US3720951A

US3720951A - Microwave absorbing wall element

Info

Publication number: US3720951A
Application number: US00093710A
Authority: US
Inventors: Y Naito
Original assignee: TDK Corp
Current assignee: TDK Corp
Priority date: 1970-05-11
Filing date: 1970-11-30
Publication date: 1973-04-03
Anticipated expiration: 1990-04-03

Abstract

This invention provides a thin microwave absorbing wall member using a ferrite plate of which complex permeability is represented substantially by a formula:

Description

United States Patent Naito 14 1March 13, 197 3 1 1 MICROWAVE ABSORBING WALL 3,596,270 7 1971 Fukui ..343/I8 A ELEMENT 3,187,331 6/1965 Beller ..343/l8 A 3,308,462 3/1967 Gluck ..343/l8 A [75] lnvemcr- Ymmyuk Japan 3,540,047 11 /1970 Walseret al ..343/l8 A [73] Assignee: TDK Electronic Company, Limited,

Tokyo, Japan Primary Examiner-Benjamin A. Borchelt Assistant Examiner-G. E. Montone 22 Fl d: No 30 1970 1 l e v Attorney-Pierce, Scheffler & Parker [21] Appl. No; 93,710

[57] ABSTRACT [30] Foreign Application Priority Data This invention provides a thin microwave absorbing May 11, 1970 Japan"; ..45 40410 member using a ferrite P1ate 0f which P permeability is represented substantially by a formula: [52] US. Cl. ..343/18 A [5i] Int. Cl. ..H0lg 17/00 I =1+ K161 [58] Field of Search ..343/18 A, 18 R 1+(jfr) [56] References Cited 3 Claims, 9 Drawing Figures UNITED STATES PATENTS 3,l9l,l32 6/1965 Mayer ..343/l8A II 7 I/ MM PATENTEU MR 1 3 I975 3, 720.951 SHEET 10F 4 (iii) PLANE WAVE INCIDENCE PATENTED MR 1- 3 I975 SHEET 4 BF 4 m0 NO MICROWAVE ABSORBING WALL ELEMENT This invention relates to improvements in a microwave absorbing wall member using magnetic materials.

The microwave absorbing wall member according to the present invention is used, for example, in an anechoic chamber required to measure the characteristics of devices for electromagnetic waves such as antennas.

In measuring the characteristics of an antenna or the like, it is desirable that there should be no electromagnetic wave at all from sources other than the device to V be measured. Therefore, it is usual to carry out the experiment on a roof or in a field instead of carrying it out in a room. However, even in such places outside the room, waves reflected by the buildings and the ground may disturb the experiment or the weather may prevent the experiment from being carried out. Therefore, microwave absorbing walls have been invented and anechoic chambers in which such absorbing wall members are put on the peripheral walls have come to be utilized.

However, recently, with the rise of the availability of such microwave absorbing walls, the frequencies to be used have come to be lower. The thickness of such microwave absorbing wall is different depending on the reflection coefficient and proposed frequency band of the wall. Thus, the wall thickness required for frequencies low in the VHF degree will be more than 1 meter. Further, there is a drawback that, when the characteristics of the microwave absorbing wall are to be improved within the same frequency band, the required wall thickness will have to be increased. The present invention has been suggested to remove such defects as discussed above.

A main object of the present invention is, therefore, to provide a microwave absorbing wall member for low-frequency applications which is thin and yet having" excellent absorbing characteristics.

Another object of the present invention is to provide a microwave absorbing wall element which is adapted to easy manufacturing of the ferrite as the material for the member.

Other objects and advantages of the present invention will become apparent from the following detailed description of a preferred embodiment of the invention in conjunction with reference to the accompanying drawings, in which:

FIGS. 1 and 2 are curve plots showing the manner in which the permeability of magnetic materials varies as a function of applied wave frequency.

FIGS. 3 to 6 are explanatory diagrams.

FIG. 7 is a sectioned view of the microwave absorbing wall member according to the present invention.

FIG. 8 is a diagram showing input impedance with respect to plane wave incidence.

FIG. 9 is a diagram showing permissible reflection coefficient.

When the frequency characteristics of the complex permeability i, "'jl (where p, is the real part and t," is the imaginary part) in the magnetic materials are measured, such measurements are generally classified into two cases as shown respectively in FIGS. 1 and 2.

Generally speaking, the characteristics shown in FIG. 1 is seen in the case of sintered ferrite or the like, in which case the value of t, will show such a phenomenon that the same rises once larger than f= (in the case of a direct current) and subsequently falls down gradually. This is generally called the resonance type phenomenon.

0n the other hand, the type that is shown in FIG. 2 occurs in the case of such ferromagnetic materials as those which consists of a powdered carbonyl iron, sendust or sintered ferrite mixed in a proper supporter of, for example, vinyl chloride butyl, neoprene, epoxy resin or the like (these materials are called generically the rubbar ferrite). The value of p, in such case will be 7 the largest at the time when f '0' and will become gradually smaller as the frequency increases. This type of phenomenon is called the relaxation type. In this case, the loss term ;1.,'' will take the forms as shown in FIGS. 1 and 2, but it is seen that there is a difference between them if these forms are observed in detail. This is for the reason that the magnetization mechanisms or loss mechanisms of those magnetic materials of which characteristics are respectively shown in FIGS. 1 and 2 are different from each other.

The term ;1.,' or a," of the sintered ferrite as represented in FIG. 1 has been called the resonance type. However, the same is actually not the pure resonance type, but rather the sum of the relaxation type as shown in FIG. 2 and the pure resonance type will take the form of each curve shown in FIG. 1. This shall now be explained in the following.

First, the complex permeability it, of the pure resonance type magnetic material is determined according to the following formula:

l mf

here,

f: frequency f,,: resonance frequency K,,,: m l where the direct current initial permeability u, is a value at the time off= 0 r coefficient which corresponds to loss Next, the complex permeability p, of the relaxation type material is determined according to the following formula:

relax iidm (2) K mf 0 K elax Next, it shall be considered which one of the above formulas (l) to (3) will explain those curves in FIGS. 1 and 2.

Since it is rather difficult to understand these formulas as they stand, the inverse number (fl,.- l)" of (;1, 1) shall be first considered. This inverse number shall be referred to hereinafter as inverse cole-cole plot, which is also represented by a formula 1 1)" g jh) The case of the above formula (1 will be the curve as shown in FIG. 3, whereas the case of the formula (2) will be a straight line such as shown in FIG. 4. In these FIGS. 3 and 4, the abscissa represents g and the ordinate represents h. Therefore, in the case when the inverse cole-cole plot is a straight line, the equation of ;2.,(f) will take the form of the formula (2), which is considered to be a pure relaxation type.

On the other hand, in the case of the sintered ferrite which corresponds to FIG. 1, its inverse cole-cole plot will be as shown in FIG. 5. The curve shown in FIG. 5 shall be considered by dividing the same into three parts (i), (ii) and (iii). Since the part (iii) has generally a higher frequency and for this reason this part is not utilized for the microwave absorbing wall, the explanation of this particular part shall be omitted. It is seen that the part (ii) is substantially of a straight line, that is, this part can be approximated to with the part of the formula (2). As regards the part (i), it is seen that this part (i) is represented by the type of the formula (3), that is, by the pure resonance type and the pure relaxation type. An extent to which the actual ones of the characteristics can be approximated will be seen in FIG. 6. This FIG. 6 shows diagrammatically the characteristics of a microwave absorbing wall element consisting of Cu-Mg-Zn series ferrite. In the drawing, the abscissa represents frequencies and the ordinate represents 1., and .1. Further, the solid lines therein show experimental values and the broken lines show theoretical values. The portion of this Cu-Mg-Zn series ferrite which can be utilized for the microwave absorb: ing wall element is, in FIG. 6, that off= 340 MHZ, around which portion the loss of the second term of the formula (3) will become small atf' and, therefore, the same is smaller than the loss of the third term (which becomes small when 3 l in f Therefore, the resonance loss in the actual ferrite can be ignored.

From the foregoing, it is seen that the microwave absorption is accomplished by the loss of the relaxation type represented by the formula (2), not only the case of the material having characteristics shown in FIG. 2, but also in the material of characteristics in FIG. 1. The wall member having almost the same characteristics as those in the case of ferrite can be obtained by using, for example, even the carbonyl iron. In order to obtain a thinner microwave absorbing wall, it is known to so select f and 7' as to be both large at the frequency where the second and third terms in the formula (3) absorb the microwaves. However, the current manufacturing art of the ferrite has not been sufficiently developed for enabling it possible to accomplish such method.

FIG. 7 shows an embodiment of the microwave absorbing wall member according to the present invention, which is constituted by a magnetic material plate 1 and a backing plate 2 of an electrically conductive metal applied to a surface of the plate 1. Examples of this embodiment are as follows:

a. Using a Ni-Cu-Zn series ferrite having a composition in mol percent:

Fe,0, 43.30 NiO 16.16 CuO 3.51s ZnO 30.74 141100, 1.277

a microwave absorbing wall member of the thickness 5.3 mm at the ferrite part was made. Its reflection coefficient was less than 0.1 in the frequency range of 540 MHZ.

b. Using a Mn-Cu-Zn series ferrite having a composition in mol percent:

Fe,0 54.53 ZnO 17.39 MnCO; 26.01 CuO 2.06

a microwave absorbing wall member of the thickness 12.1 mm at the ferrite part was made. Its reflection coefficient was less than 0.1 in the frequency range of 74-185 MHZ.

c. Using a Ni-Zn-Cu series ferrite having a composition in mol percent:

Fe O 48.54 N10 28.87 CuO 5.019 ZnO 16.01 PhD 1 .502 C00 0.4

Talc 2.0

a microwave absorbing wall member of the thickness 4.3 mm at the ferrite part was made. Its reflection coefficient was less than 0.1 in the frequency range of 1,000 1,800 MHZ.

d. Using a Cu-Mg-Zn series ferrite having a composition in mole percent:

mo, 47.42 CuO 9.76 M co, 21.66 ZnO 20.86 CoO 0.3024

a microwave absorbing wall member of the thickness 11.0 mm at the ferrite part was made. Its reflection coefficient was less than 0.1 in the frequency range of 480 1,000 MHZ.

Next, an explanation shall be made with reference to how the thickness d of the magnetic material should be determined with respect to the frequency of microwaves in the case of such microwave matching member or microwave absorbing wall member as shown in FIG. 7, wherein the microwave is to be caused to be incident upon the member at the side of the magnetic material 1 having at its one surface an electroconductive backing metal 2. In the case when a complete microwave absorption is desired, the input impedance with respect to plane wave incidence of FIG. 7 is measured. The input impedance will be as diagrammatically shown in FIG. 8 in general. In the drawing, solid lines represent those locia in the cases when the frequency f is constant, and broken lines represent those locia in the cases when the thickness d is constant. It will be seen that both f f,,, and d d are determined by observing those places where the input impedance locia pass through the point at which the reflection coefficient S 0 is satisfied.

where K In the case when, on the other hand, it is impossible to accomplish the complete absorption, it may be accomplished by so determining f and d that the input impedance track will be in a circle of the permissible coefficient SO drawn as shown in FIG. 9.

What is claimed is:

l. A microwave absorbing wall member constituted by a ferrite plate applied to a backing plate of electrically conductive metallic material, wherein the complex permeability of said ferrite plate is given substantially by a formula where the direct current initial permeability IL is a value at the time of f 0 'r: relaxation time B: a constant showing relaxation time distribution.

2. A microwave absorbing wall member as defined in 7

Claims

1. A microwave absorbing wall member constituted by a ferrite plate applied to a backing plate of electrically conductive metallic material, wherein the complex permeability of said ferrite plate is given substantially by a formula where Krelax : Mu i 1 where the direct current initial permeability Mu i is a value at the time of f 0 Tau : relaxation time Beta : a constant showing relaxation time distribution.

2. A microwave absorbing wall member as defined in claim 1 wherein said complex permeability of the ferrite plate is given substantially by a formula where fo : resonance frequency r : coefficient corresponding to loss Km : Mu i - 1 in the case when f 0.