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WO1996000454A1 - Panneau absorbant - Google Patents

Panneau absorbant Download PDF

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Publication number
WO1996000454A1
WO1996000454A1 PCT/GB1995/001496 GB9501496W WO9600454A1 WO 1996000454 A1 WO1996000454 A1 WO 1996000454A1 GB 9501496 W GB9501496 W GB 9501496W WO 9600454 A1 WO9600454 A1 WO 9600454A1
Authority
WO
WIPO (PCT)
Prior art keywords
ferrite
panel according
active material
substrate
panel
Prior art date
Application number
PCT/GB1995/001496
Other languages
English (en)
Inventor
Stephen George Appleton
Keith Charles Pitman
Original Assignee
The Secretary Of State For Defence
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by The Secretary Of State For Defence filed Critical The Secretary Of State For Defence
Publication of WO1996000454A1 publication Critical patent/WO1996000454A1/fr

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Classifications

    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01Q—ANTENNAS, i.e. RADIO AERIALS
    • H01Q17/00—Devices for absorbing waves radiated from an antenna; Combinations of such devices with active antenna elements or systems
    • H01Q17/007—Devices for absorbing waves radiated from an antenna; Combinations of such devices with active antenna elements or systems with means for controlling the absorption
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01Q—ANTENNAS, i.e. RADIO AERIALS
    • H01Q3/00—Arrangements for changing or varying the orientation or the shape of the directional pattern of the waves radiated from an antenna or antenna system
    • H01Q3/44—Arrangements for changing or varying the orientation or the shape of the directional pattern of the waves radiated from an antenna or antenna system varying the electric or magnetic characteristics of reflecting, refracting, or diffracting devices associated with the radiating element

Definitions

  • the present invention relates to absorbent panels and more particularly relates to panels for absorbing certain types of electromagnetic radiation.
  • 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 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.
  • 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.
  • the microwave permeability is limited to unity by the inertia of domain walls.
  • Ferrites 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.
  • FMR ferromagnetic resonance
  • 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.
  • the anisotropy field may be supplemented with an applied d.c. magnetic field and the FMR frequency altered accordingly.
  • 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).
  • 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.
  • uniaxial ferrites having a magnetic moment lying along the c-axis of the crystal cell
  • planar ferrites having a magnetic moment in a plane perpendicular to the crystal axis.
  • Preferred ferrites are planar hexagonal ferrites.
  • 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.
  • the invention may be used to decoy targets or disguise existing vessels.
  • the invention may be used in a switchable beacon at an airport.
  • IVS Instrument Landing System
  • 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).
  • 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.
  • 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.
  • 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.
  • 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 ⁇ .
  • 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 C ⁇ 2 [Fe 24 0 41 ].
  • 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.
  • the powders 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.
  • PVA polyvinyl acetate
  • 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.
  • 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.
  • Figure 7 illustrates plots of ⁇ ' and tan ⁇ for the same range of biasing fields (denoted using the same key) for a cobalt doped Z ferrite Ba 3 C ⁇ 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) ( ⁇ # and tan ⁇ reduced and resonance frequency increased significantly) renders it very suitable for use in a variable-performance absorber. Moreover, it can be seen that tan ⁇ for the unbiased - ⁇ - 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.

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  • Shielding Devices Or Components To Electric Or Magnetic Fields (AREA)

Abstract

Panneau en matériau absorbant les ondes hyperfréquence, comprenant au moins une couche d'un substrat à base de ferrite, un réflecteur support susceptible de réfléchir le rayonnement incident, ainsi que des moyens d'application d'un champ magnétique au substrat. Le champ appliqué modifie le comportement de résonance ferromagnétique du ferrite et par conséquent il est possible de changer les caractéristiques d'absorption du substrat. On peut produire des matériaux susceptibles d'être commutés ou polarisés afin d'annuler l'effet absorbant de la couche de ferrite, permettant ainsi au dispositif de passer d'un état essentiellement absorbant à un état essentiellement réfléchissant.
PCT/GB1995/001496 1994-06-27 1995-06-26 Panneau absorbant WO1996000454A1 (fr)

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
GB9412850.1 1994-06-27
GB9412850A GB9412850D0 (en) 1994-06-27 1994-06-27 Absorbent panel

Publications (1)

Publication Number Publication Date
WO1996000454A1 true WO1996000454A1 (fr) 1996-01-04

Family

ID=10757377

Family Applications (1)

Application Number Title Priority Date Filing Date
PCT/GB1995/001496 WO1996000454A1 (fr) 1994-06-27 1995-06-26 Panneau absorbant

Country Status (2)

Country Link
GB (1) GB9412850D0 (fr)
WO (1) WO1996000454A1 (fr)

Cited By (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
CN113437528A (zh) * 2021-07-07 2021-09-24 东莞理工学院 一种具有可调窄带反射窗口的宽频吸波超材料
WO2021231519A1 (fr) * 2020-05-12 2021-11-18 Rogers Corporation Hexaferrite de type m comprenant une céramique à faible perte diélectrique
US12406788B2 (en) 2019-10-30 2025-09-02 Rogers Corporation M-type hexaferrite comprising antimony
US12424362B2 (en) 2020-05-07 2025-09-23 Rogers Corporation M-type hexaferrite having a planar anisotropy

Citations (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3309704A (en) * 1965-09-07 1967-03-14 North American Aviation Inc Tunable absorber
DE2037329A1 (de) * 1969-08-09 1971-02-25 Philips Nv Magnetische Einrichtung
US4987418A (en) * 1987-12-28 1991-01-22 United Technologies Corporation Ferroelectric panel
WO1992016031A1 (fr) * 1991-02-27 1992-09-17 Alenia-Aeritalia & Selenia S.P.A. Structure dichroïque a selection de frequences possedant une bande passante variable et applications

Patent Citations (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3309704A (en) * 1965-09-07 1967-03-14 North American Aviation Inc Tunable absorber
DE2037329A1 (de) * 1969-08-09 1971-02-25 Philips Nv Magnetische Einrichtung
US4987418A (en) * 1987-12-28 1991-01-22 United Technologies Corporation Ferroelectric panel
WO1992016031A1 (fr) * 1991-02-27 1992-09-17 Alenia-Aeritalia & Selenia S.P.A. Structure dichroïque a selection de frequences possedant une bande passante variable et applications

Non-Patent Citations (1)

* Cited by examiner, † Cited by third party
Title
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 *

Cited By (6)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US12406788B2 (en) 2019-10-30 2025-09-02 Rogers Corporation M-type hexaferrite comprising antimony
US12424362B2 (en) 2020-05-07 2025-09-23 Rogers Corporation M-type hexaferrite having a planar anisotropy
WO2021231519A1 (fr) * 2020-05-12 2021-11-18 Rogers Corporation Hexaferrite de type m comprenant une céramique à faible perte diélectrique
GB2608921A (en) * 2020-05-12 2023-01-18 Rogers Corp M-type hexaferrite comprising a low dielectric loss ceramic
CN113437528A (zh) * 2021-07-07 2021-09-24 东莞理工学院 一种具有可调窄带反射窗口的宽频吸波超材料
CN113437528B (zh) * 2021-07-07 2022-11-11 东莞理工学院 一种具有可调窄带反射窗口的宽频吸波超材料

Also Published As

Publication number Publication date
GB9412850D0 (en) 1994-08-17

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