KR20030023899A - Thin microwave absorbers used in frequency range of mobile telecommunication - Google Patents

Abstract

PURPOSE: A thin wave absorbing member is provided to have an absorption factor over 80 percentages and a thickness of 1.0 to 1.3 mm within a frequency band of 0.8 to 2.2 GHz by using as an absorption filling material a soft magnetic metal powder and using a rubber coupling substance of an excellent forming property. CONSTITUTION: An electronic wave absorption member(10) consists of an electronic wave absorption filling material and a coupling substance. The electronic wave absorption filling material consists of a soft magnetic metal powder(12) of 40 to 65 volume%, and the coupling substance consists of an elastic material(13) of 35 to 60 volume%. The soft magnetic metal powder consists of one selected from a group of pure iron, carbon iron, iron-silicon-aluminum alloy, iron-cobalt alloy and iron-nickel alloy, and the elastic member is rubber or silicon rubber. A thickness of the absorption material is 1.0 to 1.3 mm based on an absorption factor of 80 percentages.

Description

Thin microwave absorbers used in frequency range of mobile telecommunications

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to radio wave absorbers, particularly thin absorbers for mobile communication frequency bands, and more particularly to flat soft magnetic metal powders (pure iron, carbonyl iron, iron-silicon alloys, iron-cobalt alloys, sandusts, permalloys). And a combination of the above) in a rubber or plastic support material for a thin wave absorber having an electromagnetic wave absorption rate of 80% or more and a thickness of 1.0-1.3 mm in a mobile communication frequency band (0.8-2.2 Hz). will be.

In general, the electromagnetic wave absorber is a functional material that attenuates radio wave energy by using high frequency loss characteristics of a constituent material or reduces reflected waves below a reference value and is used as a means for preventing electromagnetic interference of various electronic devices. The absorption mechanism of the electromagnetic wave absorber is fundamentally attributed to the high frequency loss characteristic of the material, and is classified into a conductive loss material, a dielectric loss material, a magnetic loss material, and a material containing two or more losses depending on the material used. Among these materials, the ferrite electromagnetic wave absorber mainly uses magnetic losses due to resonance phenomena occurring in the high frequency band of MHz or more, and is used most industrially due to its relatively thin thickness and various matching frequencies.

Ferrite electromagnetic wave absorber can be divided into sintered type and composite type according to the constituent conditions. Sintered ferrite electromagnetic wave absorbers include Mn-Zn ferrite, Ni-Zn ferrite, Ba ferrite, etc. Among these, Ni-Zn ferrite generally has excellent attenuation in the frequency range of MHz band. It is used for practical purposes and to prevent TV ghosts. The thickness of the sintered electromagnetic wave absorber for practical use in the VHF / UHF band is in the form of a tile of about 7 to 8 mm. The composite ferrite electromagnetic wave absorber is manufactured by mixing ferrite with nonmagnetic materials such as silicone rubber and plastics as a support material, and is used to prevent radar wave reflection in the band.

In addition, the performance of the electromagnetic wave absorber is generally evaluated by matching frequency, matching thickness, return loss and bandwidth. The performance of the electromagnetic wave absorber is determined by the complex permeability (μ _r = μ _r ´- j μ _r ˝) and the complex dielectric constant (ε _r = ε _r ´- j ε _r 인), which are the material constants of the absorber. By controlling the performance of the electromagnetic wave absorber.

Conventional technologies using ferrite or carbon powder as described above include Korean Patent Publication Nos. 2000-20529, 2000-22855, 2000-47782 and 2000-68775, and Korean Patent Nos. 144802 and 144803.

However, in the conventional method, the absorber incorporating ferrite or carbon powder into rubber or plastic is fundamentally impossible to be used in a narrow space such as a mobile communication terminal because the absorber thickness is 3 mm or more in the mobile communication frequency band. Do.

In addition, the most important characteristic required as an electromagnetic wave absorber for electromagnetic wave attenuation into a mobile communication terminal is that the electromagnetic wave absorption rate must be large at a communication frequency (cellular phone: 800 MHz, PCS: 1.8 GHz, IMT-2000: 2.2 GHz). It should be thinner than. In the case of PCS terminals currently on the market, an ultra-thin wave absorber having a level of 1 mm is required because the distance between the internal PCB circuit module and the inner wall of the housing is only 1-2 mm. However, when the absorber is composed of a conventional ferrite magnetic material or carbon powder, since the thickness is several mm or more in the frequency band, it is necessary to use a new material required for thinning and a radio wave absorber of a new method.

Here, in the matched radio wave absorber, the radio wave absorption is maximum when the thickness is 1/4 of the wavelength lambda, and the match thickness is inversely proportional to the square root of the permeability and permittivity of the constituent material. Therefore, in order to design a thin wave absorber, it is necessary to select a material having a high permeability and permittivity at the use frequency. In the case of the spinel ferrite magnetic material, if the Sneek limit frequency (typically less than 1 kHz) is exceeded, the specific permeability decreases rapidly, and only about 20-30 in the ㎓ band is impossible to manufacture a thin absorber having a thickness of 1 mm. Do. In the case of hexagonal ferrite, which is used as an absorber for high frequency band, the permeability in the band is only about 20. Recently, there have been attempts to use a plate-shaped amorphous magnetic powder (permeability = 10, dielectric constant = 300) as an absorbent material for mobile communication terminals, but the thickness of the absorber is only 3 mm.

The present invention is to solve the conventional problems and problems as described above, by using a plate-type iron-based soft magnetic metal powder having a higher permeability and dielectric constant than a ferrite magnetic material as the absorption filler and using a rubber binder having excellent moldability 0.8- It is an object of the present invention to provide a thin wave absorber having an absorption rate of 80% or more and a thickness of 1.0-1.3 mm in a 2.2 GHz frequency band.

1 is a schematic diagram of a radio wave absorber according to the present invention,

2 is an impedance matching root locus diagram of a radio wave absorber shorted by a conductor,

3 is a graph showing the complex dielectric constant and complex permeability of the composite material used in the present invention,

4 is a schematic diagram of a measurement tool for measuring radio wave absorption capability;

5 is a graph showing radio wave absorption characteristics of a first embodiment of the present invention;

6 is a graph showing radio wave absorption characteristics of a second embodiment of the present invention;

7 is a graph showing radio wave absorption characteristics of a third embodiment of the present invention;

8 is a graph showing radio wave absorption characteristics of a fourth embodiment of the present invention.

Explanation of symbols on the main parts of the drawing

10: radio wave absorber 11: metal conductor plate

12: soft magnetic metal powder 13: binder

In order to achieve the above object, the present invention provides a radio wave absorber composed of a soft magnetic metal powder of 40 to 65% by volume and an elastic member of 35 to 60% by volume ratio as a thin wave absorber for a mobile communication frequency band.

If the range of the radio wave absorber is less than 40%, the magnetic permeability and dielectric constant decreases, so that the radio wave absorption characteristics deteriorate.

Wherein the soft magnetic metal powder is used as an absorbent material and is selected from the group consisting of pure iron, carbonyl iron, silicon iron, iron-silicon-aluminum alloy (sandust), iron-cobalt alloy, and iron-nickel alloy (permaloy). It is preferably at least one kind and the elastic member is preferably used rubber or silicone rubber.

In addition, the soft magnetic metal powder is preferably in the form of a plate-shaped green compact having a thickness of 1 to 5 µm and a width of 10 to 100 µm, and the thickness of the absorber is preferably 1.0 to 1.3 mm based on an 80% absorption rate. Do.

In the case of the iron-based soft magnetic metal used in the present invention, since the magnetization strength is two times higher and the electrical resistivity is significantly lower than that of the ferrite, the magnetic permeability and the permittivity can be simultaneously obtained in the mobile communication frequency band compared to the ferrite magnetic material. have. In addition, in order to suppress the decrease in permeability due to eddy current loss, a plate-shaped soft magnetic metal powder was used as an absorber. In order to realize a thin wave absorber, a plate-shaped soft magnetic metal compact is used as an absorbent filler, and a composition selection and a combination method of these absorbent fillers are selected to induce impedance matching conditions.

Hereinafter, with reference to the accompanying drawings will be described in detail the present invention through the preferred embodiment of the present invention.

1 is a schematic diagram of a thin wave absorber of the present invention. The electromagnetic wave absorber 10 of the present invention is composed of a radio wave absorbing layer in which a metal conductor plate 11 is mounted on the rear surface and a plate-shaped soft magnetic metal green compact 12 is dispersed and arranged in the rubber support material 13.

In the case of a radio wave absorber short-circuited to a conductor, an antireflection condition may be obtained when an impedance match satisfying the following equation is made.

Where μ _r and ε _r are the complex permeability and the complex dielectric constant of the absorber layer, d is the thickness of the absorber layer, f is the frequency, c is the speed of light, and j = to be. Figure 2 is the solution of the complex impedance matching equations permeability (μ _r = μ _r μ _r'- j ˝) and f d (frequency and weight) on a plane, and the complex dielectric constant (ε _r = ε _r ε _r'- j ˝ ) Is a function of. The dielectric loss factor (tanδ _ε = ε _r ˝ / ε _r ´) is assumed to be 0.1. And the mobile communication terminal when the radio wave absorbent used frequency band is 0.8-2.2 ㎓ for, since the absorbing member allows the thickness to be 1 ㎜ level f and d (frequency and thickness) of the complex which satisfies the impedance matching in the range 800-2,200 ㎒ and ㎜ Permissible ranges of permeability (μ _r = μ _r ′ -j μ _r ˝) and dielectric constant (ε _r ′) are approximately equal to the hatched portion of FIG.

In this root locus diagram, impedance matching conditions can be divided into two areas. In order to satisfy the impedance matching condition in the range of μ _r ˝> μ _r ´, it is necessary to select materials with μ _r ´ = 10 or less, μ _r ˝ = 20 or more and ε _r ´ = 30 or less. Low dielectric constant materials include ferrite magnetic materials, but in the case of ferrite sintered bodies or composites, it is difficult to obtain magnetic loss characteristics of μ _r ˝ = 20 or more in the 0.8-2.2 ㎓ band.

On the other hand, in the case of μ _r ´> μ _r ˝, it is necessary to select materials with μ _r ´ = 3-10, μ _r ˝ = 0.1-10, ε _r ´ = 100-500. Even if the magnetic loss is small, if a material having a high permeability and dielectric constant is selected, a wave absorber having a thickness of 1 mm can be realized in the 0.8-2.2 kHz frequency band. Soft magnetic metal powder is suitable as an absorbent filler near these conditions. At least one member selected from the group consisting of pure iron, carbonyl iron, silicon iron, silicon-aluminum-iron alloy (sanddust), iron-nickel-based alloy (Permalloy), and iron-cobalt alloy powder is a usable material. These powders processed into a plate shape can be dispersed in the rubber support 13 to reduce the eddy current loss and to configure the flexible wave absorber 10 having flexibility. The use frequency, the thickness of the absorber, and the radio wave absorption ability can be controlled by the shape and average size of the soft magnetic metal powder 12, the mixing ratio of the dissimilar powder, and the mixing ratio of the absorbent filler and the rubber support material.

3 is a complex permeability and a complex dielectric constant of a composite prepared by mixing a soft magnetic metal powder with a rubber binder, μ _r ´ = 5.2, μ _r ˝ = 3.5, ε _r ´ = 140, ε at a frequency of 2 kHz. By showing _r ˝ = 20, it can be seen that the complex permeability and complex dielectric constant of the thin wave absorber shown in FIG.

The plate-shaped soft magnetic metal mixed powder 12 was molded with the rubber binder 13 (volume mixing ratio of the metal powder and the binder = 60: 40) to prepare a composite material having a thickness of 1.0-1.3 mm, and then the radio wave absorption ability was measured. . 4 shows the measuring fixture 20 in which the back surface of the sample 23 is short-circuited with a metal conductor 24 in a coaxial tube composed of an inner conductor 21 having a diameter of 3 mm and an outer conductor 22 having a diameter of 7 mm. After that, the return loss was measured by a network analyzer.

<Example 1>

5 is a reflection loss measured by a network analyzer after shorting the back surface of a composite material (thickness = 1.3 mm) formed by using carbonyl iron as an absorbent filler and a silicone rubber as a binder. The maximum return loss (power absorption factor = 99%) of -25 Hz is obtained at a frequency of 2.2 Hz. Based on a return loss of -10 kHz (power absorption rate = 90%), the frequency band is 1.7-2.8 GHz.

<Example 2>

6 is a reflection loss measured after short-circuiting the back surface of a composite material (thickness = 1.0 mm) formed by using a mixed powder of carbonyl iron and pure iron as an absorbent filler and a silicone rubber as a binder. The return loss of -7 ((power absorption rate = 80%) is shown at 2.2 ㎓ frequency, and the return loss of -6 ((power absorption rate = 75%) at frequency of 1.8 ㎓.

<Example 3>

7 is a reflection loss measured after short-circuiting the back surface of a composite material (thickness = 1.2 mm) formed by using a mixed powder of carbonyl iron and pure iron as an absorbent filler and a silicone rubber as a binder. It has a return loss of -8.4 전력 (power absorption rate = 86%) at the frequency of 1.8 ,, and a return loss of -7.2 ((power absorption rate = 81%) at the frequency of 2.2 ㎓.

<Example 4>

Fig. 8 is a reflection loss measured after short-circuiting the back surface of a composite material (thickness = 1.0 mm) formed by using a mixed powder of pure iron and sand dust as an absorbent filler and a silicone rubber as a binder. It has a return loss of 7 kHz (power absorption ratio = 80%) at a frequency of 1.2 kHz and a return loss of -4 ㏈ (power absorption ratio = 60%) at 800 MHz.

As described above, the present invention provides a thin wave absorber having a radio wave absorption rate of 80% or more and a thickness of 1.0-1.3 mm or less in the mobile communication frequency band (0.8-2.2 Hz). In addition, according to the present invention, by using a plate-shaped soft magnetic metal powder having a high permeability and permittivity as the absorbent filler, the thickness of the absorber can be reduced to a level of 1 mm. . Such a thin, lightweight radio wave absorber can be effectively used for preventing human harm caused by electromagnetic leakage or electromagnetic interference caused by radiation noise, which may be a problem in mobile communication terminals (cellular phones, PCS, IMT-2000).

While the invention has been described in terms of the preferred embodiments, those skilled in the art will understand that various changes and modifications can be made within the spirit of the invention as set forth in the appended claims.

Claims (4)

As a thin wave absorber for mobile communication frequency band, An electromagnetic wave absorber comprising 40 to 65% by volume of soft magnetic metal powder as a radio wave absorbing filler and an elastic member to 35 to 60% by volume as a binder. The method of claim 1, wherein the soft magnetic metal powder is selected from the group consisting of pure iron, carbonyl iron, silicon iron, iron-silicon-aluminum alloy (sandust), iron-cobalt alloy, and iron-nickel alloy (peralloy). Is at least one, wherein the elastic member is a radio wave absorber, characterized in that the rubber or silicone rubber. The radio wave absorber according to claim 1, wherein the absorber has a thickness of 1.0 to 1.3 mm based on an 80% absorption rate. The radio wave absorber according to claim 1 or 2, wherein the soft magnetic metal powder is a plate-shaped green compact.