(19)
(11) EP 0 828 313 A2

(12) EUROPEAN PATENT APPLICATION

(43) Date of publication:
11.03.1998 Bulletin 1998/11

(21) Application number: 96308050.2

(22) Date of filing: 06.11.1996
(51) International Patent Classification (IPC)6H01Q 17/00
(84) Designated Contracting States:
DE GB SE

(30) Priority: 05.09.1996 JP 283395/96

(71) Applicant: Ten Kabushiki Kaisha
Nagoya-shi, Aichi-ken (JP)

(72) Inventors:
  • Noda, Kenichi
    Nagoya-shi, Aichi-ken (JP)
  • Sakurai, Takashi
    Seto-shi, Aichi-ken (JP)

(74) Representative: Greenwood, John David et al
Graham Watt & Co. Riverhead
Sevenoaks Kent TN13 2BN
Sevenoaks Kent TN13 2BN (GB)

   


(54) Electromagnetic-wave absorber


(57) An electromagnetic-wave absorber (1) which can efficiently absorb the electromagnetic wave over broad band, with a reduced thickness and weight, which is superior in workability for attachment comprising a first ferrite sheet (3) of the predetermined thickness attached with a metal plate (15) for reflecting electromagnetic-wave on the back thereof, a second ferrite sheet (5) which has the thickness thinner than that of said first ferrite sheet (3) and is located in the front of said first ferrite sheet (3) and a dielectric means (7) which has the predetermined thickness and is located between said first and second ferrite sheets (3,5).




Description


[0001] The present invention relates to an electromagnetic-wave absorber.

[0002] In prior art electromagnetic-wave absorbers a ferrite sheet of the required thickness attached to a metal plate for reflecting electromagnetic wave on the back thereof has been used. However, since the electromagnetic wave was absorbed in different frequency bands according to the composition, thickness and the like of the ferrite sheet, it was necessary to constitute the electromagnetic-wave absorber by lapping a plurality of ferrite sheets having composition, thickness and the like corresponding to the frequency of the electromagnetic wave to be absorbed in order to absorb the electromagnetic wave from several 10 MHz to several GHz over broad band with a high damping factor.

[0003] Consequently, such prior art electromagnetic-wave absorbers which could absorb the electromagnetic wave over a broad band had an increased thickness and weight. Moreover, the electromagnetic-wave absorber required much labour and a large space was needed to attach them to radio darkrooms and buildings so that it was difficult to save the space.

[0004] The present invention is directed to solve the above-described drawbacks of the prior art absorbers and to provide an electromagnetic-wave absorber which can absorb a given electromagnetic wave with broader band in the higher damping factor with an absorber having improved reduced thickness and weight.

[0005] Embodiments of the present invention provide an electromagnetic-wave absorber which is superior in workability for attaching it and which can shorten the period of construction.

[0006] Accordingly, the electromagnetic-wave absorber according to the present invention is composed of a first ferrite sheet of the predetermined thickness attached to a metal plate for reflecting electromagnetic-wave on the back thereof, a second ferrite sheet which has the thickness thinner than that of said first ferrite sheet and is located in the front of said first ferrite sheet and a dielectric means which has the predetermined thickness and is located between said first and second ferrite sheets.

[0007] Embodiments can provide that an electromagnetic wave in the comparatively low frequency of several 10 MHz to 1 GHz is damped by the action of magnetic permeation during its passing through the first and second ferrite sheets and that an electromagnetic wave in the comparatively high frequency over 1 GHz is damped by the dielectric loss of the dielectric means as it resonates with multiple reflection between the first and second ferrite sheets.

[0008] It may, for example, be attached to the wall surface or the like of radio darkrooms and the outside wall surface or the like of buildings (tall building) to absorb electromagnetic wave. The present invention provides an electromagnetic-wave absorber which can absorb the electromagnetic wave with broader band in the higher damping factor, while improved to reduce the thickness and the weight.

[0009] Preferably, the electromagnetic-wave absorber comprises a first ferrite sheet of 3 to 5 mm thickness attached with a metal plate for reflecting electromagnetic-wave on the back thereof, a second ferrite sheet of 1 to 2 mm thickness located in the front of said first ferrite sheet and a dielectric means of 10 to 30 mm thickness located between said first and second ferrite sheets.

[0010] The dielectric means preferably comprises an air layer, a foamed resin board with low dielectric constant, or a collective body of fibres with low dielectric constant.

[0011] Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings of which:

Fig. 1 is a general perspective view of the electromagnetic-wave absorber according to the present invention;

Fig. 2 is a vertical sectional view taken on line II-II in Fig. 1;

Fig. 3 is a vertical sectional view illustrating another embodiment of the electromagnetic-wave absorber according to the present invention;

Fig. 4 is a vertical sectional view illustrating further embodiment of the electromagnetic-wave absorber according to the present invention.

Fig. 5 is a functional diagram illustrating the absorbing condition of the electromagnetic-wave absorber;

Fig. 6 is a vertical sectional view illustrating further embodiment of the electromagnetic-wave absorber according to the present invention;

Fig. 7 is a graph illustrating the characteristic of absorbing the electromagnetic wave; and

Fig. 8 is a graph illustrating another characteristic of absorbing the electromagnetic wave.



[0012] In Figs. 1 to 4, a first ferrite sheet 3 and a second ferrite sheet 5 of an electromagnetic-wave absorber 1 are made of nickel - zinc or the like and formed in rectangular sheets of substantially equal size. The first ferrite sheet 3 is of about 3 to 5 mm thickness and the second ferrite sheet 5 is of 1 to 2 mm thickness. A dielectric means 7 is placed between the first and second ferrite sheets 3, 5 which are set at an interval about 10 to 30 mm wide. The first and second ferrite sheets 3, 5 may be cut out from a ferrite board as flat sheets with the respective thicknesses as described above or may be individually formed by burning to have the above-described thicknesses. The characteristic of absorbing the electromagnetic wave may be made superior in comparatively low frequency by making the thickness of the first ferrite sheet 3 larger.

[0013] Between the first ferrite sheet 3 and second ferrite sheet 5 is provided the dielectric means 7 through which the mutual interval is kept in about 10 to 30 mm between the first and second ferrite sheets 3, 5. This dielectric means 7 is provided to form a dielectric layer and may be made of any material if it has low dielectric constant. Some suitable materials are, for example, an air layer 9 as shown in Fig. 3, a laminate (wooden material), a resin dielectric body 11 such as hard polyurethane foam or foamed styrol as shown in Fig. 4 or a collective body of fibres 13 as shown in Fig. 2.

[0014] When a air layer 9 is the dielectric means 7, as shown in Fig. 3, spacers 11a with the length equal to the above-described interval may be arranged at suitable locations between the first and second ferrite sheets 3, 5 to keep the gap of the air layer 9a. Suitable fibres to form the collective body of fibres 13 include natural fibres, organic macromelecular fibres an inorganic fibres (asbestos). These different kinds of fibres may be twined around and stuck to each other by melting or with adhesive to form a mat with thickness substantially equal to the above-described interval.

[0015] The above described first ferrite sheet 3 is attached on the back thereof with a reflector metal plate 15 of which the size is equal to that of the first ferrite sheet 3. The reflector metal plate 15 may be any metal plate having the characteristic of reflecting the electromagnetic wave such as iron, copper, brass, nickel.

[0016] In consideration of the workability for attaching the electromagnetic-wave absorber to radio darkrooms and buildings, the first ferrite sheet 3, second ferrite sheet 5 and reflector metal plate 15 are provided at the respective corners with cutaway portions 3a, 5a, 15a of a quadrant shape respectively. The respective cutaway portions 3a, 5a, 15a may be joined together to form holes for inserting the fixing screws to attach a large number of electromagnetic-wave absorbers to the wall surface when they are arranged adjacent to each other.

[0017] The above described electromagnetic-wave absorber 1 absorbs the electromagnetic wave in the operation to be described hereinafter.

[0018] In the Fig. 5 embodiment, when the electromagnetic wave with comparative low frequency (several 10 MHz to 1 GHz) is let in the electromagnetic-wave absorber 1, a part of the electromagnetic wave as shown in the solid line in Fig. 5 is absorbed due to the permeability during its penetrating through the second ferrite sheet 5. The rest of the electromagnetic wave which penetrates through the second ferrite sheet 5 is absorbed in the same way as described above during its penetrating through the first ferrite sheet 3 and thereafter reflected by the reflector metal plate 15 to be absorbed and damped during its penetrating again through the first and second ferrite sheets 3, 5.

[0019] On the other hand, when the electromagnetic wave with comparatively high frequency (over 1 GHz) is let in the electromagnetic-wave absorber 1, the electromagnetic wave as shown in the dotted line in Fig. 5 penetrates through the second ferrite sheet 5 and first ferrite sheet 3. Thereafter the electromagnetic wave resonates with multiple reflection between the first and second ferrite sheets 3, 5 due to the reflector metal plate 15 to be damped due to the dielectric loss by the dielectric means 7 between both the ferrite sheets.

[0020] Consequently, the electromagnetic-wave absorber 1 can absorb the electromagnetic wave in the broad band of several 10 GHz to several GHz in high damping factor owing to the constitution in which a small thickness dielectric means 7 is provided between the first and second ferrite sheets 3, 5.

[0021] Also, since very thin sheet can be used for the first and second ferrite sheets 3, 5 of the electromagnetic-wave absorber 1, it is possible to reduce the thickness and the weight of the electromagnetic-wave absorber 1 itself. Moreover, it is possible to efficiently perform the work for attaching those absorbers 1 to radio darkrooms and buildings so as to shorten the period of the construction.

[0022] Referring now to Fig. 6, an electromagnetic-wave absorber 1 according to the present invention is further provided with a loss dielectric body 71 through a second dielectric means 73 in the front thereof (in the front of the second ferrite sheet 5) so as to form an electromagnetic-wave absorber 75. Although the loss dielectric body 71 may be made of a ferrite sheet, it is possible to use a collective body of fibres coated with dielectric paint with the predetermined permeability or formed resin body in order to prevent the weight from increasing. The second dielectric means 73 may be the same to the above described dielectric means 7.

[0023] Although it is desirable that the characteristic of absorbing the electromagnetic wave in the electromagnetic-wave absorber 1 according to the present invention is - 20 dB or more in damping factor, it is allowable that the damping factor may be - 14 dB or more when the absorber 1 is used for radio darkrooms.

Example of a prior art absorber.



[0024] 

Thickness 6.5 mm, Flat type ferrite sheet



[0025] In the band 0.05 GHz to 0.427 GHz the damping factor was - 20 dB or more, but in the band 0.427 GHz to 2 GHz the damping factor was - 20 dB or less. Also, in the band 0.05 GHz to 0.71 GHz the damping factor was - 14 dB or more.

Example 1 in the Experiment



[0026] 

Thickness of the first ferrite sheet : 4.5 mm

Thickness of the second ferrite sheet : 1.5 mm

Thickness of the dielectric means : Air layer 22 mm



[0027] As shown in Fig. 7, in the band about 0.08 GHz to 2.011 GHz, the damping factor was - 20 dB or more. Also, in the band 0.05 GHz to 2.2 GHz, the damping factor was - 14 dB or more.

Example 2 in the Experiment



[0028] 

Thickness of the first ferrite sheet : 4.5 mm

Thickness of the second ferrite sheet : 1.5 mm

Thickness of the dielectric means : Air layer 15 mm



[0029] As shown in Fig. 8, in the bands about 0.08 GHz to 0.62 GHz and 2.07 GHz to 2.67 GHz, the damping factor was - 20 dB or more. Also, in the band 0.05 GHz to 2.8 GHz, the damping factor was - 14 dB or more.


Claims

1. An electromagnetic-wave absorber comprising a first ferrite sheet (3) of the predetermined thickness attached to a metal plate for reflecting electromagnetic-wave on the back thereof, a second ferrite sheet (5) which has the thickness thinner than that of said first ferrite sheet (3) and is located in the front of said first ferrite sheet (3) and a dielectric means (7) which has the predetermined thickness and is located between said first and second ferrite sheets (3,5).
 
2. An electromagnetic-wave absorber as claimed in claim 1 in which the first ferrite sheet (3) is 3 to 5 mm thick, the second ferrite sheet (5) is 1 to 2 mm thick and the dielectric means (7) is 10 to 30 mm thick.
 
3. An electromagnetic-wave absorber as claimed in claim 1 or 2, wherein said dielectric means (7) comprises an air layer.
 
4. An electromagnetic-wave absorber as claimed in claim 1 or 2, wherein said dielectric means (7) comprises a foamed resin board with low dielectric constant.
 
5. An electromagnetic-wave absorber as claimed in claim 1 or 2, wherein said dielectric means (7) comprises a collective body of fibres with low dielectric constant.
 
6. An electromagnetic-wave absorber as claimed in any preceding claim and further including a loss dielectric body (71) located in front of the second ferrite sheet (5).
 
7. An electromagnetic-wave absorber as claimed in claim 6 in which the loss dielectric body (71) comprises a ferrite sheet or a collective body of fibres coated with a dielectric paint or a formed resin body.
 




Drawing