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Title:
ABSORBENT PANEL
Document Type and Number:
WIPO Patent Application WO/1996/000454
Kind Code:
A1
Abstract:
A panel of microwave absorbent material comprises at least one layer of a ferrite substrate, a backing reflector capable of reflecting incident radiation, and means for applying a magnetic field to the substrate. The applied field changes the ferromagnetic resonance behaviour of the ferrite and hence the absorption characteristics of the substrate can be changed. It is possible to produce materials switchable or biasable to negate the absorbent effect of the ferrite layer, enabling the device to switch from an essentially absorbent to an essentially reflective state.

Inventors:
APPLETON STEPHEN GEORGE (GB)
PITMAN KEITH CHARLES (GB)
Application Number:
PCT/GB1995/001496
Publication Date:
January 04, 1996
Filing Date:
June 26, 1995
Export Citation:
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Assignee:
SECR DEFENCE (GB)
APPLETON STEPHEN GEORGE (GB)
PITMAN KEITH CHARLES (GB)
International Classes:
H01Q3/44; H01Q17/00; (IPC1-7): H01Q17/00; H01Q3/44
Domestic Patent References:
WO1992016031A11992-09-17
Foreign References:
US3309704A1967-03-14
US4987418A1991-01-22
DE2037329A11971-02-25
Other References:
POZAR: "A Magnetically Switchable Ferrite Radome for Printed Antennas", IEEE MICROWAVE AND GUIDED WAVE LETTERS, vol. 3, no. 3, NEW YORK US, pages 67 - 69, XP000360593
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Claims:
Claims
1. A panel of radar absorbent material comprising at least one layer of a substrate comprising a ferrite active material, a backing reflector capable of reflecting incident radiation, and means for applying a magnetic field to the substrate whereby the absorption characteristics of the substrate can be changed.
2. A panel according to claim 1 wherein the ferrite active material is doped with additives to adjust its ferromagnetic resonance frequency.
3. A panel according to claim 2 wherein the additives include cobalt (JT) ions.
4. A panel according to any one of claims 1 to 3 wherein the ferrite active material comprises an Mferrite.
5. A panel according to claim 4 wherein the ferrite active material comprises a ferrite of Ca, Sr, Mg, Pb, or Ba.
6. A panel according to claim 5 wherein the ferrite active material is doped with cobalt and titanium ions to the general formula XFe^.^COxTixO^ where X is C Sr, Mg, Pb or Ba.
7. A panel according to claim 6 wherein the level of x is selected such that the ferrite possesses anisotropy that lies near the transition from uniaxial to planar. δ. A panel according to claim 7 wherein X is barium and x is substantially 1.
8. 2.
9. A panel according to claim 7 wherein X is strontium and x is substantially 1.5.
10. A panel according to any one of claims 1 to 3 wherein the ferrite active material comprises a planar hexagonal ferrite.
11. A panel according claim 10 wherein the feirite active material is selected from the group comprising Y, X, U, Z, and W ferrites.
12. A panel according to claim 11 wherein the ferrite active material comprises a Zferrite.
13. A panel according to claim 12 wherein the ferrite active material comprises a ferrite substantially to the formula Ba3Co2[Fe24041].
14. A panel according to any preceding claim comprising a plurality of layers of different ferrite content.
15. A panel according to any preceding claim wherein the means for applying a magnetic field to the substrate is capable of generating a field level in excess of 40,000 amps per metre.
16. A panel according to claim 16 wherein the means for applying a magnetic field to the substrate is capable of generating a field level in excess of δ0,000 amps per metre.
17. A panel according to any preceding claim wherein the backing reflector is a layer of adhesive foil. lδ.
18. A panel according to any preceding claim wherein the backing reflector is a layer or layers of a conductive paint.
19. A panel of radar absorbent material substantially as hereinbefore described with reference to the accompanying drawings.
Description:
ABSORBENT PANEL

The present invention relates to absorbent panels and more particularly relates to panels for absorbing certain types of electromagnetic radiation.

It is known that certain materials can absorb incident radiation and thereby reduce the radiation scattered or reflected back to an emitter/receiver device.

A known absorber is a Salisbury screen which comprises a sheet of porous material impregnated with graphite and spaced a quarter-wavelength off a metallic backing plate. From transmission line theory, a short circuit (the metal plate) placed a quarter wavelength behind a load effectively creates an open circuit at the load. There is no reflection of the incident wave, all the power being delivered to the resistive sheet and none being reflected.

The Dallenbach layer is also a simple absorber. Most of the commercial Dallenbach absorbers are flexible and can be applied to modestly curved surfaces. The material is uniform throughout its volume and is a mixture of compounds designed to have a specific index of refraction. That design may include materials with magnetic losses as well as carbon particles responsible for electric losses. The electric and magnetic susceptances (relative permittivity and permeability) therefore have imaginary components giving a refractive index having an imaginary component resulting in attenuation of waves travelling through the material. The dielectric absorbers are typically made of a rubbery foam impregnated with carbon particles. Magnetic Dallenbach layers can be rolled from a mixture of natural or synthetic rubber loaded with carbonyl iron or ferrite powders.

The quest for improved bandwidth motivated the development of absorbing materials more complex than the Salisbury screen and the Dallenbach layer. The natural extension of the former is the Jaumann absorber which consists of resistive

sheets stacked one above the other. Optimum performance is obtained when the spacing between the sheets is a quarter wavelength and the resistivity varies from a high value at the outer sheet to a low value at the inner sheet. Graded absorbers can be made by using stacks of Dallenbach layers, each layer having a smaller intrinsic impedance the closer it is to the metallic backing foil. The thickness and electrical properties of the bonding materials used in fabrication of the absorbers must also be taken into account.

The existing radar absorbent materials only tend to be effective over a narrow frequency band of incident radiation. The present invention relates to means for enabling this absorption range to be shifted to another frequency band thereby giving a greater flexibility of use.

Thus according to the present invention there is provided a panel of radar absorbent material comprising (i) one or more layers of a substrate comprising a ferrite active material, (ii) a backing reflector capable of reflecting incident radiation and (iii) means for applying a magnetic field to the layers of substrate whereby the absorption characteristics of the layers of substrate can be changed.

In most ferromagnetic materials the microwave permeability is limited to unity by the inertia of domain walls. Ferrites, however, offer increased permeability due to the phenomenon of ferromagnetic resonance (FMR), in which energy is coupled from a microwave signal to the precessional motion of the magnetic vectors within the ferrite. The frequency at which FMR occurs is governed by the magnetocrystalline anisotropy field of the material which can be modified by partial substitution of iron ions in the crystal structure. Furthermore, the anisotropy field may be supplemented with an applied d.c. magnetic field and the FMR frequency altered accordingly. Hence materials can be designed to operate in a chosen frequency band by judicious choice of ferrite composition, and the application of an external magnetic bias field enables the loss spectrum to be modified.

Preferably the applied magnetic field has a field level in excess of 500 oersteds (40,000 amps per metre), more preferably 1000 oersteds (80,000 amps per metre). By altering the applied d.c. magnetic fields, the ferromagnetic resonance frequency can be adjusted to suit a particular requirement.

The ferrite active material is a ferromagnetic oxide and various types of ferrite may be used. For example, uniaxial ferrites having a magnetic moment lying along the c-axis of the crystal cell and planar ferrites having a magnetic moment in a plane perpendicular to the crystal axis. Preferred ferrites are planar hexagonal ferrites.

For application of a magnetic field consideration can be given to permanent magnets, electromagnets, or a combined system. However at the high field strengths preferred for operation of the invention the size and cost of permanent magnets tends to be prohibitive. The means for applying the magnetic field to the substrate is thus preferably an electromagnet in the form of one or more coils in association with yokes of ferromagnetic material. With such arrangements it is possible to produce fields either normal to or in the plane of the ferrite panel.

The backing reflector may be, for example, an adhesive foil or a layer or layers of a conductive paint.

An object of the invention is to obtain radar absorbent properties switchable or biasable to negate the radar absorbent effect. This effect can be used to produce a radar absorbent material in which it is possible to change the radar absorptive characteristics of the material for tactical strategic or other reasons. Thus for example, the invention may be used to decoy targets or disguise existing vessels.

Also it is envisaged that the invention may be used in a switchable beacon at an airport. During recent years there have been many incidents of commercial airliners becoming disorientated when approaching an airfield through the use of their Instrument Landing System (ILS) and attempting to land on taxiways and

even perimeter roads. This is a particular problem at certain modem high capacity airports which operate with multiple parallel runways operating sequentially. Use of the invention in a runway identification system as a beacon switchable from an absorbent to a reflective state allows selective identification of the active runway with by selective actuation of the reflectors.

A further example of a use for the invention is to provide a passive digital signalling device. The device may be switched between its absorbent and its reflective state in a predetermined pattern in accordance with a suitable message relaying system (such as morse code). The message is thus visible only to a remote observer who interrogates the device with a radar signal of the appropriate frequency, but is not transmitted as such.

A multilayer panel is also envisaged, the layers being within a single magnetic field. The strength of the magnetic field is sufficient to change the absorption frequency. The panels are preferably isotropic and have different concentrations of ferrite or different types of ferrites or combinations can be used.

The production of radar absorbent materials involves a knowledge of the intrinsic physical and microwave properties of the component materials. It is desirable that the effective microwave complex permeability and permittivity is equal to enable incident radar signals to enter the materials without reflection from the incident face and the imaginary components of these parameters is desirably large to obtain sufficient loss.

Ferrites have two types of hexagonal crystal structure a) uniaxial having a magnetic moment lying along the C-axis of the crystal cell and b) planar having a magnetic moment in a plane perpendicular to crystal axis. It has been found that planar hexagonal ferrites are particularly useful as they not only give a frequency shift but also give a quenching or attenuation effect at the displaced frequency.

Typical ferrites are those of the M type, Fe I2 0 19 . Examples include the M ferrites of Ca, Sr, Mg, Pb, Ba, and especially Ba-M and Sr-M The resonant frequency of the barium ferrite is 46 GHz and that of the strontium ferrite is 56 GHz.

In ferrite materials the frequency at which the FMR occurs is dependent on the magnetocrystalline anisotropy field H a ,, of the material which can be changed by partial substitution of the iron ions in the crystal structure.

The ferrite active material can be doped with additives to adjust its ferromagnetic resonance frequency such as cobalt and titanium. For example, barium ferrite [Ba Fe 12 0 I9 ] can have the two iron atoms replaced with a cobalt and a titanium atom. The doping gives a different effect by varying the position of the resonance frequency and changes the field ! .. The replacement of Fe 3 * with Co 2+ has been shown to result in a reduction in 1 , and therefore a reduction in the resonant frequency. Ti 4+ ions are included in equal proportions to Co 2+ to maintain the charge balance. Preferred M-type ferrites are thus of the form XFe 12-2x Co x Ti x 0 19 where X is Ca, Sr, Mg, Pb or Ba, and in particular of the form BaFe, 2-2x Co x Ti x 0 19 and SrFe 12-2x Co x Ti x 0 19 where x usefully has a value not exceeding 1.5. M ferrites tend to exhibit uniaxial anisotropy when undoped, but to undergo a transition to a planar anisotropy at a particular characteristic doping level. At this point response to a biasing field is particularly marked and the level of x is thus preferably selected such that the ferrite possesses anisotropy that lies near the transition from uniaxial to planar, that is near the compensation point of FL^.

Other suitable ferrites include the Y X U Z W ferrites which have hexagonal crystal cells of slightly different length. These ferrites are also preferentially doped to vary the resonance frequency. A preferred Z ferrite is Ba 3 2 [Fe 24 0 41 ].

The invention will now be described by way of example only and with reference to figures 1 to 7 of the accompanying drawings in which: figure 1 is a perspective view of a device in accordance with the invention;

figure 2 is a cross-section of an alternative device in accordance with the invention; figures 3 to 6 illustrate the properties of a Co-Ti doped Ba- and Sr- M ferrite; figure 7 illustrates the properties of a 63 3 00 2 1 6 24 0 4 ,] Z ferrite;

The ferrite specimens were manufactured by the following method. The required weights of precursor oxides and carbonates are mixed by hand and than ball-milled in water to form a fine homogeneous mixture. When dried and sieved, the powders are pressed into blocks and reaction-fired in a resistive element furnace to form the desired chemical phase. Reaction temperatures of approximately 1100°C are required. The reacted material is ground, ball-milled in water, dried and sieved as before. An 8% aqueous polyvinyl acetate (PVA) solution is added to serve as a binder, and the powder is pressed into discs and sintered at 1320°C to form dense ceramics.

In order to produce a flat radar absorbent panel, the ceramics can be ground into a powder and dispersed into a polymeric binder such as an epoxy resin. The binder/ferrite can then be pressed into a flat panel and a reflecting layer applied. Figure 1 illustrates such a ferrite panel 10 in combination with an electromagnetic arrangement of coil and ferromagnetic yoke suitable for applying a magnetic field to the panel in the plane of the panel which consists of a ferromagnetic C-frame 9 and copper coil 8. It will be readily understood that coil and ferromagnetic yoke combinations could be used to generate axial fields if desired by selecting the appropriate geometry according to established principles.

A number of layers of different ferrite content can be used. In figure 2, three layers of different ferrite content - 12, 13, 14 - form a single absorbent panel and are located adjacent to a magnet 11 capable of altering the properties of the ferrites. This enables the absorber frequency to be tailored to the specific application.

In figure 3 the relative reduction in Ba-M ferrite FMR frequency for increased doping levels (ie levels of x in BaFe^^CO x Ti x O^) is plotted. It can be seen that

variation is almost linear with x up to ~1.2. The materials in this range of x are uniaxial. Beyond x -1.2 there is a transition to planar anisotropy. The same relation can be observed in figure 4 which illustrates the relative reduction in SrFe 12-2x Co x Ti x 0 19 ferrite FMR frequency with variation in x. In this case linear behaviour predominates to x up to -1.5 before a transition to planar behaviour occurs.

It is found that the application of a biasing magnetic field has its most marked effect where levels of Co,Ti substitution are around the levels which correspond to the transition from uniaxial to planar anisotropy, ie near the compensation point of H a ,,. Figures 5 and 6 illustrate this effect for at x=1.2 and SrFe 12-2x Co x Ti x O I9 at x=1.5 respectively, plotting μ" and tanδ (the magnetic loss tangent) for a range of biassing fields up to 80kAm " ' (1000 Oe) according to the ' following key: 1 - no bias, 2 - δkAm "1 , 3 - 20kAm- 1 , 4 - 40kAm '1 , 5 - όOkAm' 1 , 6 - 80kAm " '. In both cases application of field upwards of 40kAm '1 start to produce a significant shift in resonance frequency, and the frequency is increased by a factor of about 3 with a 80kAm " ' applied field. It is also important to note that at the original (unbiased) resonance frequency the magnetic loss tangent decreases considerably with applied field. The applicability of these ferrites to the invention to produce a variable-performance absorber in accordance with the present invention is high.

Figure 7 illustrates plots of μ' and tanδ for the same range of biasing fields (denoted using the same key) for a cobalt doped Z ferrite Ba 3 2 [Fe 24 0 41 ]. This planar material exhibits a dramatic change in microwave permeability with applied field. Its high permeability levels near resonance facilitate good impedance matching with air and allow high levels of loss to be achieved in the unbiased state. It is, therefore, a useful material for incorporation into a microwave absorber. Its response to applied fields (figure 7) (μ # and tanδ reduced and resonance frequency increased significantly) renders it very suitable for use in a variable-performance absorber. Moreover, it can be seen that tanδ for the unbiased

- δ - material indicates excellent broadband loss, which may be greatly reduced by the bias field, allowing the absorber to be readily switched from a high-loss to a low- loss regime.