[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.
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.