Abstract:
An electromagnetic wave absorber and an electromagnetic wave anechoic room using the absorbers. The electromagnetic wave absorber has improved electromagnetic wave absorption characteristics at high frequencies in spite of having a hollow structure. The electromagnetic wave absorber includes a hollow shell with a bottom that is a rectangle. A part of a surface of the hollow shell and an outer face of a planar extension lie in planes that are not parallel to any side of the rectangle. At least the plane that is not parallel to any side of the rectangle is included in a surface of an electromagnetic wave absorption member.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to an electromagnetic wave absorber having a hollow structure, and to an electromagnetic wave anechoic room using the absorber. 
         [0003]    2. Description of the Prior Art 
         [0004]    As an electromagnetic wave absorber used in an electromagnetic wave anechoic room, a solid absorber of a pyramid (quadrangular pyramid) or wedge shape conventionally has been often employed. Also for weight and cost reduction, a hollow absorber of a pyramid or wedge shape conventionally has been often employed. 
         [0005]    Generally, a hollow electromagnetic wave absorber is composed of plate-like electromagnetic wave absorption members containing a resistance material as carbon etc. or plate-like electromagnetic wave absorption members including a resistance layer on a surface thereof. A foamed styrol board containing carbon inside, a corrugated board structure consisting of mixed papers or a plastic board containing carbon inside, and a foamed styrol board provided with a resistance layer on a surface thereof etc. are examples of the plate-like electromagnetic wave absorption member. 
         [0006]    When an electric field of an incident electromagnetic wave and the plate-like electromagnetic wave absorption member are parallel, both absorption and surface-reflection are large in electromagnetic wave absorption characteristics of the plate-like electromagnetic wave absorption member, and when the electric field of the incident electromagnetic wave and the plate-like electromagnetic wave absorption member are vertical, both absorption and surface-reflection are small. 
         [0007]      FIG. 10A ,  FIG. 10B  and  FIG. 10C  show a prior art of an electromagnetic wave absorber having a hollow pyramid shape and a combination of plate-like electromagnetic wave absorption members.  FIG. 10A  is a plan view as seen from an incident direction of an electromagnetic wave,  FIG. 10B  is a side view, and  FIG. 10C  is a XC-XC line cross sectional view of  FIG. 10B . In the cross sectional view shown  FIG. 10C  of the prior art, the electromagnetic wave absorption member  1  parallel to the electric field of the incident electromagnetic wave has large absorption and large surface-reflection, and the electromagnetic wave absorption member  2  vertical to the electric field of the incident electromagnetic wave has small absorption and small surface-reflection. Namely, when the incident electromagnetic wave is high frequency of short wave length, in an area A of  FIG. 10A  as seen from the incident direction of electromagnetic wave, the absorption is large because the electric field of the incident electromagnetic wave and the electromagnetic wave absorption member are parallel. On the other hand, in an area B of  FIG. 10A  the absorption is small because the electric field of the incident electromagnetic wave and the electromagnetic wave absorption member are vertical, and large reflection is caused by the area B. When a plurality of electromagnetic wave absorbers  3  of  FIG. 10A ,  FIG. 10B  and  FIG. 10C  are arranged as shown in  FIG. 11A  and  FIG. 11B , the surface-reflection is large in the area A, and also multipath reflection is increased. 
         [0008]    As a result of the above, it is apparent that the conventional hollow electromagnetic wave absorber has a problem that the electromagnetic wave absorption characteristics are low in high frequency. 
         [0009]    Conventional electromagnetic wave absorbers for constituting electromagnetic wave anechoic rooms are disclosed in Japanese Utility Model Application Laid-Open No. 1-171096, Japanese Patent No. 4988060, and Japanese Patent No. 4420253. 
         [0010]    In case of disposing a plurality of electromagnetic wave absorbers on a side wall surface (or side door), Japanese Utility Model Application Laid-Open No. 1-171096 as shown in  FIG. 12A  and  FIG. 12B  discloses a configuration in which electromagnetic wave absorbers  5  consisting of parallel arrangements of chevron-shaped electromagnetic wave absorption members are disposed on the side wall surface  7  so as to be inclined about 45° to a line of intersection between a floor surface  6  and the side wall surface  7  (or side door). 
         [0011]    In this case, whether the electric field direction of the incident electromagnetic wave is horizontal or vertical (horizontally-polarized wave or vertically-polarized wave), it seems to be expected effects of improving electromagnetic wave absorption characteristics in a high frequency range, because the electric field direction is not vertical to the outer surfaces of the electromagnetic wave absorbers  5 . However, as shown in  FIG. 12A  and  FIG. 12B , many jagged parts disable to arrange the electromagnetic wave absorbers are existing around end portions of wall surfaces and near areas of an opening as a door or the like. Because reflections of the jagged parts degrade the absorption characteristics of an electromagnetic wave anechoic room, there is a problem that must fill the jagged parts by disposing electromagnetic wave absorbers specially processed. 
         [0012]    Japanese Patent No. 4988060 discloses an electromagnetic wave absorber unit which integrally has a plurality of pyramid shape electromagnetic wave absorbers inclined at 45°. In this case, the similar problem exists because the electromagnetic wave absorber units form a jagged outline shape when the units are arranged side by side. For instance, parts disable to arrange the electromagnetic wave absorbers are existing around end portions of wall surfaces and near areas of an opening as a door or the like of an electromagnetic wave anechoic room, therefore there is a problem that must fill the parts by disposing electromagnetic wave absorbers specially processed. 
         [0013]    As shown in  FIG. 13A  and  FIG. 13B , Japanese Patent No. 4420253 discloses a electromagnetic wave absorber  10  which has side face forming parts  12  forming side faces of the hollow pyramid  11  respectively, and extending parts  13  each of which extended to protrude from one ridgeline of each part  12  in the same plane from the part  12 , wherein the extending part  13  is present only one for one ridgeline. A magnetic loss body  15  of plat-like ferrites (ferrite tiles) etc. is arranged at a bottom side of the electromagnetic wave absorber  10 . Japanese Patent No. 4420253 alleges that favorable electromagnetic wave absorption characteristics are obtained in a wide range of from low frequency to high frequency. However, because the side face forming part  12  and the extending part  13  which extends in the same plane of the part  12  are parallel or vertical to the electric field direction of the incident electromagnetic wave, decrease of electromagnetic wave absorption characteristics caused by areas vertical to the electric field direction of the incident electromagnetic wave are not ignorable. 
       SUMMARY OF THE INVENTION 
       [0014]    Under such circumstance, an object of the present invention is to provide an electromagnetic wave absorber able to improve electromagnetic wave absorption characteristics in high frequency in spite of having a hollow structure, and to provide electromagnetic wave anechoic room using the absorbers. 
         [0015]    A first aspect of the present invention is an electromagnetic wave absorber. The electromagnetic wave absorber comprises a hollow shell of which an outer shape at a bottom is a rectangle, wherein a part of a surface of the hollow shell is a non-parallel plane to each side of the rectangle, and at least the non-parallel plane includes an electromagnetic wave absorption member. 
         [0016]    In the first aspect, the electromagnetic wave absorption member may be a plate-like electromagnetic wave absorption member which includes a tip side face bent toward a distal end side, and a line of intersection between the tip side face and a cross sectional plane parallel to the bottom is non-parallel to each side of the rectangle. 
         [0017]    A second aspect of the present invention is an electromagnetic wave absorber. The electromagnetic wave absorber comprises a hollow shell including a plate-like projecting portion, and an outer shape of the hollow shell at a bottom being a rectangle, wherein the projecting portion includes a non-parallel plane to each side of the rectangle, and the non-parallel plane and at least a part of the hollow shell include an electromagnetic wave absorption member. 
         [0018]    In the second aspect, the hollow shell may include a quadrangular pyramid and the plate-like projecting portion protruding along a ridgeline of the quadrangular pyramid. 
         [0019]    In the first or second aspect, the electromagnetic wave absorption member may be a plate-like electromagnetic wave absorption member. 
         [0020]    In the first or second aspect, an angle α between one side of the rectangle and a line of intersection of the non-parallel plane and a cross sectional plane parallel to the bottom may be 30°≦α≦60°. 
         [0021]    In the first aspect, when the electromagnetic wave absorption member is a plate-like electromagnetic wave absorption member which includes a tip side face bent toward a distal end side, an angle α between one side of the rectangle and a line of intersection of the tip side face and a cross sectional plane parallel to the bottom may be 30°≦α≦60°. 
         [0022]    In the first or second aspect, the electromagnetic wave absorption member may include a dielectric and a resistance layer provided on a surface of the dielectric. 
         [0023]    In the first or second aspect, the electromagnetic wave absorption member may include a dielectric and a resistance material contained in the dielectric. 
         [0024]    In the first or second aspect, a magnetic loss material may be provided with the bottom side of the hollow shell. 
         [0025]    A third aspect of the present invention is an electromagnetic wave anechoic room. The electromagnetic wave anechoic room comprises the electromagnetic wave absorbers according to the first or second aspect, wherein a plurality of the electromagnetic wave absorbers are arranged on at least one surface of indoor side wall surfaces and a ceiling surface, so that a distal end side of the electromagnetic wave absorbers is directed to the indoor side and one side of the bottom of the absorbers is parallel with a floor surface. 
         [0026]    It is to be noted that any arbitrary combination of the above-described structural components as well as the expressions according to the present invention changed among a system and so forth are all effective and encompassed by the present aspects. 
         [0027]    According to the aspects described above, in case of having a hollow shell of which an outer shape at a bottom is a rectangle, electromagnetic wave absorption characteristics can be improved because of providing a non-parallel plane to each side of the rectangle. Also, an electromagnetic wave anechoic room of excellent electromagnetic wave absorption characteristics can be realized by using the electromagnetic wave absorber. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0028]    Embodiments will now be described, by way of example only, with reference to the accompanying drawings which are meant to be exemplary, not limiting, and wherein like elements are numbered alike in several Figures, the drawings in which: 
           [0029]      FIG. 1A  is a perspective view showing a first embodiment of an electromagnetic wave absorber according to the invention,  FIG. 1B  is a plan view as seen from an incident direction of an electromagnetic wave showing the first embodiment,  FIG. 1C  is a front view of the first embodiment,  FIG. 1D  is an enlarged cross-sectional view showing an example of a plate-like electromagnetic wave absorption member used in the first embodiment, and  FIG. 1E  is an enlarged cross-sectional view showing another example of a plate-like electromagnetic wave absorption member used in the first embodiment. 
           [0030]      FIG. 2A  is a perspective view added cross-sectional position (upper, middle, lower) showing the first embodiment,  FIG. 2B  is a cross-sectional view at the upper cross-sectional position,  FIG. 2C  is a cross-sectional view at the middle cross-sectional position,  FIG. 2D  is a cross-sectional view at the lower cross-sectional position, and  FIG. 2E  is a plan view as seen from the incident direction of the electromagnetic wave showing areas C non-parallel to the electric field of the incident electromagnetic wave are wide. 
           [0031]      FIG. 3A  is a perspective view showing a second embodiment of an electromagnetic wave absorber according to the invention,  FIG. 3B  is a plan view as seen from an incident direction of an electromagnetic wave showing the second embodiment, and  FIG. 3C  is a front view of the second embodiment. 
           [0032]      FIG. 4A  is a perspective view showing a third embodiment of an electromagnetic wave absorber according to the invention,  FIG. 4B  is a plan view as seen from an incident direction of an electromagnetic wave showing the third embodiment, and  FIG. 4C  is a front view of the third embodiment. 
           [0033]      FIG. 5A  is a perspective view showing a forth embodiment of an electromagnetic wave absorber according to the invention,  FIG. 5B  is a plan view as seen from an incident direction of an electromagnetic wave showing the forth embodiment, and  FIG. 5C  is a front view of the forth embodiment. 
           [0034]      FIG. 6A ,  FIG. 6B  and  FIG. 6C  are embodiments wherein dimensions of plate-like extensions of the first embodiment are changed.  FIG. 6A  is a perspective view showing a fifth embodiment,  FIG. 6B  is a perspective view showing a sixth embodiment, and  FIG. 6C  is a perspective view of a seventh embodiment, respectively. 
           [0035]      FIG. 7  is a graph of electromagnetic wave absorption characteristics of the first to third embodiments comparing with that of Japanese Patent No. 4420253 shown in  FIG. 13A  and  FIG. 13B . 
           [0036]      FIG. 8A  is a plan view as seen from an incident direction of an electromagnetic wave showing an eighth embodiment disclosing an electromagnetic wave anechoic room according to the invention, and  FIG. 8B  is a cross-sectional side view of the eighth embodiment. 
           [0037]      FIG. 9A  is a plan view as seen from an incident direction of an electromagnetic wave showing a ninth embodiment disclosing an electromagnetic wave anechoic room according to the invention, and  FIG. 9B  is a cross-sectional side view of the ninth embodiment. 
           [0038]      FIG. 10A ,  FIG. 10B  and  FIG. 10C  are showing a conventional example of an electromagnetic wave absorber having a hollow pyramid shape and a combination of plate-like electromagnetic wave absorption members.  FIG. 10A  is a plan view as seen from an incident direction of an electromagnetic wave,  FIG. 10B  is a side sectional view according to a section through a top, and  FIG. 10C  is a XC-XC line cross sectional view of  FIG. 10B . 
           [0039]      FIG. 11A  and  FIG. 11B  are showing continuous placements of the electromagnetic wave absorbers of the hollow pyramid shape shown in  FIG. 10A ,  FIG. 10B  and  FIG. 10C .  FIG. 11A  is a plan view as seen from an incident direction of an electromagnetic wave, and  FIG. 11B  is a front sectional view according to a section through a top. 
           [0040]      FIG. 12A  and  FIG. 12B  are showing an arrangement of electromagnetic wave absorbers disclosed in Japanese Utility Model Application Laid-Open No. 1-171096.  FIG. 12A  is a perspective view, and  FIG. 12B  is a front view showing continuous placements of the electromagnetic wave absorbers. 
           [0041]      FIG. 13A  and  FIG. 13B  are showing an electromagnetic wave absorber (comparative example) disclosed in Japanese Patent No. 4420253.  FIG. 13A  is a plan view as seen from an incident direction of an electromagnetic wave, and  FIG. 13B  is a side view. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0042]    The invention will now be described based on the following embodiments which do not intend to limit the scope of the present invention but exemplify the invention. All of the features and the combinations thereof described in the embodiments are not necessarily essential to the invention. 
         [0043]    The first embodiment of an electromagnetic wave absorber  20  of the invention is explained according to  FIGS. 1A ,  1 B,  1 C,  1 D, and  1 E and  FIGS. 2A ,  2 B,  2 C,  2 D and  2 E. As shown in those figures, a base plate  29  consists of low-permittivity dielectrics, for example foamed styrol, that is a square plate of 600 mm (one side)×600 mm (another side)×50 mm (thickness). A hollow shell  30  is obtained by combining four plate-like electromagnetic wave absorption members  31  so that an outer shape at a bottom face of the hollow shell  30  form a square of the same size as the base plate  29 . On the base plate  29  (electromagnetic wave incident side), the hollow shell  30  is placed and integrated with adhesive or the like. A magnetic loss body  40  of plat-like ferrites (ferrite tiles) etc. is placed behind the base plate  29 . The base plate  29  is attached by adhesive or the like to the magnetic loss body  40 . The base plate  29  is a member which is provided for reinforcement of the hollow shell  30  and attachment to the magnetic loss body  40 , and may be omitted. 
         [0044]    Four plate-like electromagnetic wave absorption members  31  constituting of the hollow shell  30  are connected at the bottom thereof to each one side of the base plate  29  of a square bottom face respectively. Each plate-like electromagnetic wave absorption member  31  has an outer face part that is a parallel face  31   a  of an isosceles triangle parallel to one side of the base plate  29 , and the other outer face part that is a tip side outer face bent toward a distal end side while the outer face part of the parallel face  31   a  is remained parallel. The tip side outer face is a non-parallel face  31   b  which is non-parallel to all sides of the base plate  29 , and the non-parallel face  31   b  is at an angle α (30°≦α≦60°, more preferably α=45°) with respect to one side of the base plate  29 . The four electromagnetic wave absorption members  31  are combined each other with adhesion bond, adhesion tape or the like, so as to form a hollow polyhedron (however, the bottom face only is open) and a plate-like extension  32  as a plate-like projecting portion which protrudes from the hollow polyhedron. An outer face of the plate-like extension  32  is included in the non-parallel face  31   b , and is a portion that protrudes from one of the non-parallel face  31   b  of an adjacent electromagnetic wave absorption member  31 . The height of the hollow shell  30  is 900 mm, the height of the parallel faces  31   a  of the isosceles triangle is 524 mm, the height of the plate-like extension  32  is 376 mm. Furthermore, the protruding length of the tip of the plate-like extension  32  is 354 mm. 
         [0045]    For example as shown in  FIG. 1D , each plate-like electromagnetic wave absorption member  31  consists of low-permittivity dielectric body  32  such as foamed styrol (or dielectrics of seemingly low dielectric constant, such as a corrugated board structure) and a resistance layer  33  of carbon or the like on a surface of the dielectric body  32 . Also, shown in  FIG. 1E , the electromagnetic wave absorption member  31  may consist of the low-permittivity dielectric body  32  such as foamed styrol (or dielectrics of seemingly low dielectric constant, such as a corrugated board structure) and resistance material such as carbon or the like contained and dispersed in the dielectric body  32 . In case of  FIG. 1D , it is preferable to use the electromagnetic wave absorption member  31  so that a surface provided with the resistance layer  33  is arranged to be an outer surface of the hollow shell  30 , but it is possible that the surface provided with the resistance layer  33  is arranged to be an inner surface. 
         [0046]      FIG. 2A  is a perspective view of hollow shell  30  added cross-sectional position (upper, middle, lower), and  FIG. 2B  is a cross-sectional view at the upper cross-sectional position parallel to the bottom of the hollow shell  30  (when cut by a parallel plane passing through the plate-like extension  32 ). It can be seen that the hollow pyramidal portion is formed by the non-parallel faces  31   b  inclined by the angle α (30°≦α≦60°, more preferably α=45°) with respect to one side of the bottom, and an outer face of the plate-like extension  32  projecting from the hollow pyramidal portion is formed of an extension of the non-parallel face  31   b .  FIG. 2C  is a cross-sectional view at the middle cross-sectional position (when cut by a parallel plane passing through each top of the parallel face  31   a  of the isosceles triangle), and the hollow pyramidal portion is formed by the non-parallel faces  31   b  inclined by the above mentioned angle α with respect to one side of the bottom.  FIG. 2D  is a cross-sectional view at the lower cross-sectional position (when cut by a parallel plane passing through the parallel faces  31   a  and non-parallel faces  31   b ), and there is formed an octagonal cross section in which the non-parallel face  31   b  inclined by the angle α is located between the parallel faces  31   a  of four sides parallel to the bottom sides of the hollow shell  30  respectively. Therefore, it can be seen that areas C of the hollow shell  30  non-parallel to the electric field of the incident electromagnetic wave, namely inclined by the angle α (30°≦α≦60°, more preferably α=45°) with respect to the electric field, are wide as seen from the incident direction of the electromagnetic wave. 
         [0047]      FIG. 7  is a graph of electromagnetic wave absorption characteristics data of the first to third embodiments comparing with an comparative example shown in  FIG. 13A  and  FIG. 13B . The electromagnetic wave absorber of the comparative example is the same bottom dimension (600 mm×600 mm) and the same height (900 mm, however, not including the thickness of the magnetic loss member  15 ) as that of the electromagnetic wave absorber  20  of the first embodiment, and the same electromagnetic wave absorption member consisting of the low-permittivity dielectric body  32  and the resistance layer  33  thereon as shown in  FIG. 1D  is used in both the first to third embodiments and the comparative example.  FIG. 7  shows the measurement data when the angle α is equal to 45° in all embodiments, both the embodiments and the comparative example are provided with ferrite tiles as the magnetic loss member, a plurality of the electromagnetic wave absorbers are arranged side by side, and an incident electric wave of vertical polarization is entered. 
         [0048]    In case of comparing with the first embodiment of the present invention and the comparative example, reflection attenuation (dB) of the first embodiment is better than that of the comparative example in most of the band of 1 GHz 18 GHz. 
         [0049]    According to the first embodiment following effects are obtained. 
         [0050]    (1) As can be seen from the each section of  FIGS. 2A ,  2 B,  2 C and  2 D, the hollow shell  30  of the electromagnetic wave absorber  20  has the outer face of the plate-like electromagnetic wave absorption member  31 , that is the non-parallel face  31   b , which inclined by at an angle α (30°≦α≦60°, more preferably α=45°) with respect to one side of the bottom (square) of the hollow shell  30 . Therefore, even if placing electromagnetic wave absorbers  20  on walls and a ceiling of an electromagnetic wave anechoic room so that one side of the bottom of the hollow shell  30  is made to be parallel to a boundary line of walls and a ceiling surface, or a floor and the walls, that is a general arrangement of absorbers, the non-parallel face  31   b  is at an angle α (30°≦α≦60°, more preferably α=45°) with respect to the electric field of the incident electromagnetic wave of horizontal or vertical polarization that is usually general. For the reason, almost an intermediate absorption and surface-reflection characteristics are obtained compared with characteristics in case of electromagnetic wave absorption member  31  being parallel to the electric field of the incident electromagnetic wave or being vertical to the electric field of the incident electromagnetic wave. As seen from the incident direction of the electromagnetic wave ( FIG. 2E ), because the electromagnetic wave absorber  20  is widely covered by areas of the electromagnetic wave absorption members  31  having the intermediate absorption and surface-reflection characteristics (areas C), almost no area causes large reflections in the high frequency range, also multipath reflections are reduced when a plurality of the absorbers  20  are arranged side by side, and thus better electromagnetic wave absorption characteristics are realized in the high frequency range. 
         [0051]    In case of the angle α is less than 30° and greater than 60°, it means that there are surfaces nearly vertical to the electric field of the incident electromagnetic wave, thus improvement effect of electromagnetic wave absorption characteristics is reduced. 
         [0052]    (2) In case of placing the electromagnetic wave absorbers  20  on the walls and the ceiling of the electromagnetic wave anechoic room, one side of the bottom of the hollow shell  30  may be arranged parallel to the boundary line of the walls and the ceiling surface, or the floor and the walls as a general arrangement of absorbers. Therefore, it is not necessary to adopt a special arrangement configuration disclosed in Japanese Utility Model Application Laid-Open No. 1-171096 shown in  FIGS. 12A and 12B , and it is not necessary to fill spaces existing around end portions of wall surfaces and near areas of an opening as a door or the like by disposing electromagnetic wave absorbers specially processed. As a result, it is possible to construct an electromagnetic wave anechoic room at a low cost. 
         [0053]    (3) Because the tip side outer face (the non-parallel face  31   b ) of four plate-like electromagnetic wave absorption members  31  is formed by bending toward the distal end side of the member  31  so that the line of intersection between the non-parallel face  31   b  and the cross sectional plane parallel to the square bottom is non-parallel to the each side of the square bottom, it is easy to manufacture the hollow shell  30 , and possible to reduce production cost. 
         [0054]    (4) Because the hollow shell  30  has plate-like extensions  32  protruding from the polyhedron part, it is possible to further improve the electromagnetic wave absorption characteristics. 
         [0055]    The second embodiment of an electromagnetic wave absorber  21  of the invention is explained according to  FIGS. 3A ,  3 B and  3 C. In a hollow shell  50  of the second embodiment, size relationship of a parallel face  51   a  and a non-parallel face  51   b  of an electromagnetic wave absorption member  51  is determined, so that the parallel face  51   a  of a isosceles triangle parallel to one side of the base plate  29  stand vertically from the base plate  29 , although the parallel face  31   a  of the isosceles triangle parallel to one side of the base plate  29  is inclined inside from a vertical plane to the base plate  29  in the hollow shell  30  of above mentioned first embodiment. Namely, the height of the hollow shell  50  is 900 mm, but the height of the parallel faces  51   a  of the isosceles triangle is 450 mm, also the height of the plate-like extension  52  is 450 mm. Other configurations are the same as the first embodiment described above. 
         [0056]    As shown in the electromagnetic wave absorption characteristics data of  FIG. 7 , reflection attenuation (dB) of the second embodiment is better than that of the comparative example in most of the band of 1 GHz 18 GHz, in case of comparing with the second embodiment and the comparative example. 
         [0057]    In the second embodiment, the same effects as the first embodiment described above are obtained. 
         [0058]    The third embodiment of an electromagnetic wave absorber  22  of the invention is explained according to  FIGS. 4A ,  4 B and  4 C. In the embodiment, a hollow shell  60  mounted and fixed on the base plate  29  is made of four plate-like electromagnetic wave absorption member  61  combined with each other so that the hollow shell  60  has a square quadrangular pyramid  65  and plate-like extensions  62  as plate-like projecting portions which protrude from each ridgeline of the quadrangular pyramid  65  to form an inverted triangle shape. The outer face of plate-like extensions  62  is a non-parallel face  61   b  which is at an angle α (30°≦α≦60°, more preferably α=45°) with respect to the base of the quadrangular pyramid  65  coinciding to each side of the square of the base plate  29 . The plate-like extension  62  may be a extending part of the plate-like electromagnetic wave absorption member  61  which is bended from a lateral face (parallel face  61   a ) of the square quadrangular pyramid  65 , or another plate-like electromagnetic wave absorption member of the inverted triangular shape which is integrated along the ridgeline of the square quadrangular pyramid  65  (so as to protrude from the ridgeline) with an adhesion bond or an adhesion tape or the like. The height of the hollow shell  60  and the plate-like extension  62  are 900 mm, and the protruding length of the tip side of the plate-like extension  62  is 354 mm. Other configurations are the same as the first embodiment described above. 
         [0059]    As shown in the electromagnetic wave absorption characteristics data of  FIG. 7 , reflection attenuation (dB) of the third embodiment is better than that of the comparative example in most of the band of 1 GHz 18 GHz, in case of comparing with the third embodiment and the comparative example. 
         [0060]    Also, in the third embodiment, the same effects as the first embodiment described above are obtained. 
         [0061]    The fourth embodiment of an electromagnetic wave absorber  23  of the invention is explained according to  FIGS. 5A ,  5 B and  5 C. In the embodiment, four hollow tetrahedrons  80  of one side opening are made of plate-like electromagnetic wave absorption members  71  bent in two places respectively, and a hollow shell  70  mounted and fixed on the base plate  29  is made of four hollow tetrahedrons  80  connected and integrated each other with an adhesion bond or an adhesion tape or the like so that a triangular face  80   a  opposed to the opening of the hollow tetrahedron  80  forms the side of the hollow square quadrangular pyramid  75 . The bases of the hollow square quadrangular pyramid  75  coincide respectively to the sides of the square of the base plate  29 . A pair of plate-like extensions  72  of the inverted triangle shape bent and stood from the triangular face (parallel face)  80   a  of the hollow tetrahedron  80  are matched each other at one side  72   a  of a distal end and connected with an adhesion bond or an adhesion tape or the like. The plate-like extensions  72  protrude along each ridgeline of the hollow square quadrangular pyramid  75 , and each one side  72   a  of the distal end of the plate-like extensions  72  is at an angle α (30°≦α≦60°, more preferably α=45°) with respect to the base of the quadrangular pyramid  75 . Therefore, there are four pairs of outer faces of the inverted triangle shape of the plate-like extensions  72  in total, and each outer face of the inverted triangle shape forms a non-parallel face  72   b  inclined at the angle α from the base of the quadrangular pyramid  75 . Other configurations are the same as the first embodiment described above. 
         [0062]    Also, in the fourth embodiment, substantially the same effects as the first embodiment described above are obtained because the non-parallel faces  72   b  inclined at the angle α (30°≦α≦60°, more preferably α=45°) from the base of the quadrangular pyramid  75  are provided with four pairs of the outer faces of the inverted triangle shape of plate-like extensions  72 . 
         [0063]      FIG. 6A ,  FIG. 6B  and  FIG. 6C  are embodiments wherein dimensions of the plate-like extensions of the first embodiment are changed, and  FIG. 6A  is a perspective view showing the fifth embodiment,  FIG. 6B  is a perspective view showing the sixth embodiment,  FIG. 6C  is a perspective view of the seventh embodiment, respectively. As shown in  FIG. 6A , an electromagnetic wave absorber  24  according to the fifth embodiment has plate-like extensions  32  as the plate-like projecting portions of which distal end width is made smaller than that of the first embodiment. As shown in  FIG. 6B , an electromagnetic wave absorber  25  according to the sixth embodiment has plate-like extensions  32  which has a inclination of the distal end side so as to form a pointed shape toward a center of the electromagnetic wave absorber  25 . As shown in  FIG. 6C , an electromagnetic wave absorber  26  according to the seventh embodiment has none of the plate-like extensions, namely plate-like extensions are eliminated. In the fifth, sixth and seventh embodiments, the height of the hollow shell  30  is 900 mm, the height of the parallel faces  31   a  of the isosceles triangle is 524 mm, and the height of the plate-like extension  32  is 376 mm. Other configurations are the same as the first embodiment described above. 
         [0064]    Also, in the fifth, sixth and seventh embodiments, substantially the same effects as the first embodiment described above are obtained because the hollow shell  30  has the non-parallel face  31   b.    
         [0065]    The eighth embodiment showing an electromagnetic wave anechoic room of the invention is explained according to  FIGS. 8A and 8B . An electromagnetic wave anechoic room  100  according to the eighth embodiment has a configuration in which many electromagnetic wave absorbers  20  according to the first embodiment are placed and fixed adjacent to each other on an interior side of shield panels  101  (panels provided with a conductive plate on one or both sides) constituting inner wall surfaces of the electromagnetic wave anechoic room  100 . The tip side of the electromagnetic wave absorber  20  (the side on which the hollow shell  30  is disposed) is indoor side. Normally, side wall surfaces and a ceiling surface of an electromagnetic wave anechoic room are made as shown in  FIGS. 8A and 8B . 
         [0066]    By using the electromagnetic wave absorbers  20  according to the first embodiment, it is possible to adopt a typical arrangement in which the one side of the bottom of the hollow shell  30  is arranged parallel to a boundary line of the wall and the ceiling surface, or a boundary line of the floor and the wall when placing the electromagnetic wave absorbers  20  on the wall and ceiling surface. Therefore, it is possible to construct an electromagnetic wave anechoic room of excellent electromagnetic wave absorption characteristics at low cost. 
         [0067]    The ninth embodiment showing an electromagnetic wave anechoic room of the invention is explained according to  FIGS. 9A and 9B . An electromagnetic wave anechoic room  110  according to the ninth embodiment has a configuration in which many electromagnetic wave absorbers  21  according to the second embodiment are placed and fixed adjacent to each other on an interior side of shield panels  101  (panels provided with a conductive plate on one or both sides) constituting inner wall surfaces of the electromagnetic wave anechoic room  110 . The tip side of the electromagnetic wave absorber  21  (the side on which the hollow shell  50  is disposed) is indoor side. Normally, side wall surfaces and a ceiling surface of electromagnetic wave anechoic room are made as shown in  FIGS. 9A and 9B , 
         [0068]    In case of using the electromagnetic wave absorber  21  according to the second embodiment, the parallel face  51   a  of the isosceles triangle which is parallel to one side the base plate  29  of the hollow shell  50  becomes a stand-up vertical plane to the base plate  29 . As a result, when the electromagnetic wave anechoic room  110  according to the ninth embodiment is constructed, the parallel faces  51   a  are not exposed to the electromagnetic wave incident direction. Therefore, the parallel faces  51   a  may not be electromagnetic wave absorption members. However, it is better in electromagnetic wave absorption characteristics if the parallel faces  51   a  are composed of electromagnetic wave absorption members. The other effects are the same as the eighth embodiment. 
         [0069]    Described above is an explanation based on the embodiment. The description of the embodiments is illustrative in nature and various variations in constituting elements and processes involved are possible. Those skilled in the art would readily appreciate that such variations are also within the scope of the present invention. 
         [0070]    The plate-like electromagnetic wave absorption member used in the above mentioned embodiments is not limited to a flat plate. It is possible to use those which had been previously molded so as to have a plurality of surfaces of the pyramid or polyhedron. 
         [0071]    In the above mentioned embodiments, the outer shape at the bottom of the hollow shell is a square as a example of a rectangle and made to be coincided to the square base plate. It may be a configuration in which the bottom shape of the hollow shell is a rectangle and made to be coincided to a rectangular base plate.