JPH03204998A - Tv frequency band radio wave absorbent body - Google Patents

Abstract

PURPOSE:To enable an absorbent body to be easily provided and to absorb a TV frequency band radiowave of wide band by a method wherein the imaginary number component peak value of the complex magnetic permeability of a molded body, whose main material is composed of a binder and magnetic particles which are in a specified dry weight ratio, is so set as to be lower than a TV frequency band. CONSTITUTION:A binder and magnetic particles or a binder, magnetic particles, and carbon fiber are used as main materials, and the main components are set to be in the mixing ratio by dry weight 5-25% of binder: 75-95% of magnetic particles. If the magnetic particles are above 95% by weight, a molded body is hard to set, and if below 75%, the molded body is inferior in intrinsic performance. The reason why magnetic particles are set high in compounding ratio is that an imaginary number component mu'' of a magnetic permeability is enhanced and a real number component mum'/mum'' of a magnetic permeability is set small in value. By this setup, a large molded body can be easily formed, so that a radiowave absorbent body of this design can be prevented from deteriorating in radiowave absorbing property due to the gap between panels.

Description

Detailed Description of the Invention (Field of Industrial Application) The present invention provides a radio wave for TV frequency band using a caking agent, magnetic powder, or a caking agent, magnetic powder, and carbon fiber as main raw materials. It concerns absorbents.

(Prior art) TV radio waves use frequencies around 100 MHz and a wavelength of about 3 meters, 1 ch to 3 ch, around 200 MHz,
4ch ~ 12c with a wavelength of approximately 1.5m as the operating frequency
Radio waves with frequencies (wavelengths) that are around 600 MHz and have wavelengths of about 0.5 m are used, and have frequencies (wavelengths) that differ by up to 8 times from channels 13 to 62.

Therefore, the means to prevent false images (ghosts) by absorbing TV radio waves in a wide band with a wavelength difference of more than 2 meters using a radio wave absorbing material having a specific thickness is to use a radio wave absorbing material having a high frequency, that is, a short wavelength, for example, in the radar band ( 9.4GHz, wavelength approximately 0
．． Compared to countermeasures against false images (3m), there are fewer types of radio wave absorbing materials as solutions.

There are two main solutions known at present: a method using sintered ferrite tiles and a method using pyramid-shaped block materials made of a carbon powder and expanded polystyrene composite.

However, sintered ferrite tile material is
Since the process of firing at such high temperatures is unavoidable, deformations such as uneven firing and warpage of tiles during firing occur. In order to reduce the non-uniformity of firing conditions, there are restrictions on the size of the tile shape, and the dimensions are l0C111XIOCI
It will be about I+. This value is approximately 1 of the wavelength of 100MHz.
/30, and there are many technical and especially economical problems in construction in order to obtain good absorption properties, so it is not widely used in general buildings.

In addition, in the case of pyramid-shaped block materials, the thickness of the currently known TV wave absorbing material is 1 to 2 meters, and there are many spatial problems when using it on the outer wall of a building, and when using it in an anechoic chamber etc. However, there is a problem that it occupies a large amount of space in the room.

In addition, molded products made by mixing concrete with carbon fiber carbon beads and steel fibers (Yasutaka Shimizu: EMC 1988, 8, 8 <Na2> p. 88)
It has been known. However, the reported value of the molded uniform thickness is as thick as 23.79 cm, and the reported value of the absorption frequency band is around 100 to 200 MHz, so there is a problem that it cannot be used in all TV frequency bands.

Furthermore, the volume ratio of cement:ferrite:sand was 1:4:
3 (of which ferrite lff1 accounts for 66% of the total), the value of transmission attenuation (unit: dB)
/co+) has been reported (Japanese Patent Application No. 11398/1989).
0 Publication). However, the above-mentioned publication does not mention the value of 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 economical, geometrical, and spatial disadvantages of existing radio wave absorbers, is easy to install, and can be used for TVs that cover a wide band.
The aim is to develop molded products that can absorb frequency band radio waves.

(Means for Solving the Problems) The present invention provides that the peak value of the imaginary component of the complex magnetic permeability of a molded product whose main raw materials are 5 to 25% of the binder and 75 to 95% of the magnetic powder and granules in dry weight ratio is , a radio wave absorber for the TV frequency band characterized by being below the TV radio wave frequency band, or a dry weight ratio of caking agent 5 to 25% and magnetic powder and granules 75 to 95%,
A radio wave absorber for the TV frequency band, characterized in that the peak value of the imaginary component of the complex magnetic permeability of a molded product made from 0.01 to 1% of carbon fiber as the main raw material is below the TV radio wave frequency band. It is.

More preferably, the peak value of the imaginary component of the complex magnetic permeability of the molded product, which is made mainly from 5 to 17% of caking and 83 to 95% of magnetic powder in terms of dry weight, is below the TV radio wave frequency band. A radio wave absorber for TV frequency bands, or a dry weight ratio of caking agent 5 to 17% and magnetic powder and granule material 83 to 17%.
This is a radio wave absorber for the TV frequency band, characterized in that the peak value of the imaginary component of the complex magnetic permeability of the molded product made mainly of 95% and 0.01 to 1% carbon fiber is below the TV radio frequency band. . Another feature is that at least two types of magnetic powder particles having different particle size distributions are used.

(for production) The present invention will be explained in detail below.

The binder used in the present invention includes polyvinyl acetate solution, polyvinyl acetate emulsion, polyvinyl alcohol, acrylic emulsion, cyanoacrylate, chlorobrane rubber, natural rubber, urea resin, phenolic resin, epoxy, polyurethane, starch, and cement. Although it is a general term, it often refers to a composite of the cement described below and one or more types of caking agents mentioned above, and the cement is not used alone.

Cement here refers to hydraulic cement (e.g., portland cement, white portland cement, alumina cement, pozzolan cement, truss cement, santolin cement, lime slag cement, pozzolan portland cement, silica cement, trass portland cement, blast furnace cement) , iron portland cement,
fly ash cement, solidit cement, shale pressure portland cement), air hard cement (magnesia cement), special cement (fire resistant cement, acid resistant cement, water glass cement, high sulfate slag cement), etc.
There is no particular limitation as long as it is a type of cement that is generally called cement.

The blending ratio of these is in the range of 5wt% or more to 25wt% or less of the total dry weight ratio.If it is less than 5wt%, the molded product is difficult to aggregate, and if it exceeds 25wt%, it will not work as a radio wave absorber. The original function of is inferior.

Magnetic particles include oxide magnetic materials and metal magnetic materials, such as (Fed-Fe203).

7-Fe2O,, Mn0-Fe, O,, Mn0-ZnO
-Fe2O3,CuO-ZnO-Fe20q.

CuO-Mn0-Fe2O3, NtO-ZnO-Fe2
03. N1O-CuO-ZnO-Fe20. .

MgO-Fe2O, , MgO-ZnO-Fe2O,.

MgO-Mn0-Fe20. , Li0-ZnO-Fe2
There are oxide magnetic materials such as O, etc., and metal magnetic materials such as iron powder, silicon steel powder, permalloy powder, and sendust powder. Among these, if limited to oxide magnetic materials, improvement of magnetic properties,
It is desirable to add trace elements (for example, CaO, 5i02, etc.) before firing for the purpose of promoting the sintering reaction and controlling intra-grain/grain boundary resistance.

Furthermore, Mn-based ferrites such as Mn-Zn and Mn-Mg systems are magnetic materials that are relatively easy to obtain, are available in large quantities at low prices, and have excellent absorption properties. Appropriate.

Regarding the size and particle size distribution of the magnetic powder particles, as shown in Fig. 7 (marked), for the purpose of increasing the packing density of the magnetic powder particles, Fig. 2 (b) and Fig. 4 (a) ,(b)
As shown in , it is best to use a mixture of powders of the same or different components with different particle size distribution peaks, and the difference in particle size distribution peaks should be at least twice as far apart in terms of the diameter ratio of the magnetic powders. This is desirable. The blending range of the magnetic powder will be described later.

Carbon fiber is a general term for rayon-based, acrylonitrile-based, lignin ball-based, pitch-based, and tar-based materials, and by kneading one or more types, the radio wave absorption characteristics of the molded product can be finely adjusted. do.

The blending ratio Y is 0 or 0,01≦Y≦1, and the cross-talk of a large amount of carbon fiber exceeding 1vt% is not only technically difficult, but also especially when the blending ratio of magnetic particles is In the present invention, this is rather undesirable. Further, if fine adjustment is not required, there is no need to mix it.

Regarding the shape of carbon fiber, curled or straight long fibers or short fibers may be used.
To put it bluntly, a short fiber shape is preferable for ease of cross-crossing and dispersion.

In the present invention, the materials mentioned above, that is, caking agents, magnetic powders, or caking agents, magnetic particles, and carbon fibers, are referred to as main raw materials in the present invention. AE material, dispersion material, setting/hardening accelerator, waterproofing material, antifreeze material, foaming material, coloring material, mixed material, fire resistance accelerator, etc.) are added as appropriate.

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

The mixing ratio of the main raw materials is 5% or more and 25% or less of a binder, 75% or more and 95% or less of a magnetic powder, or 5% or more and 25% or less of a binder, and a magnetic powder or granule based on dry weight. 75% to 95% or less, carbon fiber 0.01
% or more to 1% or less. Magnetic powder is 95
If it exceeds 75%, the molded product will be difficult to solidify, and if it exceeds 75%
In the range below, the original performance as a molded product is poor. The reason for increasing the blending ratio of magnetic particles is to increase the imaginary component μ of magnetic permeability (marked O in Figure 3), and to increase the real component μ' (marked in Figure 3)/μ of magnetic permeability. ’ (Mark Q in Figure 3)
Depends on the purpose of reducing the value of.

Details of the radio wave absorption mechanism are described in Yoshiyuki Naito: Radio Wave Absorber, Ohmsha (1987), page 8B, so only an approximate expression will be used and the effect of the dielectric constant will be omitted. 1st
As shown in the figure, in a magnetic loss type radio wave absorber with a conductor called a shorting plate installed on the back side, the reflection loss is 1tdB.
is dB-20・Nog l 1- (4π(, cz″+j
μ')d/λ)・・・・・・・・・・・・・・・(1)
It can be approximated by d: thickness of radio wave absorber, λ: wavelength of radio wave.

From equation (1), the no-reflection condition for completely absorbing radio waves is:
It is considered ideal that the real component of the complex magnetic permeability is as small as possible, and the imaginary component satisfies the following equation (3).

μ'/μ'<<l ・Old...Old...
(2) μ′−λ/4πd ・・・・・・・・・・・・
...(3) In other words, if you want to make the radio wave absorbing material thinner, μ' is large, and if you want good absorption characteristics, you can reduce the value of μ'/μ', and if you want to have a wide band. For this purpose, it is preferable that the conditions of the above equations (2) and (3) be satisfied over the entire absorption wavelength range.

In Fig. 3 (a) and (b), these conditions are preferably a frequency near the peak value of the imaginary component of the complex magnetic permeability (resonance frequency) indicated by the mark, and a frequency range higher than the vicinity of the mark, and the main As a result of careful study of the blending conditions of the raw materials, it was discovered that a blending amount of magnetic powder in a dry weight ratio of 75% or more or 95% or less is suitable as a radio wave absorber for the TV frequency band.

Carbon fiber is appropriately added for the purpose of compensating for the lack of radio wave extraction of magnetic particles, and is preferably added in a small amount of 0% or 0.01vt% to 1vt%.

Since the radio wave absorbing material of the present invention can be easily produced into a large-sized molded product, a decrease in radio wave absorption performance due to gaps between panels can be reduced. For example, if the area above the 4th floor of a 10-story building (assumed to be 1ooOrrr) causes radio wave interference, if 10cm square tiles are installed with a porosity of 33%,
Approximately 67,000 pieces of work will be required. Moreover, if there is a gap between each panel, radio waves (100M
Hz) increases by approximately 33 times, and the absorption characteristics decrease sharply (edited by the Japan Broadcasting Corporation: Guidebook for Countermeasures against Radio Wave Interference Using Radio Wave Absorbers, July 1980, p. 6).

However, the radio wave absorber described in the present invention, for example,
By molding to a size of x1m, the total number of installation steps can be simplified to about 330 sheets and the man-hours to about 1/200, and the problem of gaps can be solved.

(Example) The present invention will be explained below according to examples.

Example 1 A 5mm thick molded product with an inner diameter of 18.9 mm and an outer diameter of 38.8 m.
After cutting with an ultrasonic processing machine, the cylindrical cut sample was placed in a 39D coaxial tube, and the sample was cut with a network analyzer for 10 minutes.
The real (μ′) and imaginary (μ″) magnetic permeability was measured from 0 MHz to 600 MHz.

The blending ratio in terms of dry weight ratio of the molded body used for measurement is:
Mn-Zn ferrite 90%, carbon fiber〇,
05%, polyacetic acid emulsion 5%, and the balance white Portland cement.

Here, FIG. 3(a) is an example using magnetic powder particles with a particle size of 0.1 mm or more and 2 times 1 m or less (particle size distribution peak 1.5 mm), and FIG. 3(b) is an example of using magnetic powder. , fine powder (
This is an example using 72% of the same powder particles with a particle size distribution peak of about 0.75+m) and 18 vt% and a particle size distribution peak of 1.5 mm.

FIG. 4 shows a particle size distribution diagram of the magnetic powder.

The method of mixing these main raw materials is to knead cement, carbon fiber, and water into a paste, and (a) mix together magnetic powder and acetic acid emulsion.
Regarding b), first, only the fine particles were entangled with polyacetic acid emulsion, and the remaining powder and granules were further mixed to form a plate-shaped molded product with a thickness of 5 cm.

In this way, when using a caking agent that is more expensive than cement but has excellent bonding strength, it is possible to incorporate 20 to 30 wt % of fine magnetic powder into a molded product by adding a small amount. This is largely related to the magnetic properties and the density of the molded product, which will be described later. In addition, if you mix fines with cement only,
When creating a molded article, the blending amount of the fine powder was around 105 at most.

In Fig. 3, the frequencies indicated by marks indicate the resonance frequencies of each molded product, 160 MHz for (a), 160 MHz for (b)
), it was 80M)lz. For example, when comparing the values of the imaginary component μ' and μ'/μ (real component/imaginary component) of magnetic permeability at 100 MHz, (a) is μm11.5, and u'/μm1.47.
In b), the magnetic properties are significantly different from μ −17,5 and μ′/μ −0,25 even when the amount of magnetic powder is the same.

In particular, focusing on μ'/μ', sample (b) has a nearly constant μ'/μ' value of about 0.15 to 0.3 in the TV frequency range, while sample (a ) of μ′/μ′ value (0,
15 to 1.47) varies greatly depending on the frequency. TV
Since radio waves include a wide frequency range from 91) MHz to 770 MHz), μ'/
The molding conditions shown in (b) with less variation in μ' are more preferable for the radio wave absorber.

Example 2 In Fig. 5, the present invention in the TV frequency band (marked with
Also, the radio wave absorption characteristic values of the comparative product (marked with O) are shown.

In the figure, the mixing conditions of the radio wave absorber were the same as in Example 1. - Marked with fine magnetic powder (example of the present invention),
○ indicates a comparative example without fine magnetic powder, (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, in both the present invention and the comparative example, 1
Absorbs radio waves of 0d/9dB/approximately 92% or more.

These values can be said to have radio wave absorption performance equivalent to about 93% or more of the currently used ferrite sintered tile materials (return loss is about 20 dB). Also flioMtlz (
Even at the center frequency of the UHF band, the return loss value is 8.2 dB or more, which is about 85.7% or more of the absorption performance of the currently used ferrite sintered tile materials.

However, compared to the comparative product (marked O), the product of the present invention (marked with *) is 1.5 dB to 1.
2.7dB, 0.6 in UHF band (hatched area in the figure)
clB~2. [Improvement was observed in ldB characteristics. Conventionally, it was not possible to obtain high packing density and excellent magnetic properties with cement alone due to the weak bonding force and the inability to increase the blending ratio of fine particles, but by using a caking agent, even better It has become possible to create radio wave absorbers.

Further, the density of the molded product will be described later.

Example 3 FIG. 6 shows the relationship between the blending ratio of magnetic powder and the resonance frequency, and FIG. 7 shows the density of the molded product.

The blending ratio in the dry weight ratio of the molded product used for measurement was M
X% n-Zn ferrite (X-75 to 90), 0.05% carbon fiber, 5% polyacetic acid emulsion, and the balance white Portland cement. In the present invention and comparative examples, the blending conditions of the magnetic powder (particle size distribution peak) are 1.5 order (80%) and 0.75 mm (2
0%) and the O mark was 1.5 mm (100%).

Further, the procedure and method for creating the molded article were the same as in Example 1.

In the TV frequency band where the observation was conducted, the resonance phenomenon was observed in the range where the weight ratio of magnetic particles was 75% or more, and in particular, in the range where the weight ratio of magnetic particles was 83% or more, the resonance phenomenon was observed.
A resonance phenomenon was also observed at low frequencies corresponding to the HF band. Although the value of the resonance frequency is not a measure that represents all of the radio wave absorption characteristics, at least the blending conditions of the magnetic powder and granules to efficiently absorb TV radio waves are preferably in the range of 75vt% or more, and more preferably. , it can be said that it is 90vt% or less than 83vt% or more.

Here, the resonant frequency of the comparative example (marked with O) hardly changes in the region where the ratio of binder:magnetic powder is around 1 nia and the ratio of magnetic powder is high, and the present invention material (・). Also, its value is 1 of the resonant frequency (Q mark)
/2, and the magnetic properties are modified by adding a high proportion of fine powder and the addition of a caking agent to make this possible, and the (magnetic) loss associated with resonance on the low frequency side is suppressed. (The resonance frequency of this magnetic material at true density is approximately 10 MIIz)
.

Fig. 6 shows the density of the molded product used in Fig. 5, and the increase in density of the molded product indicated by O mark is maximum at around 3.24 g/cTlN compared to 87.5% or more of magnetic powder. Mn-Z
It peaks out at 66% of the true density of n-ferrite, 4.9 g-/c+tl. However, the fine powder particles indicated by 2
In the molded product with 0% addition, an increase in density equivalent to about 11% of the comparative material was observed, making it possible to create a high-density molded product as shown in FIG. 2(b). In other words, by not limiting the type of caking agent to just one type of cement, but also by incorporating components with good binding properties, we can create radio wave absorbers with high density and low resonance frequency, which have excellent properties especially in the VHF band. It becomes possible to create.

(Effects of the Invention) Since the radio wave absorbing material of the present invention is not molded into a panel material, freedom of shape is provided, and the construction process can be simplified. Also,
Since it can be mass-produced as a panel material, the economic benefit of the present invention is great compared to conventional radio wave absorbing tile materials (approximately 10 CO+ square).

[Brief explanation of drawings]

Fig. 1 is a schematic diagram of the TV frequency band radio wave absorbing product according to the present invention, Fig. 2 is a diagram showing an example (b) of the radio wave absorbing molded product of the present invention and a comparative example (a), and Fig. 3 (a) and (b
) is a graph showing the complex magnetic permeability of the present invention and comparative products from 100M) Iz to 600MHz, Figure 4 is a graph showing the particle size distribution of magnetic powder, and Figure 5 is a graph showing the particle size distribution of magnetic powder particles at thicknesses of 20m+i and 25+am. FIG. 6 is a graph showing the return loss in the TVR wavenumber band of the molded radio wave absorbing material of the present invention and the comparative product, and shows the effect of the caking material and magnetic powder on the resonant frequency of the molded product of the present invention and the comparative product. FIG. 7 is a graph showing the influence of the blending ratio of the binder and the magnetic powder on the density of the molded products of the present invention and comparative products. Figure 2 (a) (b) Figure 3 (a) (b) 00 Fish 3 Gloss 40) 50:) 6 sword 1 ■ 20:1 3 (X) 400 f (MHz) 00 00 Figure 4 Particle size (trz m) Particle size (rrh m) 0 Magnetic powder mixture ratio (wt%) 70 80 85 90 Binder: Magnetic powder (weight ratio)

Claims (6)

[Claims] (1) Caking agent 5-25% by dry weight, magnetic powder 75%
1. A radio wave absorber for a TV frequency band, characterized in that the peak value of the imaginary component of the complex magnetic permeability of a molded product made of ~95% as a main raw material is below the TV radio frequency band. (2) Caking agent 5-25% by dry weight ratio, magnetic powder 75%
~95%, the peak value of the imaginary component of the complex magnetic permeability of a molded product made mainly of carbon fiber 0.01~1% is TV
A radio wave absorbing material for a TV frequency band characterized by being below a radio wave frequency band. (3) Caking agent 5-17% by dry weight ratio, magnetic powder 83
1. A radio wave absorber for a TV frequency band, characterized in that the peak value of the imaginary component of the complex magnetic permeability of a molded product made of ~95% as a main raw material is below the TV radio frequency band. (4) Caking agent 5-17% by dry weight ratio, magnetic powder 83
~95%, the peak value of the imaginary component of the complex magnetic permeability of a molded product made mainly of carbon fiber 0.01~1% is TV
A radio wave absorbing material for a TV frequency band characterized by being below a radio wave frequency band. (5) The radio wave absorber for TV frequency bands according to claim (1), (2), (3) or (4), characterized in that two types of magnetic powder particles having different particle size distributions are used. (6) Claim (
TV described in 1), (2), (3), (4) or (5)
Radio wave absorber for frequency bands.