfies the relationship 0.65≦(P / 100)×Q, for example, an embodiment in which the thickness of the radio wave absorbing layer is 10 mm or less and the filling rate of the magnetic powder in the radio wave absorbing layer is 8 volume % or more and 35 volume % or less is preferred, an embodiment in which the thickness of the radio wave absorbing layer is 5 mm or less and the filling rate of the magnetic powder in the radio wave absorbing layer is 15 volume % or more and 35 volume % or less is more preferred, and an embodiment in which the thickness of the radio wave absorbing layer is 2 mm or more and 5 mm or less and the filling rate of the magnetic powder in the radio wave absorbing layer is 20 volume % or more and 35 volume % or less is even more preferred.

[0032] <Magnetic powder> The radio wave absorbing layer includes magnetic powder. The magnetic powder is not particularly limited, and examples thereof include powders of ferrite, iron oxide, cobalt, chromium oxide, and the like. From the viewpoint of radio wave absorption performance, the magnetic powder preferably contains magnetoplumbite hexagonal ferrite powder (hereinafter also referred to as "magnetoplumbite hexagonal ferrite powder"), and more preferably is magnetoplumbite hexagonal ferrite powder. Magnetoplumbite-type hexagonal ferrite generally has the composition formula A 1 Fe 12 O 1 9 (In the formula, A 1 represents a metal element such as Sr, Ba, Ca, or Pb. However, the concept of "magnetoplumbite-type hexagonal ferrite" in this disclosure includes the composition formula A 1 Fe 12 O 1 9 In addition to the magnetoplumbite hexagonal ferrite represented by the formula (1) below, the magnetoplumbite hexagonal ferrite also includes the magnetoplumbite hexagonal ferrite represented by the formula (1) below. A 1From the viewpoint of operability and handling, it is preferable that the metal element be at least one metal element selected from the group consisting of Sr, Ba, Ca, and Pb.

[0033] The magnetic powder preferably contains a powder of magnetoplumbite-type hexagonal ferrite represented by the following formula (1), and is preferably a powder of magnetoplumbite-type hexagonal ferrite represented by formula (1), because the magnetic powder has excellent magnetic properties and can exhibit excellent radio wave absorption performance even in high frequency bands. Hereinafter, the magnetoplumbite-type hexagonal ferrite represented by formula (1) will also be referred to as a "specific magnetoplumbite-type hexagonal ferrite." Furthermore, powder of the specific magnetoplumbite-type hexagonal ferrite will also be referred to as a "specific magnetoplumbite-type hexagonal ferrite powder."

[0034] [ka]

[0035] In formula (1), A represents at least one metal element selected from the group consisting of Sr, Ba, Ca, and Pb, and x satisfies 1.5≦x≦8.0.

[0036] In formula (1), A is at least one metal element selected from the group consisting of Sr, Ba, Ca, and Pb, and the type and number of the metal element are not particularly limited. In terms of operability and handling, A in formula (1) is preferably at least one metal element selected from the group consisting of Sr, Ba, and Ca. Furthermore, A in formula (1) preferably contains Sr, and more preferably is Sr, in that excellent radio wave absorption performance can be exhibited, for example, around 79 GHz.

[0037] In formula (1), x satisfies 1.5≦x≦8.0, preferably 1.5≦x≦6.0, more preferably 1.5≦x≦4.0, and further preferably 1.5≦x≦3.0. When x in formula (1) is 1.5 or more, radio waves in a frequency band higher than 60 GHz can be absorbed. When x in formula (1) is 8.0 or less, the magnetoplumbite-type hexagonal ferrite has magnetism.

[0038] Specific magnetoplumbite hexagonal ferrites include SrFe ( 1 0 . 4 4 ) A l (1 . 5 6 ) O 1 9 , S r F e ( 1 0 . 0 0 ) A l ( 2 . 0 0 ) O 1 9 , S r F e ( 9 . 9 5 ) Al ( 2 . 0 5 ) O 1 9 , S r F e ( 9 . 8 5 ) A l ( 2 . 1 5 ) O 1 9 , S r F e ( 9 . 7 9 ) A l ( 2 . 2 1 ) O 1 9 , S r F e ( 9 . 7 4 ) A l ( 2 . 2 6 ) O 1 9 , S r F e ( 9 . 7 0) A l ( 2 . 3 0 ) O 1 9 , S r F e ( 9 . 5 8 ) A l ( 2 . 4 2 ) O 1 9 , S r F e ( 9 . 37 ) A l ( 2 . 6 3 ) O 1 9 , S r F e ( 9 . 3 3 ) A l ( 2 . 6 7 ) O 1 9 , S r F e ( 9 .2 7 ) A l ( 2 . 7 3 ) O 1 9 , S r F e ( 7 . 8 8 ) A l ( 4 . 1 2 ) O 1 9 , S r F e ( 7. 7 1 ) A l ( 4 . 2 9 ) O 1 9 , S r F e ( 7 . 3 7 ) A l ( 4 . 6 3 ) O 1 9 , S r F e (7 . 0 4 ) A l ( 4 . 9 6 ) O 1 9 , S r F e ( 6 . 2 5 ) A l ( 5 . 7 5 ) O 1 9 , B a F e( 9 . 5 0 ) A l ( 2 . 5 0 ) O 1 9 , B a F e ( 1 0 . 0 5 ) A l ( 1 . 9 5 ) O 1 9 , C aF e ( 1 0 . 0 0 ) A l ( 2 . 0 0 ) O 1 9 , PbFe ( 9 . 0 0 ) A l ( 3 . 0 0 ) O 1 9 , Sr ( 0 . 8 0 ) B a ( 0 . 1 0 ) C a ( 0 . 1 0 ) F e ( 9 . 8 3 ) A l ( 2 . 1 7 ) O 1 9 , Sr ( 0 . 8 0 ) B a ( 0 . 1 0 ) C a ( 0 . 1 0 ) F e ( 8 . 8 5 ) A l ( 3 . 1 5 ) O 1 9 etc. For example, SrFe ( 1 0 . 0 0 ) A l ( 2 . 0 0 ) O 1 9 is a specific magnetoplumbite-type hexagonal ferrite with a resonance frequency around 76.5 GHz, and SrFe ( 9 . 70 ) A l ( 2 . 3 0 ) O 1 9 is a specific magnetoplumbite-type hexagonal ferrite having a resonance frequency around 85.0 GHz.

[0039] The method for producing the specific magnetoplumbite hexagonal ferrite powder is carried out by the method described in JP-A-2023-145469.

[0040] The composition of the magnetoplumbite-type hexagonal ferrite is confirmed by high-frequency inductively coupled plasma (ICP) emission spectroscopy. Specifically, a pressure-resistant container containing 12 mg of sample powder and 10 mL of 4 mol / L (liter; the same applies hereinafter) hydrochloric acid solution is placed in an oven set at 120°C for 12 hours to obtain a solution. Next, 30 mL of pure water is added to the resulting solution, which is then filtered using a 0.1 μm membrane filter. Elemental analysis of the filtrate thus obtained is performed using an inductively coupled plasma (ICP) optical emission spectrometer. Based on the results of the elemental analysis, the content of each metal atom relative to 100 atomic percent iron atoms is calculated. The composition is confirmed based on the calculated content. As an ICP optical emission spectrometer, for example, ICPS-8100 (model number) manufactured by Shimadzu Corporation can be suitably used, but the ICP optical emission spectrometer is not limited to this.

[0041] The crystalline phase of the magnetoplumbite hexagonal ferrite may or may not be a single phase, but when the magnetoplumbite hexagonal ferrite is a specific magnetoplumbite hexagonal ferrite, the crystalline phase is preferably a single phase. When the aluminum content is the same, a powder of the specific magnetoplumbite-type hexagonal ferrite having a single crystal phase tends to have a higher coercive force and better magnetic properties than a powder of the specific magnetoplumbite-type hexagonal ferrite having a non-single crystal phase (for example, a two-phase crystal phase).

[0042] In the present disclosure, the term "single-phase crystal" refers to a case in which only one diffraction pattern showing the crystal structure of a magnetoplumbite-type hexagonal ferrite of any composition is observed in a powder X-ray diffraction (XRD) measurement. On the other hand, in the present disclosure, the case where "the crystal phase is not a single phase" refers to a case where a plurality of magnetoplumbite-type hexagonal ferrites of arbitrary compositions are mixed and two or more types of diffraction patterns are observed, or a diffraction pattern of a crystal other than magnetoplumbite-type hexagonal ferrite is observed.

[0043] When the crystalline phase is not a single phase, a diffraction pattern having a main peak and other peaks is obtained. Here, the "main peak" refers to the peak with the highest diffraction intensity in the observed diffraction pattern. When the radio wave absorbing layer contains magnetoplumbite-type hexagonal ferrite powder as the magnetic powder, the ratio (Is / Im) of the diffraction intensity value of the main peak (hereinafter referred to as "Im") to the diffraction intensity value of the other peaks (hereinafter referred to as "Is") obtained by powder X-ray diffraction (XRD) measurement of the magnetoplumbite-type hexagonal ferrite powder is preferably 1 / 2 or less, and more preferably 1 / 5 or less, from the viewpoint of being able to produce a radio wave absorber with more excellent radio wave absorption performance. When two or more diffraction patterns overlap and each peak has a maximum, the respective maximum values ​​are defined as Im and Is, and the ratio is calculated. When two or more diffraction patterns overlap and a peak other than the main peak is observed as a shoulder, the maximum intensity value of the shoulder is defined as Is, and the ratio is calculated. If two or more other peaks are present, the sum of the diffraction intensities is defined as Is, and the ratio is calculated.

[0044] For the assignment of diffraction patterns, for example, the database of the International Centre for Diffraction Data (ICDD, registered trademark) can be referenced. For example, the diffraction pattern of magnetoplumbite-type hexagonal ferrite containing Sr can be found in "00-033-1340" of the International Center for Diffraction Data (ICDD). However, when part of the iron is replaced with aluminum, as in certain magnetoplumbite-type hexagonal ferrites, the peak position shifts.

[0045] As described above, it is confirmed by powder X-ray diffraction (XRD) measurement that the crystalline phase of the magnetoplumbite-type hexagonal ferrite is a single phase. Specifically, the powder X-ray diffraction (XRD) is measured under the following conditions. As a powder X-ray diffraction (XRD) device, for example, X'PertPro (product name) manufactured by PANalytical Inc. can be suitably used, but the powder X-ray diffraction (XRD) device is not limited to this.

[0046] -Conditions- X-ray source: CuKα ray [Wavelength: 1.54 Å (0.154 nm), Output: 40 mA, 45 kV] Scan range: 20° < 2θ < 70° Scan interval: 0.05° Scan speed: 0.75° / min

[0047] Whether the radio wave absorbing layer contains magnetoplumbite-type hexagonal ferrite powder can be confirmed, for example, by the following method. The electromagnetic wave absorbing layer is finely chopped, soaked in a solvent (e.g., acetone) for 1 to 2 days, and then dried. The dried electromagnetic wave absorbing layer is further ground into fine powder and subjected to powder X-ray diffraction (XRD) measurement to confirm the structure. Furthermore, after cutting out a cross section of the radio wave absorbing layer, the composition can be confirmed by using, for example, an energy dispersive X-ray analyzer.

[0048] The shape of the particles constituting the magnetic powder is not particularly limited, and examples thereof include spherical, rod-like, needle-like, tabular, and irregular shapes. The shape of the particles constituting the magnetoplumbite-type hexagonal ferrite powder may be, for example, tabular or irregular.

[0049] The size of the particles constituting the magnetic powder is not particularly limited. The magnetic powder (preferably magnetoplumbite-type hexagonal ferrite powder, more preferably specific magnetoplumbite-type hexagonal ferrite powder; the same applies hereinafter) has a mode diameter of 5 μm, where the most frequent value is the mode diameter, the cumulative 10% diameter is D10, and the cumulative 90% diameter is D90, in a number-based particle size distribution measured by a laser diffraction scattering method. It is preferable that the mode diameter is 5 μm or more and less than 10 μm and (D90-D10) / mode diameter≦3.0, it is more preferable that the mode diameter is 5 μm or more and less than 10 μm and (D90-D10) / mode diameter≦2.5, it is even more preferable that the mode diameter is 5 μm or more and less than 10 μm and (D90-D10) / mode diameter≦2.0, it is particularly preferable that the mode diameter is 5 μm or more and less than 10 μm and (D90-D10) / mode diameter≦1.5, it is most preferable that the mode diameter is 5 μm or more and less than 10 μm and (D90-D10) / mode diameter≦1.0.

[0050] Magnetic powder having a mode diameter of 5 μm or more and (D90-D10) / mode diameter≦3.0 tends to produce a radio wave absorber with superior radio wave absorption performance, since it contains relatively few fine particles with inferior magnetic properties. Magnetic powder having a mode diameter of less than 10 μm and (D90−D10) / mode diameter≦3.0 tends to produce a radio wave absorber with superior strength because it contains relatively few coarse particles.

[0051] The particle size of the magnetic powder (i.e., mode diameter, D10, and D90) can be controlled by classification using a sieve, a centrifuge, etc., or by pulverization using a mortar and pestle, an ultrasonic disperser, etc. For example, when controlling the particle size of the magnetic powder by pulverization, it is possible to adjust the particle size to the desired value by selecting the pulverization means, pulverization time, media material, media diameter, etc. For example, when pulverizing using media, the particle size of the magnetic powder tends to become smaller. Also, for example, the longer the pulverization time, the smaller the particle size of the magnetic powder tends to become. Also, for example, the smaller the media diameter, the smaller the particle size of the magnetic powder tends to become. The value of "(D90-D10) / mode diameter" can be adjusted to a desired value by sorting the particles after pulverization, for example, by classification using a sieve, a centrifuge, or the like.

[0052] The mode, cumulative 10% diameter, and cumulative 90% diameter of the magnetic powder are values ​​determined based on the particle size distribution based on the number of particles measured by the laser diffraction scattering method. Specifically, they are values ​​measured by the following method. 10 mg of magnetic powder is diluted with 500 mL of cyclohexanone, then stirred for 30 seconds using a shaker. The resulting liquid is used as a sample for particle size distribution measurement. Next, the particle size distribution of the sample is measured using the laser diffraction scattering method. A laser diffraction / scattering particle size distribution analyzer is used as the measuring device.

[0053] As a laser diffraction / scattering particle size distribution measuring device, for example, Partica LA-960 (product name) manufactured by Horiba, Ltd. can be suitably used. However, the laser diffraction / scattering particle size distribution measuring device is not limited to this.

[0054] The particle size of the magnetic powder contained in the radio wave absorbing layer can be confirmed, for example, by the following method. The electromagnetic wave absorbing layer is chopped into small pieces and then ultrasonically dispersed in a solvent (e.g., acetone). The particle size of the magnetic powder can be confirmed by measuring the obtained dispersion liquid as a sample using a laser diffraction scattering method.

[0055] The coercive force (Hc) of the magnetic powder is not particularly limited, but is preferably 2.5 kOe or more, more preferably 4.0 kOe or more, and even more preferably 5.0 kOe or more. When the coercive force (Hc) of the magnetic powder is 2.5 kOe or more, a radio wave absorber with more excellent radio wave absorbing performance can be produced. There is no particular upper limit to the coercive force (Hc) of the magnetic powder, but it is preferably, for example, 18 kOe or less.

[0056] The saturation magnetization (δs) per unit mass of the magnetic powder is not particularly limited, but is preferably, for example, 10 emu / g or more, more preferably 20 emu / g or more, and even more preferably 30 emu / g or more. When the saturation magnetization (δs) per unit mass of the magnetic powder is 10 emu / g or more, a radio wave absorber with more excellent radio wave absorbing performance can be produced. There is no particular upper limit to the saturation magnetization (δs) per unit mass of the magnetic powder, but it is preferably, for example, 60 emu / g or less.

[0057] The coercive force (Hc) and saturation magnetization per unit mass (δs) of the magnetic powder were measured using a vibrating sample magnetometer under the conditions of an ambient temperature of 23°C, a maximum applied magnetic field of 50 kOe, and a magnetic field sweep rate of 25 Oe / s (seconds). As a vibrating sample magnetometer, for example, a TM-TRVSM5050-SMSL model (model number) manufactured by Tamagawa Manufacturing Co., Ltd. can be suitably used. However, the vibrating sample magnetometer is not limited to this.

[0058] The radio wave absorbing layer may contain only one type of magnetic powder, or may contain two or more types.

[0059] The mass content of the magnetic powder in the radio wave absorbing layer is not particularly limited as long as the filling rate of the magnetic powder in the radio wave absorbing layer is 35% by volume or less.

[0060] (Effectiveness verification) The inventors of the present application used the following simulation model. The antenna sizes of the first and second antenna elements 2 and 3 were optimized based on the transmission frequency and the relative dielectric constant of the dielectric substrate 1. The center-to-center distance between the first and second antenna elements 2 and 3 was set to a length equivalent to the wavelength λ1 of the transmission signal, and the thickness of the dielectric substrate 1 was set to 0.05 times the wavelength λ1 of the transmission signal. The simulation was performed at a transmission frequency of 78 GHz. Note that the relative dielectric constant εr of the dielectric substrate 1 is usually in the range of 2.0 to 5.0, so the relative dielectric constant εr was set to 3.0. In the simulation, the amount of radio waves received by the second antenna element due to leakage of radio waves emitted from the first antenna element was compared as isolation [dB] for cases with and without a radio wave absorber (isolation structure, 30 mm high). The isolation without the wave absorber was -30dB, while the isolation with the wave absorber was -35dB, demonstrating a significant improvement in isolation.

[0061] (Another configuration example 2) Fig. 3 shows another example of the configuration. The radio wave absorber (isolation structure) 5 in Fig. 3 is almost the same as the radio wave absorber in Fig. 2. However, it is formed in a lattice pattern. This is because surrounding the first and second antenna elements 2 and 3 in a lattice pattern makes it easier to absorb radio waves.

[0062] (Effectiveness verification) As in the simulation described above, a comparison was made between the presence and absence of a radio wave absorber (isolation structure, 30 mm high) 5, in terms of the amount of radio waves that leak from the first antenna element and are received by the second antenna element, expressed as isolation [dB]. The isolation without the wave absorber was -30dB, while the isolation with the wave absorber was -38dB, demonstrating a significant improvement in isolation.

[0063] (Other configuration example 3) Fig. 4 shows another example of the configuration. The radio wave absorber (isolation structure) 5 in Fig. 4 is almost the same as the radio wave absorber in Fig. 3. However, it is formed by being embedded in the dielectric substrate 1. This is to efficiently absorb radio waves propagating through the substrate and to make it difficult to absorb radio waves transmitted upward from the first antenna element, thereby increasing radiation efficiency.

[0064] (Effectiveness verification) As in the simulation described above, a comparison was made between the presence and absence of a radio wave absorber (isolation structure, embedded in a dielectric substrate) 5, in terms of the amount of radio waves that leak from the first antenna element and are received by the second antenna element, expressed as isolation [dB]. The isolation without the wave absorber was -30dB, while the isolation with the wave absorber was -36dB, demonstrating a significant improvement in isolation. [Industrial Applicability]

[0065] In the present disclosure, the effect of improving isolation by the radio wave absorber (isolation structure) is large, and therefore it is useful for improving the performance of an antenna device, for example. [Explanation of symbols]

[0066] 1. Dielectric substrate 2 First antenna element 3 Second antenna element 4. Grounding conductor 5. Radio wave absorber (isolation structure)

Claims

1. a dielectric substrate and first and second antenna elements; The antenna device further comprises a radio wave absorber between the first and second antenna elements.

2. 2. The antenna device according to claim 1, wherein the radio wave absorber includes a magnetic powder and a binder, and selectively absorbs a specific frequency.

3. 2. The antenna device according to claim 1, wherein the height of the radio wave absorber is 30 mm or less.

4. 2. The antenna device according to claim 1, wherein the wave absorber has a lattice shape and is disposed in contact with a dielectric substrate.

5. The antenna device according to claim 1 , wherein the magnetic powder includes magnetoplumbite hexagonal ferrite powder.

6. 2. The antenna device according to claim 1, wherein the dielectric substrate has a relative dielectric constant of 1 to 40.

7. A radio wave absorber used in the antenna device according to any one of claims 1 to 6.

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