DE1014616B - Adaptation layer with a high refractive index for the reflection-free transition of electromagnetic radiation from one medium such as air to another medium - Google Patents

Description

.Adapting layer with a high refractive index for the non-reflective Transition of electromagnetic radiation from one medium such as air to another Medium The invention relates to a high refractive index matching layer for the reflection-free transition of electromagnetic radiation from a medium like air in another medium, the layer also acting as an absorbent layer can be formed.

It has been proposed to use a non-reflective material Embedding of highly permeable iron in a binder made of ortho- and disilicates to produce, as an embodiment, a substance with a value of the effective Permeability 20 and the effective dielectric constant 25 is called. Here the adaptation condition is thus approximately fulfilled.

In contrast, the invention solves the problem of achieving As thin as possible, highly effective layers of the type mentioned have as high a level as possible Value of the effective permeability and dielectricity.

It is also a body that absorbs electromagnetic radiation has been proposed that of substances of high dielectric constant such as titanium oxide and composed of materials of high permeability such as ferromagnetic metals is. In this case it is advisable to use a few dielectric substances bound and has no way of doing this for the desired purpose, z. B. also in terms of ease of processing, the most suitable material as a dielectric to choose.

The invention consists in that a layer of the aforementioned Kind of a ferromagnetic of great permeability #ts, preferably in powder form in Finely and evenly distributed as possible, in a dielectric with a low dielectric constant EB contains, whereby the fill factor of the ferromagnetic should be as high as possible and all ferromagnetic particles are separated from each other by the dielectric should. Such a layer can also be absorbent by using known per se Substances be designed as an absorption layer. The layer can be used for camouflage serve against the location of an object by electromagnetic waves; this application is described in more detail below.

Absorbent layers, which, however, have relatively low values of the effective Have dielectric and permeability are known per se, e.g. B. such from Foam with embedded carbon. However, these layers have to be in order to achieve a to ensure good adaptation, be made relatively thick. The layers after In contrast, the present invention, since it has high values of the effective dielectricity and have permeability, very effective in terms of being even very thin to be made for good adjustment.

For the behavior of a homogeneous layer towards electromagnetic In addition to their geometric shape and size, waves are primarily four material constants decisive: the dielectric constant e, the permeability, u, the electrical Loss factor tg 8 (), the magnetic loss factor tg 8 (, u).

The total losses of an electromagnetic passing through the layer In a known way, waves are made up of electrical and magnetic waves Losses together.

The refractive index n of the layer is due to neglecting the losses given. The wavelength A, of electromagnetic radiation in the layer is A, = 4 / n, where A, = cl f is the wavelength in a vacuum or also in air, c is the speed of light in a vacuum and f is the frequency of the radiation.

The term homogeneous is understood here to mean that any discontinuities of the layer material have an extension which is at most equal to half the wavelength of the radiation is within the layer, so that the layer with respect to this radiation acts like a uniform material.

Regarding the geometric shape and size, it is only assumed that the expansion of the layer in two dimensions is large compared to the wavelength im Vacuum is; the expansion in the third dimension, i.e. the thickness, is unlimited, but will with most Applications in the order of magnitude of the wavelength lie in the layer.

From interference phenomena as a result of reflection on the back of the Layer is ignored in the following considerations, even if it is also in the practical application must be taken into account in the manner familiar to the person skilled in the art have to.

Of particular interest is the behavior of the electromagnetic Waves on the face of the layer that is adjacent to the propagation medium.

As is well known, an electromagnetic wave can only be reflection-free at an interface between two media with the material constants ei,, u1, tg b (El), tg 8 (, u1) and e2, JC62, tg a (s2), tg 8 (, u2) happen when the following conditions are met: El // @ ß @ l = --2 / ß2 tg S (El) tg s 4 '' ° 1) = tg a (E2) / tg S (i'42). In the following it should be assumed, for example, that one medium is air, although the invention is not restricted to this case.

With sufficient approximation, the same constants apply to air as to the vacuum, i.e.: For the reflection-free or low-reflection adaptation of a layer to air, it must be required that the electrical and magnetic losses of the layer on the front side are as equal as possible and that the ratio a = elu, referred to below as adaptation ratio, is as close as possible to 1.

If one initially assumes that the first condition is fulfilled by the fact that the layer on the front side is loss-free, then the reflection factor r of the interface against air, based on the amplitude of the electric or magnetic field strength e or 55 through the adaptation ratio a determined as follows: It turns out that it is not possible in principle to achieve the value a = 1 in this way, but that this limit value can be approached very well.

The ferromagnetic can, for example, iron, nickel or an alloy derr two metals such. B. Permalloy, Mumetall od. Like. Be. The distribution of the ferromagnetic in the dielectric can outwardly be the same as in the production of the so-called high-frequency iron.

Because of the eddy current losses and those that occur at high frequencies To displace the current, the ferromagnetic particles must be kept as small as possible in order not to result in excessive magnetic losses. But also the permeability jus will eventually decrease more or less sharply at very high frequencies, if this effect cannot be achieved by sufficiently reducing the size of the individual particles Part is suppressed.

The absolute lower limit of the particle size is the smallest Elementary area that still shows ferromagnetic properties.

In the context of the invention, especially at very high frequencies of the incident Radiation appropriate size of the ferromagnetic particles should be as close as possible the size of such an elementary range to be also at these frequencies sufficient permeability to guarantee u3.

In order to obtain a high effective permeability, ueff of the mixed body, the filling factor F related to the ferromagnetic should be as high as possible. Fill factors of 97 % can easily be achieved according to the experience gained in the manufacture of the so-called high-frequency iron.

The dielectric then only takes up a volume of 3% by it coats the ferromagnetic particles in the form of a thin film and like a Binders together.

With a further increase in the fill factor, metallic ones finally appear Contact between neighboring particles and possibly chain formation on that As is apparent to the person skilled in the art in detail, on the losses, the effective Impact permeability and the effective dielectric constant of the mixing body would and must be avoided in any case.

The effective permeability, ueff, of such a mixed body depends on the permeability y3 of the individual particles, but is much more dependent on the fill factor. As a result of the demagnetization factor, the effective permeability through the dielectric interspaces is greatly reduced and is below 100 for filling factors of 90 to 97 ° / o a further increase in permeability y3 is not increased by more than a few percent. For F = 97% z. B. already a permeability of = 3000 , while an increase from ß3 to the theoretical value ß3 = oo only brings an increase of »eff by a few percent.

This fact is significant in that, firstly, one does not necessarily must use a ferromagnetic material of the highest initial permeability, but this also according to other aspects, e.g. B. after the manufacturability of a fine-grained Powder, can choose.

On the other hand, the permeability increases, u3 of a ferromagnetic value of the initial permeability As with increasing magnetic field strength S,) in general strongly and reaches a maximum permeability, which in some cases is up to 10 times as high lies like the initial permeability, and then again at even greater field strengths 5 to gradually decline. The above-mentioned task therefore requires a substance that consists of Reasons for adapting to the adjoining medium from which the waves are incident and in order to achieve the lowest possible layer thickness, they are identical to one another and has the highest possible values of dielectric constant and permeability.

Substances with a high dielectric constant, so-called dielectrics, just like substances with high permeability, so-called ferromagnetics, are in large Number and variation available.

In contrast, there are no such suitable substances available for which the adaptation ratio a = a /, u ps is 1; but in general a is significantly greater than 1. But a value of a = 4 already means a reflection factor of 330 / " while practical applications generally only allow a reflection factor of 100 /" or less.

For the purpose of solving the problem formulated at the outset, the invention aims to build a homogeneous mixed body from a dielectric and a ferromagnetic in such a way that it receives the highest possible effective dielectric constant eeff and the highest possible effective permeability, ueff, and that the adaptation ratio a = Eefflueff is as small as possible, ie as close as possible to 1. However, the effective permeability reff of the mixing body changes with the field strength .5 by only a few percent, so that one can count on a constant value. The effective dielectric constant seff of the mixed body is greatly increased by the embedded metal particles and therefore also depends on the fill factor; on the other hand, it is directly proportional to the dielectric constant aB of the binder.

The adaptation ratio a = seff / @ ceff is, as can be shown theoretically, equal to the dielectric constant sB of the binder and multiplied by a factor dependent on the fill factor, which tends asymptotically to 1 for high fill factors and high ß.9. For fill factors above 90 ° / o and, u,> 3000, a = sB is a good approximation.

This means that for a dielectric constant of the binder 8s = 1.4 ... 2.0, an adaptation ratio of a = 1.4 ... 2.0 can be achieved, so that, according to the above considerations, the reflection factor: y = 8.4 ... is 17.1. In order to keep the reflection factor small, it is necessary to use a binder with a dielectric constant 8B that is as low as possible.

The refractive index n of the mixing body is So if you reach z. B. with a fill factor F = 97 % and a binder with sB = 1.4 an effective permeability #teff = 100, then: n = 118. The closer the dielectric constant of the binder eB is to 1, the smaller the reflection factor . However, if binders with a dielectric constant es <2 are not available, the reflection factor of the layer can be further reduced according to the invention by the following measure.

The one built up from the powdery ferromagnetic and the binding agent Layer is traversed in a net-like manner by air slots of width ö, so that between the slots prisms of the mixing body remain.

The slots preferably form an orthogonal and equidistant one Network, so that the prisms have a square cross-section of edge length a.

The slot width ö is small compared to the edge length a, so that the Network is periodic in a + ö, or approximately in a.

So that the arrangement remains homogeneous in the sense of the above definition, the distances a + δ or a must be less than half a wavelength of the radiation in the arrangement. The wavelength A in the arrangement results, as explained above, from the vacuum wavelength 2 and the mean refractive index n of the slotted layer as follows: A = A, / n. So it should be: a < .1, / 2n.

The dielectric constant and permeability are determined by the air slots the layer is greatly reduced, which in the event that the magnetic field strength .5 and the electric field strength l2 run in the direction of the slots, easily can be seen. But it can be shown that for any angle between the field vectors and the direction of the slots result in a decrease in the dielectric constant and permeability takes place. The net-like slit layer has a middle one Dielectric constant s and an average permeability, u, each of which depends on the effective dielectric constant ff or the effective permeability V.eff the mixed substance and the ratio of the edge length a to the slot width ö, thus depend on the value x = a / ö.

It can be shown that for a given seffund #teff, where sefflt.eff = a> 1 , with decreasing x, e.g. B. increasing slot width, the mean dielectric constant E and the mean permeability -u decrease more and more and finally approach the value a. As a result, of course, the mean refractive index n = also increases more and more from. At the same time, however, the reflection factor y is also reduced, since the dielectric constant is reduced more than the permeability, so that the adaptation ratio ä = ä /. closer and closer to the value 1.

By choosing the slot width δ or the quotient x, one has a means in hand to lower the reflection factor as desired, in which case, however, the refractive index drops. As is apparent from the following numerical example, but can be reached by the inventive use of louvers still considerable values of n at a relatively small reflection factors, the mixing substance possess - without slots - a reff = 200 and #teff = 100, so that a = 2 and y = 17.10 / 0, where n = 141.

By introducing an orthogonal equidistant slot network with a = 8 mm and ö = 1 mm, s 7.8 and _ - 7.5 are achieved, so that ä - 1.04, r Fe 0.9 % and n. @ 7 , Turns 65.

According to the above considerations, this layer is for vacuum wavelengths / LO> 14 cm to be regarded as homogeneous, with the wavelength in the slotted layer ., = Ao / n> 1.8 cm.

One will of course, in practice, come from the smallest in question Choose the wavelength of the radiation, the ratio x as small as possible, in order to achieve a to obtain high refractive index n, but only so small that the allowable reflection factor y is not exceeded.

The value x is on the front of the layer, i.e. on the The side on which the radiation is incident should be selected to be small in order to have a small reflection factor there but this value depends on the depth of the layer, i.e. in the direction of propagation to let the incident radiation grow continuously or in small steps, by changing the slot width ö and / or the edge length a accordingly. As a result, the refractive index n increases towards the depth, so that the entire layer has a higher refractive index on average than at the front.

The adjustment ratio ä thus also increases. How yourself can be shown theoretically and practical measurements have resulted in this increase of ä, if it is not too steep, no significant increase in the reflection factor the arrangement. If the ascent from ä becomes too steep with depth, then eventually step reflections on the inside of the layer.

You can follow the described layer - with or without slits a further inventive idea for the absorption of a more or less large Use part of the radiation by making it more suitable by choice or addition Substances are certain electrical and magnetic losses. You just have to do it ensure that the adjustment condition mentioned above is met at the front of the layer is fulfilled with sufficient accuracy, Electrical and magnetic dissipation factor tgb (e) or tgb (@C) should be the same as possible on the front.

There are two ways of doing this. Either the electric and made the magnetic losses at the front very small. Then let one or both loss factors increase in a similar way towards depth, as described above for the adaptation ratio a, and avoids by choosing a not too steep increase in losses, the occurrence of impermissible high reflections from inside the layer. Reducing losses at the The front side is facilitated by the introduction of the air slots according to the invention. The change in the slot width proposed in the foregoing acts with the depth of course not only on the adjustment ratio, but also on the losses the end.

The second option is to use the electrical dissipation factor tg8 (s) should be as close as possible to the magnetic tg8 (#t), at least on the front. Then it is not necessary to keep the losses at the front as low as possible to keep, but you can now also have high losses at the front of the layer without causing a reflection of the incident waves will. You can keep the losses constant throughout the shift, you can but also change one or both loss factors depending on the depth. It must just pay attention again to the ratio of the electrical dissipation factor to the magnetic loss factor, i.e. the expression tg8 (e) / tgö (p.), not too steep is changed towards the depth, which inadmissibly high reflections from the inside the shift would cause.

A layer constructed according to the invention with the inventive concept above The chosen loss factors only need to be chosen so thick that the desired absorption is achieved when the radiation passes through.

As for a certain absorption with an optimal choice of the loss factors not the geometric thickness of the layer, but the number of those occurring in the layer Wavelength is decisive, it can be seen that an increase in the refractive index n a reduction in the minimum layer thickness required for a certain absorption means.

So if you need z. B. for an absorption of 90 ° / o, based on the amplitude, a layer thickness of one wavelength, the geometric layer thickness in the above example for a vacuum wavelength is 2, = 15 cm and a refractive index n = 7.65 only D , - 2 cm.

An increase from n to 15 would reduce the minimum layer thickness to 1 cm to decrease.

The figures correspond to practical measurements that were carried out according to the invention built-up layers were carried out.

Claims (7)

PATENT CLAIMS: 1. High refractive index matching layer for the reflection-free transition of electromagnetic radiation from a medium like air in another medium, the layer also acting as an absorbent layer can be formed, characterized in that the layer is a ferromagnetic material high permeability V., s, preferably in powder form in as fine and uniform a form as possible Distribution, in a dielectric with a small dielectric constant sB, where the fill factor of the ferromagnetic material should be as high as possible and all ferromagnetic Particles should be separated from one another by the dielectric. 2nd layer after Claim 1, characterized by such a high fill factor of the ferromagnetic material, preferably over 90% that when using a ferromagnetic material with an initial permeability from 1000 to a maximum of 10,000 a further increase in the initial permeability and / or the increase in permeability with increasing magnetic field strength S5 no more significant increase in the effective permeability V.eff of the layer ' "causes:` 3. Layer according to claim 1 or 2, characterized in that the grain size of the powdery ferromagnetic is kept so small that at the highest Frequency of the incident radiation not yet inadmissibly high losses, e.g. B. caused by eddy currents and current displacement, and that the opposite of the field strength of the magnetic alternating field .5 effective permeability # L8 not yet inadmissible sinks. 4. Layer according to one of claims 1 to 3, characterized in that they to further improve the adaptation perpendicular to the direction of incidence of the radiation network-like of two preferably orthogonal families of essentially parallel and equidistant air slits are traversed, the spacing a of which is at most equal half the wavelength of the radiation within the layer and its width 8 is so small that the mean refractive index n of the arrangement is as high as possible, but at least so large that the permitted reflection factor r of the arrangement is not exceeded, the width ö of the air slots in the direction of the incident Radiation decreases so that the refractive index of the layer increases with depth. 5. Layer according to one of claims 1 'to 4, characterized in that it is used for simultaneous absorption of incident radiation with electrical and magnetic It is prone to losses that are very small at the front and at the bottom rise towards. 6. Layer according to Claim 5, characterized in that the rise the losses towards the 'riefe are at most so steep that no impermissible ones Reflections occur from inside the layer. 7th shift after one of the Claims l to 4, characterized in that they are for simultaneous absorption the incident radiation is afflicted with electrical and magnetic losses which are as similar as possible to one another, at least on the front side of the layer. B. layer according to claim 7, characterized in that the electrical and / or magnetic losses change gradually towards the depth, but at most so steep that no impermissible reflections occur from inside the layer. Considered Pamphlets Radio Mentor, 1949, pp. 438-450; “Radar system Engineering «by Louis N. Riden`our, New York and London, 1947, pp. 69 to 73.