FR2783359A1 - Wideband electromagnetic wave absorbing structure, using two layers with ferrite compact on lower ground side, and composite material on exterior with wave impedance greater than that of air - Google Patents

Abstract

The two layer absorber comprises a ferrite compact on the 'earthed' side, on top of which is a composite material having a high wave impedance, considerably above that of air. Preferably, the composite material is isotropic and non-homogeneous, such as a layer incompletely, but regularly, filled with a ferrimagnetic material such as powdered, large-grained, flaked or other form of ferrite in a low permittivity matrix. Alternatively, ferrite material with pores can be used, especially a porous ferrite frit. (No drawing - figures not suitable for reproduction).

Description

BROADBAND ELECTROMAGNETIC WAVE ABSORBENTS

  In my USA patent (US Serial n 08 / 942.166 dated October 1, 1997), I described broadband absorbents, using two layers of ferrite or one layer of ferrite and a composite of

  ferrites, separated by a layer with high wave impedance.

  The importance of the concept of "decoupling" between layers, by a

  such an intermediate layer ("buffer" layer) has been described.

  In this context, I found the astonishing result that the intermediate layer can be removed: Then, the internal layer (against the mass), produced by a compact ferrite, is not followed by a buffer layer, but simply a second external magnetic layer, composed of a magnetic composite

new, described in this patent.

  This new magnetic composite must represent at the

  times the characteristics of the buffer layer, and the characteristics

  ticks of the outer ferrite (or ferrite composite) layer, i.e. representing the necessary condition of wave impedance

  high (Hi-Z), as well as the necessary additional absorption.

  This new composite can be produced by particles of one or more ad hoc ferrites in the form of powder, grains, more or less circular, elongated or acicular grains, scales, sticks or bars, immersed in an air or dielectric foam medium (o 6 * is close to 1 - jO), or vice versa, produced by a ferrite, compact, comprising a large number of pores, such as porous sintered ferrites, or else ferrites / ferrites with

  fine porosity, with elongated macroscopic holes.

  I found that the permeability spectra Pû * = P2 '- j p2 "of this composite layer, called Hi-Z, first - 2 -

-2- 2783359

  must correspond as closely as possible to an "ideal spectrum": This is determined by a "reconstruction" of 2 *, to obtain a zero wave reflection, for the frequency domain considered. Consider a typical soft ferrite, Ni-Zn, compact, with the permeabilities of Figure 1, and having a permittivity

constant of t1 * = 15 - jO.

  Figure 2 reproduces such spectra of "ideal" permeabilities,

  for the outer layer, with this ferrite, placed against the ground.

  For these "ideal" permeabilities, I have considered that * = 1 - jO. They are shown for the three thicknesses of the layer

  absorbent external, 9, 12.9 and 16 mm.

  I was able to demonstrate that the shape of the dispersion, and the values of P 'and P "are physically possible, between some 300 MHz and some 8 GHz: In other words, a perfect two-layer absorbent becomes possible, according to the terms of my invention, under the condition of producing spectra v 2 * at the precise pace,

for the outer layer.

  For all these curves, the root of the ratio of complex permeabilities divided by the permittivity, indicates a module greater than unity, in the whole spectrum, that is to say, the external absorbent layer represents well the characteristics " hi-Z "described: This represents a new line of materials, which I have not found described in the literature. It is thus clear, that the invention excludes, a priori for the outer layer, all "rubber-ferrites" or other composites

  with a dielectric matrix, described in the literature.

  However, I found it possible to produce such composite materials, having essentially a permittivity equal to or close to ú * = I - jO in the whole frequency range, and having the low values of the required permeabilities, with concentrations

  in overall ferrite volumes as reduced as 10 to 25%.

  Unexpectedly, I found that these 2 * spectra do not correspond

  not due to the shapes and values of the spectra of the ferrite (s) used in the compact state: in fact, for such reduced values of the filling coefficient, the permeabilities observed are linked to the shape and the geometric distribution of the ferrite, where a high number of fine grains are not equivalent to a

  reduced number of larger compact pieces of ferrite.

  In the same way, grains (non-spherical) oriented in the direction of propagation or in the perpendicular plane, show different spectra: in reality therefore, the spectra U2 * of this external layer can be modified, by playing on the isotropy or the oriented anisotropy of the structure, carried out with essentially isotropic soft ferrites, in the compact state. In a first example, I made a painting with such a Mn-Zn sub-stoichiometric ferrite (with the spectra of P2 * in Figure 3 in the compact state: I therefore use another soft ferrite, with a Snoek spectrum (saturation magnetization), extending at frequencies slightly higher than those of ferrite

Figure 1.

  This ferrite, in the form of a fine powder (particle size less than 0.2 mm), is incorporated into any paint base, with a volume concentration of around 68% for the dry extract. The paint is applied with a spray gun or by soaking to a tangle of horsehair or fibers, like airy horsehair, to obtain an overall density of the order of unity, which corresponds to a volume concentration of ferrite of 1 'order

0.2.

  This Hi-Z composite represents the permeability spectrum of the plots 4a in FIG. 4. An identical composite embodiment, but with the ferrite of FIG. 1, shows the spectra of the permeabilities of the plots 4b in FIG. 4: I indicate these results in graphical form, making the

differences in spectra.

  Thus, superimposing these measured spectra on the ideal spectra (Figure 2), we can verify, first, that the general appearance of the spectra obtained corresponds well to the appearance of the "ideal" spectra, above some 200 MHz. We can then verify a very approximate correlation of the spectra 2 'and f2 for d

2 I2 '2

  = 12.9 mm, especially at high frequencies, for composite at

Mn-Zn ferrite (Traces 4a).

  Finally, we can see that for similar real permeability spectra, the loss permeabilities can vary a lot. Figure 4 clearly shows the first of the means that I found to "model" the spectrum of the outer layer, by modifying the composition of the composite ferrite. Thus, a ferrite or a mixture of ferrites, of different saturation composition and magnetization, allows the permeability of the losses to be placed at will (with little change in P ')

in the frequency spectrum.

  It is understood that these different ferrites have a low conductivity.

  vity, to approach the condition ú * - 1 - jO. I noted however that slightly higher permittivities, for example 1.5 or 2, still make it possible to realize the condition "hi-Z", by taking values 2 'and P2 "multiplied roughly by The simulation of the reflectivity obtained by the inner layer of the compact NiZn ferrite (Figure 1) (d1 = 5.3 mm) with the adjacent layer of the Hi-Z composite of the Mn-Zn ferrite, is shown in Figure 5: - a - We obtain a reflectivity approaching 15 dB, from around 15 MHz to 5 GHz, with variations related to the thickness d2 of the outer layer. Other means make it possible to better "clone" the spectra P 2 'and P 2 " ideal, with in particular the approach too

  perfect as possible, of the unit permittivity, (for the realization

  tion of the hi-Z condition, on a spectrum as wide as pos-

  sible). I found, in particular, that larger grains (for the Hi-Z composite), the orientation of these grains in the direction of propagation, an oblong shape in the direction of propagation, up to the use of bars or buchettes (always in the direction of propagation), allow the realization

of these conditions.

  In such "hi-Z" composites, an "uniaxia-" anisotropy

  the "is thus introduced, due to the structure of the composite (without the use of anisotropic ferrites!): we then observe the possibility of maximizing the wave impedance, and spreading it over the entire spectrum theoretically possible, that is to say

up to 5 to 8 GHz.

  As a third example, I cite below the realization

  tion of a "hi-Z" composite with ferrite particles in Figure 3, between 0.5 mm and 2 mm in size, with a volume concentration between 0.1 and 0.25: these grains are oriented in the direction of propagation, for the application of a continuous magnetic field at the time of solidification of the composite, produced for example by a dielectric foam matrix (of permittivity 1). Permeability in the direction of propagation is thus

  favored at the expense of permeability in the plane perpendicular to

  laire; I observed at the same time, that the effective permittivity

is more reduced, as desired.

-6 - 2783359

  The permeability spectra of this composite are shown in Figure 6, superimposed on the "ideal" permeabilities: for a thickness of the hi-Z outer layer between 12.9 mm and 16 mm, there appears an even better "cloning" (than for the examples in Figure 4). This is verified by direct measurement of the reflectivity: for an internal layer of ferrite (from Figure 1), of thickness slightly increased to 6.3 mm (which corresponds to the optimum thickness for the absorbent layer), with the anisotropic composite above, we obtain a reflectivity equal to 15 dB or better

  than 15 dB, up to 5 GHz (Figure 7).

  The figure also indicates the comparison with the performance of the first layer alone, demonstrating the obtaining of the extension of the reflection towards the high frequencies, for the absorbent

two layers.

  It is interesting to note in passing, that this solution is of the "retrofit" type, that is to say an existing anechoic chamber, equipped with the first ferrite alone, can be improved, by adding, after the fact, the layer "hi -Z ". This additional layer is itself very light (about a quarter of the weight

of the first absorbent).

  It is also interesting to note that the addition of the hi-Z composite outer layer, with its "optical index" (square root of the product of 2 *. 2 *), modifies the angle of incidence of a wave. vertical incident, in the inner layer: this considerably improves the reflectivity for an oblique incidence. This is particularly true, at high frequencies, o for example, for an incident angle of 30, the reflectivity

  reached 13 dB for both polarizations, at 5 GHz.

  The anisotropy of the external layer can go up to the maximum, represented by a structure in "needles" or "bars" - 7 - of ferrite, aligned in the direction of propagation of the incident wave, over the entire thickness of the outer layer. Such a structure is equivalent, if we consider the connected bars, to a

  grid or waffle type structure, for this outer layer.

  I observed that a cloning of the ideal H * spectra, better still, is obtained by a partial anisotropy as carried out in the example above, and also carried out with aligned bars, plus grains (aligned or no) added to the composite. "Planar" ferrites, intrinsically having

  uniaxial permeability, can also be used.

  These solutions make it possible to optimize the hi-Z condition, in particular at the frequencies at the extreme end of the band.

frequencies.

  As a fourth example, I cite the case of a grid or waffle composite, mentioned above. Given the low values of P * required, obviously such a structure must be very hollow (to obtain the filling coefficient of the order of 0.1 to 0.25): this makes the structure fragile, with an external ferrite compact. I observed that we can then start from a normally sintered ferrite (to represent the intrinsic P * of the compact), but representing a high porosity: We thus approach

  partial realization of the Hi-Z impedance.

  In this example, I made a ferrite of the composition MnZn (From Figure 3), with a porosity of 50%, and I made circular holes close together in the direction of propagation, to finish obtaining the Hi-Z characteristic: This makes it possible to obtain exactly the "negative" of the structure in described bars: the diameter of the toroids makes it possible to adjust the overall ferrite concentration, that is to say the permeability P 2 * obtained. The shape of such holes is unimportant, provided that they are symmetrical in the direction perpendicular to the propagation of the waves, and provided that, for such inhomogeneous structures, the pitch of the irregularities (such as for example the distance between bars, or again, the distance between holes) should only represent a fraction of the wavelength corresponding to the maximum frequency of the useful spectrum of the absorbent (a fraction of a centimeter). Figure 8 shows the excellent correlation obtained for

  the 2 * permeability of this composite, for d2 = 9 mm.

  I thus obtained a two-layer absorbent, with a reflectivity equal to or greater than 20 dB, from 20 MHz to 5 GHz (ferrite table

  1, thickness 6.3 mm as the first layer).

  In a fifth example, I mention that my invention is described for reasons of ease of understanding, with a first (internal) layer of given ferrite. It is understood, of course, that other ferrites can be used, providing a

  interesting reflectivity characteristic, at low frequencies.

  By the procedure described, other "ideal" P * spectra are established, that is to say describing the outer layer, showing a theoretical reflection equal to zero. We therefore proceed in the same way to

  determine this "ideal" outer layer.

  In addition, it is obvious that "cloning" can favor certain frequency domains, and thus to place

  maximum reflectivity, at will, in the spectrum considered.

  I also mention, that the procedure described, allows the realization of absorbents up to some 8 GHz, because of the

  internal compact ferrite used. To go further, the same process

  hard is used, with a third layer, and another "hiZ" composite. Three-layer absorbents are thus possible to achieve usable reflectivities up to around 18 GHz and beyond. In this case, even higher Snoek frequency materials are used (ultrafine ferromagnetics,

  highly anisotropic ferrimagnetics, etc.).

  Finally, an additional layer of air can be inter-

  calibrated, to better "clone" the ideal characteristics of {4 *:

  indeed, such a layer introduces a positive phase shift on the

  pedance brought to the surface, surface in contact with the "hiZ" composite: the ideal permeability curves are then deformed, which can facilitate the precision of the "cloning" to obtain

of zero wave reflection.

- 10 -

Claims (10)

Claims   1. Absorbent of electromagnetic waves, using two absorbent layers, adjacent to each other, in which the first layer (against ground) is produced by a ferrite   compact, and in which the outer layer is produced by a material   composite channel representing a high wave impedance, substantially higher than that of air, up to frequencies   high of the useful frequency range.   2. Two-layer electromagnetic wave absorbent, according to 1, in which the composite material is isotropic and non-homogeneous, such as a layer incompletely, but regularly filled with a ferri-magnetic material, the latter can be a powder of ferrites, large grains of ferrite, bars, flakes, or any other geometrical shape, in a low-permittivity matrix, or vice versa, the latter being able to be made of ferrite with holes, pores, and in particular, a porous sintered ferrite.   3. Two-layer electromagnetic wave absorbent,   according to 1, in which the composite material is uniaxially ani-   sotropic and non-homogeneous, such as a layer incompletely filled with a ferrimagnetic material, which may be oblong grains, bars, ferrite flakes, or any other geometric shape, in a matrix with low permittivity, oriented with their axis easy magnetization in the direction of propagation, or vice versa, this one can be a   ferrite with holes, pores, oriented.   4. Two-layer electromagnetic wave absorbent, according to 1 and 2, in which the composite matrix comprises a magnetic medium with a controlled degree of anisotropy, which with   the ad-hoc choice of the composition of the ferrites, their concentration - He -   volume, the dimensions of the composite layer, allow obtaining a high reflectivity (reflection close to zero)   electromagnetic waves, in a frequency range of which than 10 MHz to 8 GHz and beyond.   5. Two-layer electromagnetic wave absorbent,   according to one of the preceding claims, 1, 2, 3 and 4, in   which the high wave impedance is essentially higher than the vacuum wave impedance, and is achieved by a relatively low volume concentration of the ferrimagnetic and / or ferrimagnetic material (volume concentration about 10 to 25%), combined with a matrix material with permittivity equal to or close to unity, making it possible to obtain a reduced effective permittivity of the composite, and the realization of the wave impedance high.   6. Two-layer electromagnetic wave absorbent,   according to one of the preceding claims, 1, 2, 3, 4 and 5, in   wherein the first layer (against ground) is essentially a conventional absorbent for the band from about 30 MHz to 1 GHz, and in which the second composite layer with impedance   high, is magnetic and absorbent.   7. Two-layer electromagnetic wave absorbent,   according to one of the preceding claims, 1, 2, 3, 4, 5 and 6,   in which the external composite material demonstrates a magnetic permeability spectrum, with its real and imaginary terms, as close as possible to the "image spectra", as determined by the optimization of the reflectivity vs the characteristics of   the first (internal) layer of compact ferrite.   8. Two-layer electromagnetic wave absorbent,   according to one of the preceding claims, 1, 2, 3, 4, 5, 6 and - 12 -   7, in which the optical index of the outer layer is lower than the index of the inner layer (compact ferrite), thus improving the reflectivity by oblique incidences of the electromagnetic waves. 9. Two-layer electromagnetic wave absorbent,   according to one of the preceding claims, 1, 2, 3, 4, 5, 6, 7   and 8, in which a layer of air or other medium with high impedance   wave is inserted between the two layers.   10. Two-layer electromagnetic wave absorbers,   according to one of the preceding claims, 1, 2, 3, 4, 5, 6, 7,   8 and 9, in which an additional third layer is placed outside, as determined according to claim 7, to extend the absorption range beyond some 8 GHz, and up to 18 GHz and above.