JPH0291998A - Radiowave-absorbing body - Google Patents

Abstract

PURPOSE:To obtain a radiowave-absorbing body having excellent characteristics by forming a plurality of tapered protruding ridges or projections on one surface of a mat shaped fiber assembly. CONSTITUTION:In a radiowave absorbing body, polar polymer of nylon and the like is coated with carbon. The carbon and polyvinyl chloride are dissolved and dispersed in toluene. The solution is splayed and applied, and the carbon coating is provided. Entangled fibers are bonded with a bonding agent. A mat shaped fiber assembly incorporates at least two fibers having at least two kinds of thicknesses, e.g., 0.1mm phiand 1mmphi. The kinds of the thicknesses are increased for radiowave absorption in a broad range. The thickness of the carbon coating is 10-30mum. Tapered protruding ridges or projections are provided on one surface of the met so that the tips are sharp. The projections are directed toward the radiowave input direction (a). At this time, it is considered that the conductivity is changed apparently in the thickness direction in the absorbing body. The conductivity is low at the incident surface. The conductivity becomes higher toward the inner part. Since the conductivity at the incident surface is low, the difference between said conductivity and that of air is small. The amount of the radiowave which is reflected from the incident surface is decreased, and the excellent radiowave absorbing characteristics can be achieved.

Description

[Detailed description of the invention]

[Industrial Application Field 1] The present invention relates to a radio wave absorber that absorbs radio waves (particularly microwave, millimeter wave, and submillimeter wave bands). [Prior Art] Conventionally, as a radio wave absorber used in the above wavelength range, a mat-like material in which horse tail hair and palm fibers are intertwined and fixed with adhesive, and then coated with carbon is known. ing. The reason why coconut fiber is mixed with the horse's tail hair is that horse hair is in short supply and difficult to obtain, so coconut fiber is the best way to replenish that supply, and to improve the radio wave absorption properties.
This is because it is convenient for increasing the space area. A radio wave absorber made of randomly assembled fibers of this type is used, for example, as a transmitting antenna by attaching it to the side surface of a parabolic reflector of a parabolic antenna to prevent interference with an adjacent parabolic antenna. In this case, in addition to the radio waves emitted by the antenna in one direction, there are also radio waves that wrap around behind the parabolic reflector due to diffraction, which causes radio energy loss and interference with other radio waves. Therefore, to prevent this, the diffracted waves are absorbed by a mat-like radio wave absorber. [Problems to be Solved by the Invention] By the way, since horse tail hair and palm fiber are natural, it is difficult to obtain them with the same quality, and it is difficult to obtain them due to the amount of supply. Furthermore, although there may be some differences in the thickness of the hair and fibers of the horse's tail, when viewed macroscopically, there is only one type of hair from a horse's tail and the fibers of a palm tree, and both constitute a radio wave absorber. There are at most only two types of fibers. - When fishing on a boat, if there are restrictions on the type of fiber thickness, the radio wave absorption band will also become narrower. Therefore,
In order to obtain a wide radio wave absorption bandwidth, it is desirable to use fibers of various thicknesses. In general, conventional radio wave absorbers made of horse tail hair and palm fibers, especially palm fibers, are made of hydrocarbons, so they have the property of being difficult for carbon to adhere to their surfaces. Therefore, it is difficult to form a carbon coating of uniform thickness on these hairs and fibers. However, if the thickness of the carbon coating on these hairs and fibers is uneven, it is not preferable as a radio wave absorber because a predetermined wavelength band such as extremely high frequency waves is not absorbed at a similar level over the entire band. Based on the above-mentioned circumstances, the present inventors believe that in order to improve the radio wave absorption characteristics of a mat-like radio wave absorber, it is necessary to make it composed of fibers of various thicknesses and to form a uniform carbon coating on the surface of the fibers. We focused on the following: It has been found that in order to obtain better radio wave absorption characteristics due to roughness, the radio wave absorber needs to have a structure that does not produce reflected waves on the radio wave incident surface. The object of the present invention is to - have fibers of uniform thickness, have fibers of various thicknesses, form a carbon thin film of a constant thickness on the fibers, and have a structure so as not to generate reflected waves. The purpose of the present invention is to provide a radio wave absorber with improved characteristics. [Means for Solving the Problems] In order to achieve the above object, the radio wave absorber of the present invention is a mat-like fiber assembly in which fibers made of polar polymers of at least two different thicknesses are entangled with each other and coated with carbon. The radio wave absorber is characterized in that a plurality of tapered protrusions or protrusions are formed on one surface of the mat-like fiber aggregate.

Structure of the Invention The radio wave absorber of the present invention is a mat-like material made of polar polymer and carbon-coated fibers intertwined with each other. Because we use artificial fibers instead of artificial fibers, each fiber has a uniform thickness, and fibers of various thicknesses can be easily obtained, and fibers of the same quality can be used. The thickness of the carbon coating can also be made uniform. The polar polymer used in the present invention is not particularly limited, and examples include polyvinylidene chloride, nylon, polyester, and acrylic. In addition, carbon coating is done by spraying carbon and an adhesive such as polyvinyl chloride or polyvinyl acetate as a binder dissolved or dispersed in an appropriate solvent, such as a hydrocarbon such as toluene or xylene, or an alcohol such as methanol or ethanol. This is done by applying by means such as coating. Note that the entangled fibers may unravel if left as they are, so it is desirable to bond them with an adhesive or the like so that they do not come apart. Further, the mat-like fiber aggregate only needs to be composed of fibers with at least two types of thickness, for example, the minimum thickness is 0.1.
rnrRφ fiber content is 10 to 90% by weight, maximum width], O#φ fiber content is 90 to 10% by weight2
It can be composed of different types of fibers and their content. Needless to say, if broadband radio waves are to be absorbed, the mat-like fiber aggregate may be constructed by increasing the types of fiber thicknesses. The thickness of the Naori-Bon coating is usually 5 to 50 nm, preferably 10 to 30 μm. On the other hand, in order to obtain excellent radio wave absorption characteristics over a wide band, it is necessary to prevent the radio waves incident on the radio wave absorber from being reflected at the incident surface of the radio wave absorber. However, it is virtually impossible to eliminate the reflection of radio waves at the radio wave incidence surface, and a portion of the radio waves that are destined to be incident on the absorber are reflected at the incidence surface. This is because when radio waves propagate through space, if they encounter a point where the conductivity suddenly changes, they tend to be reflected at that point. If the radio wave incident surface of the absorber is flat, some of the reflected radio waves cannot be completely absorbed and the radio wave absorption characteristics deteriorate. Therefore, in the present invention, a plurality of tapered protrusions or protrusions are provided on one surface of the mat-like fiber aggregate, and the radio wave absorption characteristics are improved by arranging the protrusions toward the radio wave incident direction. In other words, even if a part of the radio wave were to be reflected by a sloped surface of one protrusion or protrusion, it would not be reflected in the opposite direction because it is a sloped surface.
The radio waves will be incident on other protrusions or protrusions again. Therefore, some of the radio waves that are initially reflected without being absorbed are eventually absorbed as they are repeatedly reflected on the sloped surfaces of the protrusions or protrusions, so that it is possible to obtain good radio wave absorption characteristics. Note that since it is preferable that the radio wave incident surface of the absorber does not have a flat portion, it is desirable that the protrusions or protrusions are tapered and provided with sharp tips. Although the thickness of the radio wave absorber of the present invention is arbitrary, the thickness is usually set to 1/2 of the wavelength of the lowest absorption frequency. For example, 3
When absorbing radio waves of GHz or higher, the wavelength is 1
Since the IR is 00 mIR, the thickness of the apex portion of the protrusion or protrusion may be set to 50 mm. Further, if the protrusions or protrusions are tapered with sharp tips, it can be considered that the conductivity of the upper absorbent body changes in the thickness direction. In other words, it can be said that the conductivity of the radio wave incident surface is low, and the conductivity increases toward the inside. As mentioned above, radio waves are reflected at points where there is a sudden change in conductivity, but with such a configuration, the conductivity of the incident surface is low, so the difference in conductivity with the air is small, and therefore the radio waves are reflected at points where there is a sudden change in conductivity. The amount reflected at the radio wave incident surface is reduced, and good radio wave absorption characteristics are achieved. [Example] Hereinafter, the radio wave absorber of the present invention will be specifically described based on Examples. Figure 1 shows the appearance of one block of the radio wave absorber of the present invention. The radio wave absorber is made of various types of fibers F or mat-like fiber aggregates intertwined randomly, and the fibers F are bonded to each other at bonding parts 1 with adhesive so that they do not come undone, and are coated with carbon. There is. In this example, the fiber F is 0. L
mmφ, 0.15mmφ, 0, 2 mmφ, 0, 2
5 mmφ, 0, 3 mmφ, 0, 4 nrmφ,
Using 8 types of wood of 0, 7 mmφ, 1,0 mrnφ, fir x width = 300 rrrm x 300 a
m d) Create a sample. Note that the density of the fibers F and the specific gravity of the zigzag mat-like fiber aggregate were set to a constant value of 0.05 g/ci, and a plurality of approximately wedge-shaped protrusions 11 with sharp tips of 30 mm in height were provided every 50 mm on the radio wave incident side. The thickness from the top of the ridge 11 to the bottom was 50 mm. ? ! The r-fiber F has a carbon coating with an average thickness of 25 mm, and therefore is conductive with a constant resistance value as a radio wave absorber. For example, the radio wave absorber created in this example is
When the resistance across 50 mm at the center is measured using a tester, it is set at 0.2 to 0.4 k (2). Since such radio wave absorbers are made of artificial fibers, each fiber The thickness of the fibers is uniform, and since the fibers are polar polymers, carbon easily adheres to them, and the thickness of the carbon coating is also uniform.Therefore, radio wave absorption remains at a constant level over the target wavelength range (especially ultrashort waves). Next, a method for manufacturing the radio wave absorber shown in Fig. 1 will be described.The thickness of each fiber made of a polar polymer as a base material is
■0.1 cocoonφ, ■0.15mrIIφ, ■0.2tnm
φ, ■0, 25 mmφ, ■0, 3 nmφ, ■0
, 4 mmφ, ■0, 7 mtnφ, ■1.0 prisonφ
Prepare fibers. These various fibers were added at 0.5% by weight, 0.
10% by weight, 015% by weight, 015% by weight, 015% by weight, 015% by weight, 010% by weight, 015% by weight.
As, fir x width x thickness = 300 mtnX 300
After producing a mat-like fiber aggregate of men x 50 am, substantially wedge-shaped grooves with a depth of 3051I11 are provided on one side of the mat-like fiber aggregate every 50 m by a known method. After that, an adhesive made of a vinylidene chloride binder is sprayed onto the mat to bond and fix each fiber, and then diluted with a binder of toluene and polyvinyl chloride. The radio wave absorber shown in FIG. 1 was manufactured by spraying and drying a carbon paint prepared by dissolving the paint at a weight ratio of 1:2. The radio wave absorption characteristics of the radio wave absorber thus manufactured were evaluated using a measurement system as shown in FIG. In the figure, a metal plate 4 is laid on the back surface (the surface with a dense fiber density) of the radio wave absorber 5 under test, and a horn antenna 2 is placed toward the front surface (the surface with a low fiber density) of the radio wave absorber 5 under test. .2° is placed perfectly. Here, the horn antenna 2 is on the transmitting T side,
゛ is considered to be the receiving R side. Note that 3 is horn antenna 2
゛ is a detector that detects the received radio waves. With this configuration, the degree to which the radio waves emitted from the horn antenna 2 toward the test radio wave absorber 5 are reflected by the metal plate 4 and returned to the horn antenna 2', that is, the amount of attenuation (reflection), is 2.8 to 4. ．．
The investigation was conducted in the 4GIIz frequency band. Uramigo Z As in the previous example, 8 types of fibers are entwined in the same weight ratio to create a mat-like fiber aggregate with fir x width x thickness = 300 oboro x 300 tai x 50 #, and a specific gravity of 0.05 g/cffl. After producing the mat fiber aggregate, approximately wedge-shaped grooves with a depth of 30 mm were formed at intervals of 50 mm on one side of the mat fiber aggregate in the tangential direction of the fir tree, using known means, and carbon was spray-coated using the same method. A radio wave absorber having pyramid-shaped protrusions 12 as shown in FIG. 2 was manufactured. The radio wave absorber produced in this way was
Using the measurement system shown in the figure, the return loss of radio waves was investigated in the frequency band of 2.8 to 4.4 GHz in the same manner as in Example 1. Isamu Shikasa Various fibers of the same type as in the above example were intertwined in the same ratio, and the specific gravity was set to a constant fiber density of O-05g/ci, and a thickness of 50% was obtained.
m++, tu X n = 300 mm X 300
A sample of mm was prepared. The fibers were coated with carbon in the same manner as in the previous example, and the resistance was set at 2 to 4 Ω when measured with a tester for 50 minutes at the center of the sample. The test radio wave absorber manufactured in this way was subjected to the third test in the same manner as above.
The return loss was investigated in the frequency band of 2.8 to 4.4G)1z using the measurement system shown in the figure. FIGS. 4 and 5 show the radio wave absorption characteristics of the samples produced in Example 1.2 and Comparative Example, respectively. As is clear from this figure, the return loss of radio waves at any frequency was 20 dB or more, confirming that it is an extremely good radio wave absorber. Furthermore, as is clear from comparison with the comparative example, the product of the present invention provided with tapered protrusions or protrusions,
It was confirmed that the product had superior radio wave absorption characteristics compared to the comparative example product with a flat radio wave incident surface. . Although not shown in the figure, in the frequency band of 4.4 GIIz or higher, both the actual survey 1 and 2 and the comparative example product had excellent radio wave absorption characteristics (although the comparative example product was inferior, both Both had a return loss of 20 dB or more). Therefore, the product of the present invention exhibits good radio wave absorption characteristics even in the low frequency band near the lowest absorption frequency (3 GHz because the thickness of the absorber is 50 m+), and has excellent radio wave absorption over a wide band compared to the comparative example product. It has been revealed that this material is a radio wave absorber with special characteristics. [Effects] As explained above: According to the radio wave absorber of the present invention, since it is composed of artificial fibers made of polar polymer, each fiber itself has a constant diameter, and various types of fibers can be easily obtained. is,
Since fibers of uniform quality can be used, carbon can easily adhere to it, and it is easy to form a carbon film with a uniform thickness, a radio wave absorber with excellent radio wave absorption properties can be realized. . In addition, the radio wave absorber of the present invention has a convex strip or protrusion formed on its lower surface, and when used with the convex surface facing the radio wave incident direction, the apparent conductivity of the radio wave incident surface of the upper absorber Because the amount of radio waves is low, the amount of radio waves reflected on the incident surface is originally small, and the radio waves reflected by one protrusion or protrusion are eventually absorbed by other protrusions or protrusions, so it is extremely good. It becomes possible to achieve radio wave absorption characteristics.

[Brief explanation of the drawing]

1 and 2 are perspective views showing the external appearance of one embodiment of the radio wave absorber of the present invention, FIG. 3 is a schematic diagram showing a measurement system for determining radio wave absorption characteristics, and FIGS. FIG. 5 is a characteristic diagram showing the radio wave absorption characteristics of the radio wave absorbers shown in FIGS. 11 and 2, respectively, determined by the measurement system shown in FIG. F...Fiber, 1...Joining portion, 11...Convex strip, 12
... Convexity, 2.21 ... Pone antenna, 3 ... Detector, 4 ... Metal plate, 5 ... Test radio wave absorber, T.
...Transmission, R...Receiving gutter patent application Person: Representative Director, Mitsubishi Cable Industries, Ltd. Yuki Jozo Figure 3, 4rIj Shield Wave. Number [GH! ] Fig. 1 Fig. 5 (eHxl

Claims (1)

[Claims] A radio wave absorber made of a mat-like fiber aggregate coated with carbon and made of polar polymer fibers of at least two different thicknesses intertwined with each other, and one surface of the mat-like fiber aggregate is provided with a tapered convex strip or A radio wave absorber characterized by having a plurality of protrusions formed.