JPS641080B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a radio wave absorber, and particularly to a radio wave absorber effective for microwaves. FIG. 1 shows an example of the structure of this type of radio wave absorber, where 1 is a metal plate and 2 is a radio wave absorbing layer. As the radio wave absorbing layer 2, epoxy resin or rubber mixed with ferrite, carbonyl iron, etc. is used. With such a configuration, the material constant and thickness of the absorption layer 2 are appropriately selected, and the surface reflected wave b caused by the incident wave a and the radio wave b' (second If the second reflected wave and the second reflected wave are made to have equal amplitudes and a phase difference of 180° on the surface of the absorption layer 2, the reflection of electromagnetic absorption will be suppressed and it will work well as a radio wave absorber. As is clear from the above explanation, this type of radio wave absorber has (1) moderate reflection (50% or less) on the surface, and (2) attenuates the electromagnetic waves that enter the absorber and the surface of the absorber. (3) ensuring that the amplitudes are equal;
It is required that the phase of the secondary reflected wave differs by 180° from the phase of the surface wave due to phase changes due to propagation within the absorber. The characteristics of such a radio wave absorber are given by the complex magnetic permeability (μ〓), the complex dielectric constant (ε〓), and the thickness (d) of the absorber at a predetermined frequency. That is, it is known that if the complex reflection coefficient of the absorber is Γ〓, it can be given by the following formula. however

[Formula] λ; Free space wavelength μ〓=μ′−iμ″ ε〓=ε′−iε″ Therefore, by substituting the material constant at each frequency into equation (1), the reflection coefficient can be determined. However, this does not mean that the optimal thickness of the absorber
It is difficult to know the limiting frequency etc. of a radio wave absorber from the perspective of frequency characteristics and material constants. Therefore, mechanically

【formula】

It is known that ε^'' hardly contributes to a thin absorber given by [Formula], so if this is omitted, the following formula is obtained. Next, put y≡ε^′/μ^′ and y≡μ^″/μ^′ (2
) is further transformed to obtain the following equation. What is needed in equation (3) is the absolute value of the reflection coefficient, so we rationalize it and plot the vertical axis of the logarithmic graph paper as y,
If x is plotted on the horizontal axis and a region where |Γ〓| is below a certain value is found, a closed loop as shown in FIG. 2 can be obtained. Figure 2 shows a chart in which nickel zinc ferrite with a volume ratio of 0.4 is dispersed in an epoxy resin. The parameter fd in the figure is the product of the frequency f and the thickness d of the absorber, and corresponds to kd in equation (3), and the unit is G.
It is Hz・mm. Each closed loop corresponding to the parameter fd exhibits a range of Γ-15 dB. The straight line extending diagonally is an approximate line of the locus of Γ=0 when fd is changed. The marks for each frequency from 3 GHz to 12 GHz are shown in the figure by converting the material constant of the absorber at each frequency into ε^'/μ^' and μ^''/μ^''. FIG. 2 shows that if the material constant is Γ -15 dB (in a closed loop), it will operate as an absorber with a reflection amount of -15 dB or less. For example, the 6GHz point is
Since it is included in fd=25, the thickness d of the absorber
If d = 25/6 = 4.17 mm, the absorber will have a reflection amount of -15 dB or less. This thickness is only 1/12 of the free space wavelength of 50 mm, which is extremely thin compared to the length of general absorbers, which are approximately one wavelength long. However, since the material constant at 12 GHz is outside the -15 dB region, it is clear that an absorber below -15 dB cannot be constructed. In this way, according to the chart shown in FIG. 2, it can be easily determined whether the material can be used as an absorber depending on whether the material constant exists within or outside the region. Further, according to the chart shown in FIG. 2, it is possible to predict the frequency band. For example, in Figure 2, the material constants at 5.5GHz are in the region of fd = 25,
That is, it is expected to exist in the area indicated by the arrow in the figure. Therefore, the expected optimal thickness d of the absorber is d=25/5.54.5 mm. If we increase the frequency to 4GHz while maintaining this thickness, fd=4.5×
4=18. Therefore, the material constant at 4GHz is
It is within the region of fd=18, and although the region of fd=18 is not shown, it is expected to be located near fd=20. On the other hand, at 7.5GHz it is fd34 and fd
It is expected to be located near the area of =35.
The material constant in these frequency bands is −15 dB
However, as is clear from Figure 2, it is not possible to suppress reflection to below -15 dB with a thickness of 4.5 mm in other frequency bands. The chart explained above is a patent application filed in 1973 by the applicant.
- This is detailed in No. 129944. The present invention is a further development of Japanese Patent Application No. 54-129944, and it is possible to determine the center frequency of the absorber (the frequency at which the amount of radio wave absorption is maximum) by adjusting only the thickness of the radio wave absorber. It is an object of the present invention to provide a microwave radio wave absorber having a composition, thereby making it possible to simplify the manufacturing process of the radio wave absorber, improve the yield, and reduce the manufacturing cost. The features of the present invention to achieve this objective are:
In a microwave absorber having a radio wave absorbing layer made of rubber or plastic in which a magnetic material is dispersed and a metal plate provided on one surface of the layer, the magnetic material is ferrite and carbonyl iron, and the volume ratio is smaller than that of the rubber or plastic. In the microwave absorber, the radio wave absorbing layer is formed by dispersing and mixing 0.15 to 0.35 ferrite and 0.1 to 0.25 carbonyl iron. Examples will be described below with reference to the drawings. FIG. 3 is a chart when nickel zinc ferrite is dispersed in epoxy resin, and υ in the figure indicates the volume ratio of nickel zinc ferrite to epoxy resin. The straight line indicated by the dashed line in the figure is fd
This is an approximation line showing the locus of the point where the reflection coefficient Γ=0 when changing . As is clear from FIG. 3, the frequency characteristics of the material constants behave as if partially parallel to the locus of Γ=0, so it can be seen that it can be used as a broadband absorber. FIG. 4 is a chart when carbonyl iron is dispersed in epoxy resin, and υ in the figure indicates the volume ratio of carbonyl iron to epoxy resin. As is clear from this chart, the material constant in the case of carbonyl iron changes almost orthogonally to the locus of Γ=0, so it can be seen that the absorber essentially has a narrow band. By the way, in a ferrite-based absorber, as shown in Figure 3, x≡ at a given frequency of 9 GHz or higher, regardless of the volume ratio of ferrite.
μ^''/μ^' decreases rapidly. On the other hand, in the absorber containing carbonyl iron, x≡μ^''/μ^' does not decrease at a given frequency as shown in FIG. Therefore, if carbonyl iron is mixed into a ferrite-based absorber to form an absorber, the frequency characteristics can be changed to Γ
Since it is possible to approach the locus of =0, it is possible to obtain a good absorber. Figure 5 is a chart based on this perspective.
The case is shown in which nickel zinc ferrite with a volume ratio of υ = 0.18 and carbonyl iron with a volume ratio of υ = 0.22 are dispersed in an epoxy resin. As is clear from the figure, the frequency characteristics of the material constants move along the locus of reflection coefficient Γ=0 (indicated by the broken line), indicating that the material behaves well as an absorber. FIG. 6 shows the frequency characteristics of an absorber constructed by dispersing nickel zinc ferrite and carbonyl iron in chloroprene rubber at the same volume ratio as in FIG. 5, using the thickness t of the absorber as a parameter.
As shown in the figure, the center frequency of the absorber (the frequency at which the amount of radio wave absorption is maximum) changes according to its thickness. Therefore, by simply adjusting the thickness without changing the composition of the absorber, the center frequency can be changed. It can be seen that adjustment is possible. Therefore, the conventional method of determining the center frequency by adjusting both the amount of ferrite and the thickness of the absorber can be eliminated, and the center frequency can be determined by simply adjusting the thickness of the absorber. becomes possible. When such characteristics are achieved, the amounts of ferrite and carbonyl iron are not limited to the above-mentioned volume ratio, but have a certain range defined by the charts in FIGS. 3 and 4. In other words, if the frequency range is 4 GHz to 12 GHz, a good absorber cannot be expected unless each material constant in 4 GHz to 12 GHz falls inside the solid line indicating the reflection amount -20 dB. According to the chart in FIG. 4, the volume ratio of ferrite and carbonyl iron is determined so that the material constant is within the region of -20 dB.
In other words, the material constant when ferrite and carbonyl iron are mixed in a dispersed manner is the addition of the material constant when ferrite is mixed (Fig. 3) and the material constant when carbonyl iron is mixed (Fig. 4). The range of volume ratios of ferrite and carbonyl iron for which the material constant is within the range of -20 dB when dispersing and mixing carbonyl iron is from a combination of ferrite with a volume ratio of 0.15 and carbonyl iron with a volume ratio of 0.25 to a combination of ferrite with a volume ratio of 0.35. It can be seen that it is a combination of ferrite and carbonyl iron at a volume ratio of 0.1. Therefore, if this composition is prepared, the center frequency of the absorber can be determined simply by adjusting its thickness. Although the above description has been made using nickel zinc ferrite as the ferrite, it is also possible to use manganese zinc ferrite or lithium ferrite, which have high insulating properties. In particular, the former has hardly been used in the past, and the present invention makes effective use of its properties. As explained above, according to the present invention, the center frequency can be determined simply by adjusting the thickness of the absorber, which has a great effect on simplifying the manufacturing process, improving yield, and reducing manufacturing costs. Obtainable.

[Brief explanation of the drawing]

Figure 1 is an example of the structure of a microwave absorber, Figure 2 is an explanatory diagram of a chart, Figure 3 is a chart in which nickel zinc ferrite is dispersed in epoxy resin, and Figure 4 is a diagram in which carbonyl iron is dispersed in epoxy resin. FIG. 5 is an example chart of the microwave absorber according to the present invention, and FIG. 6 is an example of the frequency characteristics of the microwave absorber according to the present invention. 1: Metal plate, 2: Radio wave absorption layer.

Claims (1)

[Scope of Claims] 1. A microwave absorber comprising a radio wave absorbing layer made of rubber or plastic in which a magnetic material is dispersed and a metal plate provided on one surface of the layer, wherein the magnetic material is ferrite and carbonyl iron, A microwave absorber characterized in that the radio wave absorbing layer is constituted by dispersing and mixing ferrite in a volume ratio of 0.15 to 0.35 and carbonyl iron in a volume ratio of 0.1 to 0.25 to rubber or plastic. 2. The microwave absorber according to claim 1, wherein the ferrite is nickel zinc ferrite. 3. The microwave absorber according to claim 1, wherein the ferrite is a highly insulating manganese zinc ferrite.