JP2735914B2 - Radio wave absorber for TV frequency band - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial application field) The present invention relates to a radio wave for a TV frequency band using a binder and a magnetic powder or a binder, a magnetic powder and carbon fiber as main raw materials. It is about an absorbent.

The (prior art) TV radio waves, with a frequency use 100MHz near a wavelength of about 3m, 1ch~3ch _o 200MHz vicinity, with a frequency use wavelength of about 1.5m, 4ch~12ch _o 600MHz vicinity, about 0.5m as the wavelength Radio waves of different frequencies (wavelengths) of up to slightly more than eight times from 13 ch to 62 ch are used.

Therefore, means for absorbing TV radio waves having a specific thickness and absorbing a wide range of TV radio waves having a wavelength difference of more than 2 m and preventing false images (ghosts) can be achieved by using a high operating frequency, that is, a short wavelength, such as a radar band ( There are fewer types of electromagnetic wave absorbers as a solution than countermeasures against false images at 9.4 GHz and a wavelength of about 0.3 m).

Currently, there are two main solutions, and there are a method using a sintered ferrite tile material and a method using a pyramid type block material made of a carbon powder and a polystyrene foam composite.

However, since a step of firing at a high temperature of 1000 ° C. or more is inevitable for the sintered ferrite tile material, deformation such as uneven baking and warping of the tile during firing occurs. For the purpose of reducing the non-uniformity of the firing conditions, the shape of the tile is limited in size, and the size is about 10 cm × 10 cm. This value is about 1/30 of the wavelength of 100 MHz. In order to obtain good absorption characteristics, there are many technical and particularly economical problems in construction, and it is not widely used in general buildings.

In the case of pyramid-shaped block materials, the thickness of currently known TV radio wave absorbers is 1 to 2 m, and there are many spatial problems in adopting them on the outer walls of buildings. However, there is a problem that the space of the room is occupied greatly.

Also, a molded product in which carbon fiber, carbon beads and steel fiber are mixed with concrete (Yasutaka Shimizu: EMC 1988.6.6 <No. 2>, page 86) is known.
However, since the reported value of the molded product thickness is as thick as 23.79 cm and the reported value of the absorption frequency band is around 100 to 200 MHz, there is a problem that it cannot be used in all TV frequency bands.

In addition, the cement: ferrite: sand volume ratio of 1: 4: 3
The transmission attenuation value (unit: dB / cm) has been reported for electromagnetic wave absorbing building materials (of which the ferrite weight is 66% of the total) (Japanese Patent Application No. 47-113980 → Japanese Patent Publication No. 52-27355). Gazette). However, the publication does not describe the value of the return loss (unit: dB), and the amount of radio waves directly reflected on the surface of the molded product is completely unknown.

(Problems to be Solved by the Invention) The present invention overcomes the economic, shape, and space disadvantages of existing radio wave absorbers, is easy to construct, and has a wide band TV.
The purpose is to develop molded products that can absorb frequency band radio waves.

(Means for Solving the Problems) The present invention relates to a molded product containing, as a main raw material, 5 to 17% of a binder in a dry weight ratio and 83 to 95% of two types of magnetic particles having different particle size distributions, or The imaginary number of the complex magnetic permeability of a molded product mainly composed of 5 to 17% of a binder by dry weight ratio, 83 to 95% of two kinds of magnetic powder having different particle size distribution, and 0.01 to 1% of carbon fiber. A gist of the invention is a TV frequency band radio wave absorber, wherein the component peak value is equal to or lower than the TV radio frequency band.

The magnetic powder is preferably Mn-based ferrite.

(Operation) Hereinafter, the present invention will be described in detail.

The binder in the present invention includes polyvinyl acetate solution, polyvinyl acetate emulsion, polyvinyl alcohol, acrylic emulsion, cyanoacrylate, chloroprene rubber, natural rubber, urea resin, phenol resin, epoxy, polyurethane, starch and cement. Although it is a collective term, it often refers to a composite of the cement described below and one or more kinds of binders described above, and is not used alone.

The cement mentioned here is a hydraulic cement (for example,
Portland cement, white Portland cement, alumina cement, pozzolan cement, truss cement,
Santrin Cement, Lime Slag Cement, Pozzolan Portland Cement, Silica Cement, Trasport Cement, Blast Furnace Cement, Iron Portland Cement, Fly Ash Cement, Solid Cement, Shale Ash Portland Cement), Air-Hardening Cement (Magnesia Cement), Special Cement (Refractory cement, acid-resistant cement, water glass cement, high sulfate slag cement)
The cement is not particularly limited as long as it is generally called cement.

These compounding ratios are in the range of 5 wt% or more to 17 wt% or less in terms of dry weight ratio, and when less than 5 wt%, the molded product is hard to coagulate. The original function of is inferior.

The magnetic powder and granular material is an oxide magnetic material and the metallic magnetic, for example _{(FeO-Fe 2 O 3,} γ-Fe 2 O 3, MnO-Fe 2 O 3, MnO
−ZnO−Fe ₂ O ₃ , CuO−ZnO−Fe ₂ O ₃ , CuO−MnO−Fe ₂ O ₃ , NiO−
ZnO-Fe ₂ O ₃ , NiO-CuO-ZnO-Fe ₂ O ₃ , MgO-Fe ₂ O ₃ , MgO-Zn
_{O-Fe 2 O 3, MgO} -MnO-Fe 2 O 3, LiO-ZnO-Fe 2 O 3, an oxide magnetic material such as iron powder, silicon steel powder, permalloy powder, metallic magnetic material such as sendust powder There is. Of these, if limited to oxide magnetic materials, trace elements (for example, CaO, SiO _2, etc.) are fired before firing for the purpose of improving magnetic properties, accelerating the sintering reaction, and controlling intracrystalline and intergranular resistance. It is desirable to add

Furthermore, magnetic materials that are relatively easily available at present, are available in large quantities and inexpensively on the market, and have excellent absorption characteristics include Mn-based ferrites such as Mn-Zn-based and Mn-Mg-based ferrites. Appropriate.

See Figure 7 for the size and particle size distribution of magnetic particles.
As shown by the mark, in order to increase the packing density of the magnetic particles, as shown in FIG. 2 (b) and FIGS. 4 (a) and 4 (b), the same or different particle size distribution peaks are used. It is preferable to use a mixture of powders of different components, and it is desirable that the difference between the particle size distribution peaks be at least twice as large as the diameter ratio of the magnetic powder. The mixing range of the magnetic powder will be described later.

The carbon fiber is a general term for rayon-based, acrylonitrile-based, lignin-poval-based, pitch-based, and tar-based materials, and finely adjusts the electromagnetic wave absorption characteristics of a molded body by kneading one or more types.
The compounding ratio Y is 0 or 0.01 ≦ Y ≦ 1,
It is not only technically difficult to knead a large amount of carbon fiber exceeding 1 wt%, but it is rather unfavorable especially in the present invention where the mixing ratio of magnetic powder is high. When fine adjustment is not required, it is not necessary to mix.

Regarding the shape of the carbon fiber, curled or straight long fibers or short fibers may be used, and in short, the shape of short fibers is more desirable than the ease of kneading and dispersion.

As described above, the materials described in the text, that is, the binder, the magnetic powder, or the binder, the magnetic powder, and the carbon fiber are referred to as main raw materials in the present invention. AE material, dispersing material, setting / hardening promoting material, waterproofing material, antifreezing material, foaming material, coloring material, mixture material, fire resistance promoting material, etc.) are appropriately added.

FIG. 1 shows a schematic diagram of the present invention. A conductor called a short circuit plate is provided on the back surface of the radio wave absorber. This plate can be replaced by a metal plate such as an iron plate or a copper plate, or a wire mesh or a reinforcing bar, and has a function of reducing the thickness of the electromagnetic wave absorber described in the present invention.

The mixing ratio of the main raw materials is 5% to 17% of binder, 83% to 95% of magnetic powder, or 5% to 17% of binder, and 5% to 17% of magnetic powder 83% to 95% or less, carbon fiber 0.01% to 1
% Or less. If the amount of the magnetic powder exceeds 95%, the compact is hardly coagulated, and if it is less than 83%, the original performance of the compact is inferior. As described above, the reason for increasing the mixing ratio of the magnetic particles is that the imaginary component μ ″ of the magnetic permeability is used.
(Indicated by ○ in FIG. 3), and the real component μ ′ of the magnetic permeability
(Marked by ● in FIG. 3) / μ ″ (marked by ○ in FIG. 3).

The details of the radio wave absorption mechanism are described in Yoshiyuki Naito: Radio Wave Absorber, Ohmsha (1987), page 86, and only the approximate expression is used, and the effect of the dielectric constant is omitted. As shown in FIG. 1, in a magnetic loss type electromagnetic wave absorber in which a conductor called a short-circuit plate is provided on the back surface, the return loss dB is dB = 20 · log | 1− ｛4π (μ ″ + jμ ′) d / λ｝ | (1) d: Thickness of radio wave absorber, λ: Wavelength of radio wave From Equation (1), the non-reflection condition for completely absorbing radio wave is the real component of complex magnetic permeability Is ideally small, and the imaginary component satisfies the following equation (3).

μ ′ / μ ″ << 1 (2) μ ′ = λ / 4πd (3) That is, when the thickness of the radio wave absorber is to be reduced, μ ″ is large, and when the absorption characteristics are desired to be improved. μ '/
When it is desired to reduce the value of μ ″ and make the band broader, it is preferable that the conditions of the above-mentioned expressions (2) and (3) be satisfied in the entire absorption wavelength range.

In FIGS. 3 (a) and 3 (b), these conditions are desirably a frequency (resonance frequency) near the peak value of the imaginary component of the complex magnetic permeability indicated by a mark and a frequency range higher than the vicinity of the mark. As a result of thorough examination of the mixing conditions of the raw materials, it was found that a range of 83% or more and 95% or less in terms of the dry weight ratio of the magnetic powder was appropriate as a radio wave absorber for the TV frequency band.

The carbon fiber is appropriately blended for the purpose of compensating for the shortage of the radio wave loss of the magnetic powder, and a small addition range of 0% or 0.01 wt% to 1 wt% is preferable.

Since the radio wave absorber of the present invention can be easily formed into a large molded product, it is possible to reduce the deterioration of the radio wave absorption performance due to the gap between the panels. For example, if 10-story 4 floors or more areas of the building (1000 m ² and assumed) is interference, when mounting the 10cm square tiles with 33% porosity, and needs work about 67,000 sheets Become. Moreover, the gap between the panels
The presence of each 1 mm increases the reflectivity of radio waves (100 MHz) by about 33 times, and dramatically reduces the absorption characteristics. (Japan Broadcasting Corporation): Guidebook for measures against radio interference by radio wave absorbers July 6, 1981
page).

However, the radio wave absorber described in the present invention is, for example, 3m ×
By molding into a size of 1m, the total number of installations is about 33
The man-hour for 0 sheets can be simplified to about 1/200, and the problem of the gap can be solved.

(Examples) Hereinafter, the present invention will be described with reference to examples.

Example 1 After cutting a molded product having a thickness of 5 mm to an inner diameter of 16.9 mm and an outer diameter of 38.8 mm using an ultrasonic machine, a cylindrical cut sample was charged into a 39D coaxial tube, and a network analyzer was used.
, The real (μ ′) and imaginary (μ ″) magnetic permeability were measured.

The compounding ratio of the molded body used for the measurement in the dry weight ratio was Mn-Zn ferrite 90%, carbon fiber 0.05%.
%, Polyacetic acid emulsion 5%, and the balance was white Portland cement.

Here, FIG. 3 (a) shows that the particle size is 0.1 mm or more and 2 mm.
FIG. 3 (b) shows an example using a magnetic powder having a particle size distribution peak of 1.5 mm and a fine powder (particle size distribution peak of about 0.75 mm) of 18 wt% and a particle size distribution peak of 1.5%. This is an example using 72% of the same granular material of mm.

FIG. 4 shows a particle size distribution diagram of the magnetic powder. As for the method of mixing these main raw materials, while kneading cement, carbon fiber, and moisture into a paste, (a) is mixed with magnetic particles and acetic acid emulsion, and (b) is mixed. First, only the fine fraction, entangled with the polyacetic acid emulsion, further mixed with the remaining powder and granules, 5m
An m-thick plate-like molded product was prepared.

As described above, when a binder that is more expensive than cement and has excellent bonding strength is used, it is possible to mix 20 to 30% by weight of fine magnetic particles in a molded product with a small amount of addition. This is greatly related to the magnetic properties described below and the density of the molded product. In addition, when the fines are mixed only with cement to form a molded product, the blending amount of fine particles is 10% at most.
Before and after.

In FIG. 3, the frequency indicated by the symbol ▼ indicates the resonance frequency of each molded product, which was 160 MHz for (a) and 80 MHz for (b). For example, comparing the values of the imaginary component μ ″ and μ ′ / μ ″ (real component / imaginary component) of the magnetic permeability at 100 MHz, (a) shows that μ ″ = 11.
5, μ ′ / μ ″ = 1.47 and (b) shows μ ″ = 17.5, μ ′
/Μ″=0.25, the magnetic properties are significantly different even with the same amount of magnetic particles.

Paying particular attention to μ ′ / μ ″, sample (b) is in the TV frequency range, and the value of μ ′ / μ ″ is approximately 0.15 to 0.3, which is almost constant, whereas the value of μ ′ of sample (a) is / Μ ″ value (0.15 to 1.4
7) varies greatly depending on the frequency. TV signal from 90MHz
By including a wide frequency range up to 770 MHz, the molding condition shown in FIG. 4B with little fluctuation of μ ′ / μ ″ with respect to the TV frequency range is more preferable as a radio wave absorber.

Embodiment 2 FIG. 5 shows the present invention in the TV frequency band (marked with ●).
In addition, the radio wave absorption characteristic value of the comparative material (marked with ○) is shown.

In the figure, the mixing conditions of the radio wave absorber were the same as those in Example 1. ● mark is fine magnetic particles (Example of the present invention), ○ mark is no fine magnetic particles (Comparative Example),
(A) shows the case where the sample thickness is 20 mm, and (b) shows the case where the sample thickness is 25 mm.

According to the figure, both the present invention and the comparative example are in the VHF band and 10 d
・ 9dB ・ Absorbs about 92% or more radio waves. These values are
It can be said that the electromagnetic wave absorption performance is equivalent to about 93% or more of the ferrite sintered tile material currently used (the return loss is about 20dB). Also at 600MHz (center frequency of UHF band),
The return loss value is 8.2dB or more, which is about 85.7% of the absorption performance of currently used ferrite sintered tile materials.

However, the product of the present invention (marked with ●) is a comparative product (marked with ○).
1.5 to 12.7 dB in the VHF band (hatched area in the figure)
In the UHF band (hatched area in the figure), improvement was observed in the 0.6 dB to 2.0 dB characteristics. Conventionally, cement alone did not provide a high packing density and excellent magnetic properties because the bonding strength was weak and the mixing ratio of fine powders could not be increased. It has become possible to create radio wave absorbers. As for the density of the molded product,
It will be described later.

Example 3 FIG. 6 shows the relationship between the mixing ratio of the magnetic powder and the resonance frequency, and FIG. 7 shows the density of the compact. The mixing ratio in the dry weight ratio of the molded product used for the measurement was Mn-Zn ferrite X% (X = 75 to 90), carbon fiber 0.05%,
The polyacetic acid emulsion was 5%, and the balance was white Portland cement. In the present invention and comparative examples, the mixing conditions of the magnetic powder (particle size distribution peak) are as follows:
%) And 0.75 mm (20%), and the circle mark is 1.5 mm (100%). The procedure and method for forming a molded product were the same as those in Example 1.

In the observed TV frequency band, the range where the resonance phenomenon was observed is the range where the weight ratio of the magnetic particles is 75% or more, and especially, the range where the weight ratio is 83% or more corresponds to the VHF band. A resonance phenomenon was observed even at a low frequency. Although the value of the resonance frequency is not a measure representing all of the radio wave absorption characteristics, at least the blending conditions of the magnetic powder to efficiently absorb the TV radio wave are more than 83 wt% or more than 90 wt%.
It can be said that:

Here, the resonance frequency of the comparative example (indicated by ○) shows that the ratio of the binder to the magnetic particles is about 1: 7, and the material of the present invention (indicated by ●) is hardly changed in a region where the magnetic particles are high. And a difference is observed. The value is 1/1 of the resonance frequency (marked by ○).
The magnetic properties are improved by the addition of a high content of fine powder and the addition of a binder that enables this, and the (magnetic) loss accompanying resonance on the low frequency side is likely to occur (note that The resonance frequency at the true density of this magnetic material is about 10 MHz).

FIG. 6 shows the density of the molded product used in FIG.
The increase in the density of the molded product indicated by the mark is maximum at around 3.24 g / cm ³ from 87.5% or more of the magnetic particles, and reaches a peak at 66% of the true density of Mn-Zn ferrite of 4.9 g / cm ³ . However, in the molded product to which 20% of the fine particles indicated by the black circles was added, an increase in the density corresponding to about 11% of the comparative material was observed, and as shown in FIG. A molded article can be created. That is, the type of the binder is not limited to one type of cement, and by further blending a component having good binding properties,
It is possible to produce a radio wave absorber having a high density and a low resonance frequency, particularly having excellent characteristics in the VHF band.

(Effect of the Invention) By forming the radio wave absorber of the present invention into a panel material,
The freedom of the shape is given, and the construction process can be simplified. In addition, since it can be mass-produced as a panel material, the present invention has a large economic benefit as compared with a conventional radio wave absorption tile material (about 10 cm square).

[Brief description of the drawings]

FIG. 1 is a schematic diagram of a TV frequency band radio wave absorber according to the present invention, FIG. 2 is a diagram showing an example of a radio wave absorption molded product of the present invention (b) and a comparative example (a), and FIG. And (b)
Is a graph showing the complex magnetic permeability of the present invention and the comparative material from 100 MHz to 600 MHz, FIG. 4 is a graph showing the particle size distribution of the magnetic powder, FIG. 5 is a thickness of 20 mm and 25 mm
FIG. 6 is a graph showing the return loss in the TV frequency band of the radio wave absorber and the comparative product of the present invention molded into a binder, and FIG. Graph showing the effect of the compounding ratio with
The figure shows the effect on the density of the moldings of the invention as well as of the comparisons.
It is a graph which shows the influence of the compounding ratio of a binder and a magnetic powder.

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Takeshi Yamamoto 1618 Ida, Nakahara-ku, Kawasaki-shi, Kanagawa Prefecture Nippon Steel Corp. (72) Inventor Hideo Tanaka 1-25-1 Nishi-Shinjuku, Shinjuku-ku, Tokyo Taisei Construction Co., Ltd. (72) Kenji Sugimoto 1-25-Nishi-Shinjuku, Shinjuku-ku, Tokyo 1. Inside Taisei Corporation (72) Inventor Tetsuo Yamada 1-25-1 Nishishinjuku, Shinjuku-ku, Tokyo Inside Taisei Corporation (72) Inventor Tetsuzo Morita 1-25-1 Nishishinjuku, Shinjuku-ku, Tokyo Taisei (72) Inventor Masao Kosaka 2-14-2 Nagatacho, Chiyoda-ku, Tokyo Showa Mining Co., Ltd. (72) Inventor Tadashi Ito 2-2-2 Matsumoto, Urawa-shi, Saitama (56) Bibliography JP Akira 61-37832 (JP, A) JP Akira 58-108603 (JP, A) Tokuoyake Akira 52-27355 (JP, B2)

Claims (3)

(57) [Claims] 1. The imaginary component peak value of the complex magnetic permeability of a molded product mainly comprising 5 to 17% of a binder in a dry weight ratio and 83 to 95% of two types of magnetic powder having different particle size distributions. A radio wave absorber for a TV frequency band, which is below the TV radio frequency band. 2. The complex permeability of a molded product mainly composed of 5 to 17% of a binder in dry weight ratio, 83 to 95% of two kinds of magnetic powder having different particle size distribution, and 0.01 to 1% of carbon fiber. A radio wave absorber for a TV frequency band, wherein an imaginary component peak value of a magnetic susceptibility is equal to or lower than a TV radio frequency band. 3. The method according to claim 2, wherein the two types of magnetic powder having different particle size distributions are Mn.
The electromagnetic wave absorber for a TV frequency band according to claim 1 or 2, which is a base ferrite.