Background Art A broad-band ferrite electromagnetic wave absorber is widely used to construct anechoic chambers for the electromagnetic interference (EMI) tests of electronic devices, the characteristic tests of antennas, etc. , and are used as wall materials for preventing obstruction to televisions and radars caused by electromagnetic waves reflected from architectural structures or other structures, such as buildings or bridges.

As conventional electromagnetic wave absorbers made of sintered ferrite magnetic material, having a thickness of 5-8 mm, have excellent characteristics that can absorb electromagnetic waves with, for example, a low frequency of about 30 MHz, they are widely used to construct anechoic chambers for measuring electromagnetic waves radiated from electronic devices, and are used as wall material for preventing electromagnetic waves from being reflected by buildings.

In the meanwhile, for an example of a technique for a broad-band design of an electromagnetic wave absorber made from a layer of ferrite material, there has been proposed a technique of floating and arranging tile-typed ferrite magnetic material

over a layer of air (actually, it is vertically spaced apart from a reflection plate by use of a foamed polyurethane sheet). As an example of this technique, when ferrite tiles of NiZn series having a height (a thickness in a direction perpendicular to a surface on which ferrite tiles are disposed) of 7 mm are arranged over the reflection plate, with a layer of air of 10 mm interposed between them and the reflection plate, that is, with an entire height of 17 mm from the reflection plate, an electromagnetic wave absorber having a return loss more than 20 dB for electromagnetic waves of 300-800 MHz is obtained.

Typically, assuming that a reflection coefficient of electromagnetic waves on a surface of the electromagnetic wave absorber is S, a power absorption coefficient ap of the electromagnetic wave absorber can be expressed as the following Equation (1).

(Xp=l_IS12 (1) Accordingly, an electromagnetic wave absorber having a low value of ISI can be said to have excellent characteristics. Typically, the following Equation (2) is used as a measure of characteristic evaluation of the electromagnetic wave absorber. lSl<0. 1 (2) In other words, a return loss (-20 log S) more than 20 dB and an absorption ability more than 99% are employed.

The most basic ferrite electromagnetic wave absorber has a structure in which tile-typed ferrite magnetic material F is attached to a reflection plate M, as shown in Fig. 15. Absorption characteristics of an electromagnetic wave absorber having such a structure are shown in Fig. 16.

In Fig. 16, a horizontal axis represents a frequency f and a vertical axis represents a reflection coefficient ISl. As can be seen from this drawing, assuming that lower and upper limit frequencies are respectively fL and fH, both of which yield a

value of ISI of 0.1, a frequency bandwidth B satisfying the value of lSl of 0.1 is expressed as the following Equation (3).

B=fH-fL (3) Many studies have been made of such a frequency bandwidth B. As a result, for example, the following facts have been discovered.

(A) All of ferrites, which are used when the lower limit frequency fL is 30 MHz, are ferrites of sintered NiZn or MnZn series. In this case, the upper limit frequency fH generally has a range of 300-400 MHz.

(B) All of ferrites, which are used when the lower limit frequency fL is 90 MHz, are ferrites of sintered NiZn or MnZn series. In this case, the upper limit frequency fH generally has a range of 350-520 MHz.

As one application, when the electromagnetic wave absorber is applied to the wall material of anechoic chambers for measuring electromagnetic waves radiated from the electronics devices, fL = 30 MHz, and fH in both of the above cases (A) and (B) is lower than fH of 20 GHz proposed in the 1998 as a regulation limit by Comite Internationale Special des Perturbations Radioelectrique II. In addition, in the case of wall material for preventing the reflection of television waves from buildings, it is required in Japan that fL and fH are respectively 90 MHz and 800 MHz, so the electromagnetic wave absorber in the above case (B) also has insufficient characteristics to meet this demand.

Accordingly, many attempts have been made to improve the electromagnetic wave absorber. For one recent example of a broad-band design of the electromagnetic wave absorber made of only sintered ferrite, there is disclosed an electromagnetic wave absorber in which ferrite is arranged in a lattice form on the reflection plate, in US Patent No. 5,276, 448 by some of the present inventors. In the

electromagnetic wave absorber having the lattice form, fL of 30 MHz and fH of 800 MHz are obtained.

For another example of the broad-band design of the electromagnetic wave absorber made of only sintered ferrite, there is disclosed an electromagnetic wave absorber in which tile-typed ferrite magnetic material is arranged on the reflection plate, ferrite magnetic material of the same thickness is superimposed in a lattice form on the tile-typed ferrite magnetic material with the lattices repeatedly arranged at constant intervals, and slots are formed in the direction of the height of the ferrite magnetic material on the superimposed ferrite magnetic material, in Korean Patent No. 144,802 by present inventors. In this electromagnetic wave absorber having the lattice form, fL of 30 MHz and fH of 1000 MHz are obtained.

However, the electromagnetic wave absorption characteristics of the two above-mentioned electromagnetic wave absorbers are satisfactory for the wall material of buildings, but they are unsatisfactory for the anechoic chambers and significantly lower than the above-described regulation limit. In addition, in the case where a thickness of the ferrite in a portion of the structure must be made relatively thin, and the width and thickness of the slots must be made small, there occur unavoidable problems in which the deterioration of flow of the material, the deterioration of the removal of a molded product from a mold and the non-uniformity of molding pressure, etc. occur when ferrite material is injected into a mold to form an entire structure integrally in an actual manufacturing process, the molded ferrite material is susceptible to deformation or brittleness when sintered, and manufacturing cost is increased due to difficulty in setting desirable manufacturing conditions.

In order to overcome the above problems, there is proposed an electromagnetic wave absorber in which tile-typed ferrite magnetic material is

arranged on a reflection plate and cutting cone-shaped ferrite magnetic material is disposed lengthwise and crosswise on the tile-typed ferrite magnetic material at constant intervals, in Korean Patent application Laid-Open No. 2001-103,241. In this electromagnetic wave absorber, fL of 30 MHz and fH of 50 GHz are obtained.

However, although such an electromagnetic wave absorber satisfies the electromagnetic wave absorption characteristics required for anechoic chambers, there remain problems that sharp projections formed due to non-uniformity of molding pressure of the molded product, the non-uniformity of stroke of lower and upper molds, etc. are broken or deformed when an entire structure is integrally formed during an actual manufacturing process, which results in an increase in the defect rate of products. Particularly, in recent years, as electromagnetic wave environments (EMI/EMC) gain popularity, broader-band electromagnetic wave absorbers become desired and frequencies to be used for the electromagnetic wave environments in the future will be higher, so the upper limit frequency fH should accordingly become higher.

Disclosure of the Invention Accordingly, the present invention has been made keeping in mind the above problems occurring in the prior art, and an object of the present invention is to provide a broad-band ferrite electromagnetic wave absorber which satisfies a need for broader- band electromagnetic wave absorbers and can be more economically manufactured through the easy setting of manufacture conditions.

In order to accomplish the above object, the present invention provides a broad-band ferrite electromagnetic wave absorber, wherein a first layer of tile-typed

ferrite magnetic material is disposed upon a reflection plate, a second layer of cylindrical ferrite magnetic material is disposed upon the first layer of tile-typed ferrite magnetic material lengthwise and crosswise at constant intervals"a", a third layer of cutting cone-shaped ferrite magnetic material is disposed upon the second layer of cylindrical ferrite magnetic material, a fourth layer of cylindrical ferrite magnetic material is disposed upon the third layer of cutting cone-shaped ferrite magnetic material, each of the constant intervals"a"of the second layer of cylindrical ferrite magnetic material is shorter than a wavelength of the electromagnetic wave used, and the following relationships are satisfied: r2 < ri <"a", 5mm < r2 < 15mm, 4mm zahl < 8mm, lmm < h2 < 5mm, 10mm < h3 < 20mm, 0. 1mm < h4 < 2. 5mm, where ri is a diameter of the second layer of cylindrical ferrite magnetic material and the bottom of the third layer of cutting cone-shaped ferrite magnetic material, r2 is a diameter of the top of the third layer of cutting cone-shaped ferrite magnetic material and the fourth layer of cylindrical ferrite magnetic material, hl is a height of the first layer of tile-typed ferrite magnetic material, h2 is a height of the second layer of cylindrical ferrite magnetic material, h3 is a height of the third layer of cutting cone- shaped ferrite magnetic material, and h4 is a height of the fourth layer of cylindrical ferrite magnetic material.

In addition, the present invention provides a broad-band ferrite electromagnetic wave absorber wherein a first layer of tile-typed ferrite magnetic material is disposed upon a reflection plate, a second layer of cylindrical ferrite magnetic material is disposed upon the first layer of tile-typed ferrite magnetic material lengthwise and crosswise at constant intervals"a", a third layer of cutting cone-shaped ferrite magnetic material is disposed upon the second layer of cylindrical ferrite magnetic material, a fourth layer of cylindrical ferrite magnetic material and a

fifth layer of lower truncated-rotation elliptical ferrite magnetic material are disposed upon the third layer of cutting cone-shaped ferrite magnetic material, each of the constant intervals"a"of the second layer of cylindrical ferrite magnetic material is shorter than a wavelength of the electromagnetic wave used, and the following relationships are satisfied: r2 < ri <"a", 5mm < r2 < 15mm, 4mm hl < 8mm, lmm < h2 5mm, 12mm < hs 22mm, 0. 1mm h4 # 2. 5mm, lmm # hs < 5mm, where ri is a diameter of the second layer of cylindrical ferrite magnetic material and the bottom of the third layer of cutting cone-shaped ferrite magnetic material, r2 is a diameter of the top of the third layer of cutting cone-shaped ferrite magnetic material and the fourth layer of cylindrical ferrite magnetic material, r3 is a diameter of a bottom of the fifth layer of lower truncated-rotation elliptical ferrite magnetic material, hl is a height of the first layer of tile-typed ferrite magnetic material, h2 is a height of the second layer of cylindrical ferrite magnetic material, h3 is a height of the third layer of cutting cone-shaped ferrite magnetic material, h4 is a height of the fourth layer of cylindrical ferrite magnetic material, and h5 is a height of the fifth layer of lower truncated-rotation elliptical ferrite magnetic material.

The present invention relates to an electromagnetic wave absorber, in which the tile-typed ferrite magnetic material is disposed upon the reflection plate and protrusion-typed absorption layers having different material constants are continuously and vertically disposed upon the tile-typed ferrite magnetic material.

The material constants can be varied to be set values, thus giving an equivalent permeability and an equivalent permittivity when viewing the protrusions by varying the configuration of the protrusions. In other words, by adjusting the interval, shape, height and width of the protrusions, the equivalent permeability and equivalent dielectric constant of the protrusions can be varied.

In particular, the ferrite magnetic material is desirably shaped and easily manufactured, which results in improved manufacturing efficiency, by forming the protrusions lengthwise and crosswise at constant intervals. In addition, sintered NiZn series, MnZn series, etc. can preferably be used as the ferrite magnetic material.

Brief Description of the Drawings Fig. 1 is a perspective view of a first embodiment of an electromagnetic wave absorber according to the present invention; Fig. 2 is a plan view of the first embodiment of the electromagnetic wave absorber according to the present invention; Fig. 3 is a side view of the first embodiment of the electromagnetic wave absorber according to the present invention; Fig. 4 is a view showing the absorption characteristics of the first embodiment of the electromagnetic wave absorber according to the present invention; Fig. 5 is a perspective view of a second embodiment of an electromagnetic wave absorber according to the present invention; Fig. 6 is a plan view of the second embodiment of the electromagnetic wave absorber according to the present invention; Fig. 7 is a side view of the second embodiment of the electromagnetic wave absorber according to the present invention; Fig. 8 is a view showing an absorption characteristic of the second embodiment of the electromagnetic wave absorber according to the present invention; Fig. 9 is a principle view for obtaining an equivalent permittivity and an equivalent permeability when the electromagnetic wave absorber according to the

present invention is divided into multiple layers and homogenized; Fig. 10 is a view showing a unit structure for obtaining a synthesized capacitance of the electromagnetic wave absorber according to the present invention; Fig. 11 is a view showing a model of synthesized capacitance of Fig. 10; Fig. 12 is a view showing a unit structure for obtaining a synthesized inductance of the electromagnetic wave absorber according to the present invention; Fig. 13 is a view showing a model of synthesized inductance of Fig. 12; Fig. 14 is a view showing a model of multiple structures for obtaining a reflection coefficient of the electromagnetic wave absorber according to the present invention; Fig. 15 is a side view of a basic tile-typed ferrite electromagnetic wave absorber; and Fig. 16 is a view showing the absorption characteristics of the basic tile-typed ferrite electromagnetic wave absorber.

Best Mode for Carrying Out the Invention The absorption characteristics of electromagnetic wave absorbers of embodiments to be described below will be evaluated through measurements using TEM waves through the use of rectangular coaxial lines.

In addition, in Figs. 1 to 3 and 5 to 7 that show structures of electromagnetic absorbers of the embodiments, M designates a reflection plate, F designates a sintered ferrite, and Cu designates a unit structure of the electromagnetic wave absorber. The electromagnetic wave absorber of the present invention is configured such that layers of the unit structure Cu are disposed in parallel on the same plane to have a required area

in contact with each other. In addition, rl is the diameter of a second layer of cylindrical ferrite magnetic material and the bottom of a third layer of cutting cone- shaped ferrite magnetic material, r2 is the diameter of the top of the third layer of cutting cone-shaped ferrite magnetic material and a fourth layer of cylindrical ferrite magnetic material, r3 is the diameter of the bottom of a fifth layer of lower truncated- rotation elliptical ferrite magnetic material, and hl, h2, h3, h4, and hs are the heights of the first to fifth layers, which are common in all drawings.

[Embodiment 1] In a first embodiment of the present invention, an equivalent permeability and an equivalent permittivity can be controlled by disposing a first layer of tile-typed ferrite magnetic material on a reflection plate M, a second layer of cylindrical ferrite magnetic material on the first layer of tile-typed ferrite magnetic material lengthwise and crosswise at constant intervals"a", a third layer of cutting cone-shaped ferrite magnetic material on the second layer of cylindrical ferrite magnetic material, and a fourth layer of cylindrical ferrite magnetic material on the third layer of cutting cone- shaped ferrite magnetic material, as shown in Figs. 1 to 3, and adjusting heights hi, h2, h3, and h4 of the first to fourth layers, the diameter ri of the second layer and the bottom of the third layer, and the diameter r2 of the top of the third layer and the fourth layer.

Although the first, second, third and fourth layers are shaped integrally in the manufacture of the first embodiment, the first to fourth layers are separately shown in the drawings for the purpose of easy distinction.

The electromagnetic wave absorption characteristics of the first embodiment of the present invention are obtained as described below.

As the first layer consists of tile-typed ferrite magnetic material, the equivalent permittivity Eeff is as below.

#eff=#r where Sr of Equation (4) is a relative permittivity of the ferrite magnetic material.

In addition, the equivalent permeability pleff iS as below.

, Ueff = pr (5a) where p. r of Equation (5a) is a relative permeability of the ferrite magnetic material and can be expressed as below.

Ei 1+ l+jf/fm (i) where tli of Equation (5b) is an initial permeability of the ferrite magnetic material, fm is a relaxation frequency of the ferrite magnetic material, f is a frequency, and j is an imaginary number unit.

As a portion F occupied by the ferrite magnetic material and a portion A occupied by air are mixed together in a range from the second layer to the fourth layer in the first embodiment, as shown in Fig. 9 (a) showing a single period of the array of periodic arrangement of Fig. 1, a homogenized equivalent effective permittivity set an a homogenized equivalent effective permeability Heq should be obtained for homogenization, as shown in Fig. 9 (b), in order to obtain the absorption characteristics.

This is referred to as an equivalent material constants method (see"IEEE Transactions on Electromagnetic Compatibility, "Vol. 38, No. 2, pp. 173#177, May 1996), which was proposed by the present inventor et al.

As can be seen from the plan view of Fig. 3, the structures of Figs. 1 and 9 (a) are symmetrical, so 1/4 of one period of the array, i. e. , a unit structure Cu, can be a

representative. Then, using a unit structure diagram as shown by a virtual line in Fig.

3, when each of layers is divided into 1, 2,3,..., i,..., and n layers, as shown in Fig.

9 (a), the equivalent effective permittivity F,, q and the equivalent effective permeability leq are obtained for each of layers, and then replaced with Fig. 9 (b) for homogenization.

By obtaining synthesized capacitance in Fig. 11 and synthesized inductance in Fig. 13 using model diagrams (Figs. 10 and 12) for the calculation of the equivalent material constants, the equivalent permittivity Seffand the equivalent permeability Ueff for the unit structure Cu of the second layer can be obtained by the following Equations.

The ferrite magnetic material is divided into the third#(n-1)-th layers as shown in Fig. 9(a), using the unit structure of the third layer, and the equivalent permittivity and the equivalent permeability are obtained for each of layers as below, using those shown in Figs. 10 to 13, <BR> <BR> <BR> <BR> <BR> a[(a-#t)#r+#t]+[(a-xn)(xn+1-xn)]#r<BR> <BR> <BR> <BR> <BR> <BR> <BR> a(xn+1-xn)#r<BR> (8)<BR> <BR> <BR> <BR> <BR> <BR> <BR> <BR> <BR> <BR> <BR> <BR> <BR> <BR> a[(a-xn)µr+xn]+#t(a-xn)µr µeff a#tµ , thus homogenizing each of layers as shown in Fig. 9(b).

In Equations (8) and (9), Xn is the length of the ferrite magnetic material up to each of steps in Figs. 10 and 12 of i-th layer in an axis direction.

In the case of the fourth layer, h2 and r1 in the Equations (6) and (7) are replaced with h4 and r2, respectively.

Since the equivalent model obtained as shown in Fig. 9 (b) corresponds to a multi-structure model of Fig. 14, a reflection coefficient #S# in this case is as below. <BR> <BR> <BR> <BR> <BR> z-1<BR> <BR> <BR> <BR> +'1 1 (10) In Equation (10), Zn is an input impedance viewed from a surface of n-th layer to the reflection plate. By using Fig. 14, Z1 is obtained by the following Equation (11), Zz-Zn by the following Equation (12).

In Equations (11) and (12) and Fig. 14, dl, d2,..., and dn are the thicknesses of divided layers.

The ferrite magnetic material used in the first embodiment is sintered ferrite magnetic material of NiZn series and has a relative permittivity prof14 and a relative permeability p, r of 2,500.

In Figs. 2 and 3, the height hl of the first layer is 6. 2mm, the height h2 of the second layer is 2mm, the height h3 of the third layer is 16mm, the height h4 of the fourth layer is 0. 5mm, the diameter rl of the second layer and the bottom of the third layer is 18mm, the diameter r2 of the top of the third layer and the fourth layer is 1 Omm, and the lengthwise and widthwise interval"a"of the second layer is 20mm.

The absorption characteristics of the electromagnetic wave absorber having the above-described structure are obtained as shown in Fig. 4 for electromagnetic waves incident normally on a surface of the ferrite magnetic material toward the reflection plate. As can be seen from the drawing, a return loss more than 21 dB is obtained for a range of frequency band from 30 MHz to 100 GHz.

This result shows that the absorption characteristic obtained in the first embodiment of the present invention is further improved over that of the case where the tile-typed ferrite magnetic material (hl = 6. 5mm) is disposed on a conductive plate M, and the cutting cone ferrite magnetic material having the height h2 of 18mm, a disposition period a of 20mm, the bottom diameter rl of 18mm, and the top diameter r2 of 7mm is simply disposed on the tile-typed ferrite magnetic material in Korean Patent Laid-Open No. 2001-103,241, which is proposed by the present inventor et al. In addition, the electromagnetic wave absorber of the first embodiment can be easily manufactured because edge damage can be prevented when the absorber is formed integrally by disposing the cylindrical ferrite magnetic material on the top of the cutting cone ferrite magnetic material.

[Embodiment 2]

In a second embodiment of the present invention, an equivalent permeability and an equivalent permittivity can be controlled by disposing a first layer of tile-typed ferrite magnetic material upon a reflection plate M, a second layer of cylindrical ferrite magnetic material upon the first layer of tile-typed ferrite magnetic material lengthwise and crosswise at constant intervals"a", a third layer of cutting cone-shaped ferrite magnetic material upon the second layer of cylindrical ferrite magnetic material, and a fourth layer of cylindrical ferrite magnetic material and a fifth layer of lower truncated-rotation elliptical ferrite magnetic material upon the third layer of cutting cone-shaped ferrite magnetic material, as shown in Figs. 5 to 7, and adjusting heights hl, h2, h3, h4, and hs of the first to fifth layers, a diameter rl of the second layer and the bottom of the third layer, a diameter r2 of the top of the third layer and the fourth layer, and a diameter r3 of the bottom of the fifth layer of lower truncated- rotation elliptical ferrite magnetic material.

The absorption characteristics of the second embodiment of the present invention can be obtained for the first to fourth layers by Equations (4) to (12), in same manner as in the first embodiment. For the fifth layer of the lower truncated-rotation elliptical ferrite magnetic material, since the basic principle applied to the lower truncated-rotation elliptical ferrite magnetic material is similar to that of the cutting cone ferrite magnetic material of the first embodiment except that a radius of the lower truncated-rotation elliptical ferrite magnetic material is reduced as its height increases, the electromagnetic wave absorption characteristic of the second embodiment can be obtained by changing only an expression of xn in Equations (8) and (9) using an elliptical equation.

Although the first to fifth layers are shaped integrally in the manufacture of the second embodiment, the first to fifth layers are separately shown in the drawings for the purpose of easy distinction.

The ferrite magnetic material used in the second embodiment is sintered ferrite magnetic material of NiZn series and has a relative permittivity Er of 14 and a relative permeability ptr of 2,500.

In Figs. 6 and 7, the height hl of the first layer is 6.2mm, the height h2 of the second layer is 2mm, the height h3 of the third layer is 16mm, the height h4 of the fourth layer is 0. 5mm, the height hs of the fifth layer is 2.3mm, the diameter rl of the second layer and the bottom of the third layer is 18mm, the diameter r2 of the top of the third layer and the fourth layer is 10mm, the diameter r3 of the bottom of the fifth layer is 6.8mm, and the lengthwise and widthwise interval"a"of the second layer is 20mm The absorption characteristics of the electromagnetic wave absorber having the above-described structure are obtained as shown in Fig. 8 for electromagnetic waves incident normally upon a surface of the ferrite magnetic material toward the reflection plate. As can be seen from the drawing, a return loss more than 25 dB is obtained for a range of frequency band from 30 MHz to 100 GHz.

The absorption characteristics obtained in the second embodiment is further improved by more than 4 dB and can be easily manufactured, compared to the first embodiment. In addition, since the electromagnetic wave absorber of the second embodiment has a convex end portion by which a surface reflection is remarkably reduced, a performance can be significantly improved in not only a lower frequency band region but also a higher frequency band region.

As described above, the present invention provides a broad-band ferrite electromagnetic wave absorber, wherein the tile-typed ferrite magnetic material is

disposed upon the reflection plate, the cylindrical ferrite magnetic material is disposed upon the tile-typed ferrite magnetic material lengthwise and crosswise at constant intervals"a", the cutting cone-shaped ferrite magnetic material is disposed upon the cylindrical ferrite magnetic material, the cylindrical ferrite magnetic material and the lower truncated-rotation elliptical ferrite magnetic material are in turn disposed upon the cutting cone-shaped ferrite magnetic material. Such a broad-band ferrite electromagnetic wave absorber has a low defect rate and improved manufacturing efficiency, which results in lower manufacture cost, because of ease of manufacturing conditions when setting the ferrite electromagnetic wave absorber. In addition, since the equivalent permeability and the equivalent permittivity can be easily controlled to be set to a desired value, the ferrite electromagnetic wave absorber can exhibit super broad- band electromagnetic wave absorption characteristics adaptable to wall material of buildings and, particularly, anechoic chambers for preventing the virtual images of televisions or radars, or absorbing jamming.

Industrial Applicability As described above, the present invention provides a broad-band ferrite electromagnetic wave absorber, which is widely used to construct anechoic chambers for the electromagnetic interference (EMI) tests of electronic devices, the characteristic tests of antennas, etc. , and is used as wall material for preventing obstruction to a television and a radar by electromagnetic waves reflected from architectural structures or other structures such as buildings or bridges.