The present invention relates to magnetic field concentration apparatus, particularly signal enhancement and/or apparatus for confining magnetic flux within an aperture, and/or to shield magnetic flux from components, and/or to means for improving signal reception. The present invention has a particular application to passive devices, such as tokens, transponders or smart cards. ₁₀ PRIOR ART

It is well known that metal placed in the proximity of a magnetic field receiving coil will substantially reduce the amount of magnetic field received by the coil. For example, metal placed between a source of the magnetic field _ις and the coil can operate to prevent any magnetic field from being picked up by the coil as shown in Figure 1. The metal sheet serves to absorb and deflect the flux radiated from the driver coil or magnetic field source.

Figure 2 shows another situation where a metal ₂₀ sheet serves to reduce signal reception. The metal sheet is placed behind the coil. The metal sheet acts to reduce the amount of flux received by the coil by radiating an opposing flux field. The closer the metal sheet, to the coil, the larger the opposing flux field and the less signal flux is -r received by the coil. In effect the metal sheets serve to proportionately nullify the driver coil radiated flux.

U.S. Patent 4,373,163 discloses an electrostatic shield with an antenna loop therein (Figure 1). The antenna is surrounded by a metal shield. The specification does not , _n disclose a conductor plate proximate and in substantially the same plane as an antenna to enhance signal reception, in accordance with the present invention as will be described in more detail hereinafter.

U.S. Patent 4,486,731 discloses a signal , enhancement apparatus in the form of a coil having magnetically permeable material disposed in overlapping relationship with a coil. The coil is influenced by strips 23 and 24 when the coil is oriented parallel to the

direction of impinging flux (column 2, lines 23 to 41). However, U.S. Patent 4,486,731 relates to the problem of coil reception when the coil is in the same plane as the impinging flux, whereas the present invention relates to _ς increasing the reception of flux by a coil by providing a concentrator in juxtaposition to and in substantially the same plane as the coil.

U.S. Patent 4,754,284 discloses an automobile antenna system for use in receiving high frequency bands in - - excess of 50 MHz.

SUMMARY OF INVENTION

An object of the present invention is to provide a concentrator which can be associated or juxtaposed a coil without deleterious effect on the coil's ability to receive . _f . signals or magnetic fields.

Another object of the present invention is to provide improved concentrator performance.

The present invention in one form provides a concentrator adapted to concentrate signals for reception by a receiver means, said concentrator comprising an

20 electrically conductive portion juxtaposed said receiver means, said concentrator being aligned so as to receive said signals and cause further signals to impinge on said receiver means, and thereby enhance reception of said signals by said receiver means..

25

The present invention further provides a magnetic field concentrator comprising an electrically conductive portion, said concentrator being orientated to receive a magnetic field and cause flux to be radiated from said portion, the concentrator being juxtaposed a receiver means,

30 adapted to also receive said field, such that said receiver means operates to receive said flux in addition to said field.

The electrically conductive portion is preferably made of metal, for example aluminium or copper.

35

The present invention also provides a magnetic field concentrator adapted to surround a receiver means, the concentrator comprising a loop having first and second interconnected portions, wherein :

the first portion has first and second regions, the first portion being continuous between said first and second regions, the first region being juxtaposed and spaced from said second region, the first portion being adapted to c surround said receiver means, the second portion having third and fourth regions, the second portion being continuous between said third and fourth regions, the third region being juxtaposed and spaced from said fourth region, the second portion being adapted to ₁₀ substantially encircle said first portion, first and second interconnection portions, the . first interconnection portion being adapted to couple said first region to said third region, the second interconnection portion being adapted to couple said second _jc region and said fourth region, the loop being formed in a continuous manner and having a space provided between said first and second interconnection portions.

The concentrator described above may, upon being 2 _Q incident with a magnetic field, have substantially no magnetic field pass between said first and second portions, the field being diverted through said first portion and incident said receiver means.

The present invention also provides a magnetic ₂₅ field concentrator adapted to enhance field reception by a receiver means, the concentrator being juxtaposed and adapted to influence said receiver means, wherein the concentrator comprises a metal portion adapted to substantially surround said receiver means in one plane, the ₃₀ metal portion being further adapted to encircle the receiver means in the plane in a discontinuous manner, wherein, upon radiation by said field, the concentrator generates flux which is received by the receiver means in addition to said field. ,,. The present invention may provide a shield for alleviating radiation from magnetic fields within a predetermined area, the shield comprising a loop having first and second portions and third and fourth interconnecting portions,

the first portion having first and second regions, the first portion being continuous between said first and second regions, the first region being substantially opposingly juxtaposed and spaced from said second region, the first portion substantially encircling said predetermined area, the second portion having third and fourth regions, being aligned with the first and second regions respectively, the second portion being continuous between said third and fourth regions, the third reg ^a ion being ^y substantially opposingly juxtaposed and spaced from said' fourth region, the second portion being adapted to substantially encircle said first portion, the first interconnection portion being adapted to couple said first and fourth regions and the second interconnecting portion being adapted to couple said second and third regions, wherein said magnetic fields are substantially excluded from the area bounded by the first portion.

The portions of the shield and concentrators described above may in one form be substantially "C" shaped in configuration.

The present invention also provides a shield for substantially eliminating magnetic field radiation from within a predetermined area comprising a first conductive portion substantially encompassing said area and a second conductive portion substantially encompassing and being spaced from said first portion, end regions of said first and second portions being coupled in an overlapping arrangement so as to form one continuous strip, whereby, upon radiation by a magnetic field, the field is substantially diverted from said area.

The present invention may also provide a magnetic field concentrator which may be used to confine an alternating magnetic flux within an aperture and/or 5 concentrate a magnetic flux in a coil and/or shield components from the flux. The concentrator may be useful where a large coil for collecting flux is more expensive

than a concentrator and smaller coil or where the coil size presents problems. The concentrator may preferably be made from a material with good electrical conductivity which thereby improves the concentrator's performance. The degree of conductivity may determine the amount of flux radiated from the concentrator. Non-magnetic or magnetic conductors may also be contemplated.

Preferred embodiment(s) of the present invention will now be described with reference to the accompanying drawings, wherein : 0

Figures 1 and 2 show prior art arrangements.

Figures 3 to 14 show various exemplary (only) forms of concentrator according to the present invention.

Throughout this document, the term "coil" should be construed in a non-limiting way. The term "coil" may 5 include, for example, any signal receiving apparatus or magnetic field receptor as the present invention has many applications. As can be appreciated, the shape of the metal herein described should not be limited to a particular configuration. The shape of metal is dependent on its 0 application or use.

Also, throughout this document, the terms "signal" or "signals" include within their scope any form of electromagnetic radiation. The signal may, for example, be a powering signal or a data or informational signal.

Although metal placed proximate a receiving coil is known to reduce the coil's receptive ability, a metal sheet placed in juxtaposition and/or in substantially the same plane as the coil as will be hereinafter detailed will not

„ have a negative effect on the coil's receptive ability. 0

Figure 3 shows a metal sheet placed in the same plane as the coil. The metal serves to increase the amount of flux, impinging the coil. The increase of flux on the coil due to the metal is inversely proportional to the spacing between the metal and the coil. 5

Figure 4 shows a coil surrounded in the coil's plane by metal. As can be seen, the additional fluxes produced by circulating currents, opposes and reduces the

applied currents. Accordingly, no flux enhancement is produced for the coil the metal surrounds.

Figure 5 shows metal similar in shape to that previously mentioned, however, a slot or gap is provided in the metal so that the metal surrounds the coil in a discontinuous manner.

The gap in the metal surrounding the coil causes the eddy currents (produced in response to impinging flux) to produce a field that serves to increase the flux impinging the coil.

Figure 6 shows a concentrator similar to that -. hereinbefore described.

The concentrator may be preferably constructed in two formε :-

(i) A metal plate with a hole cut to allow the magnetic flux to pass through. A slot or slots are cut from the hole out to the perimeter to alleviate circulating currents, which causes a drastic reduction in flux, from encircling the hole. The slot(s) may overlap, as long as there is substantially no continuous conduction path around the central hole (Figures 5, 6 and 7). Co-pending Applications PI 7198 ANTENNA STRUCTURE AND METHOD OF MANUFACTURE and PJ 1693 INDUCTIVE ELEMENT FOR USE AS AN ANTENNA IN TRANSPONDERS filed in the name of Magellan Corporation (Australia) Pty. Limited disclose a method of simultaneously fabricating electrical coils and capacitors. Conveniently, the plates of capacitors so fabricated may take a substantially "C" shaped configuration and be disposed to surround the associated coils as hereinbefore described. This technique permits the area consumed by said capacitor plates to contribute towards the flux gathering ability of said coils.

(ii) A wire loop concentrator using high conductivity wire, for example bent to follow the perimeter of the metal plate shown in Figures 7, 9 and 10, may

perform the same concentrating function provided the wire forms a continuous conducting path. The operation of both exemplary forms as shown in the drawings can be described thus :- c (i) Circulating currents induced on the surface of a metal plate prevent an alternating magnetic flux from penetrating below the skin depth. For lOOKHz on copper, this is about 0.18mm. Consequently, an alternating flux cannot penetrate thick metal , _Q plates and flows around the conducting obstacle.

With a hole cut in the metal plate, some of the" flux interrupted by the plate is diverted through the hole increasing the flux density in that area, while the balance goes around the outer edge of the ₁₅ plate (Figure 8). Without the slot, the metal plate acts as a one turn short circuit. This may maintain an almost equal in magnitude, oppositely directed flux in the central hole cancelling most of the flux trying to pass through it. This may _ ₀ have a negative effect for magnetic field concentration purposes, but may be used to substantially exclude flux from an area, (ii) The wire loop acts as a one turn short circuit. The back emf generated in the wire loop ensures _ ₅ that the total flux passing through the space between the inner and outer loops is very small, only enough to account for ohmic losses. The flux intercepted by the loop configuration is concentrated in the inner loop in substantially the -,„ same manner as for the metal plate (Figure 9).

The wire loop concentrator may also be used to substantially exclude flux from an area. By crossing the wire connections between the inner and outer loops, without allowing them to touch, the flux passing through the inner _3C . loop is drastically reduced. Figure 10 shows an example of this configuration.

Figure 11 shows an example of a field concentrator acting as an electrostatic Faraday shield. The coil is

- 8 - shown partially surrounded by a conventional Faraday shield. The Faraday shield is extended to form a field concentrator, or may be coupled to an existing concentrator.

Electrostatic shielding reduces the capacitive sensitivity of the coil to objects in the vicinity of the coil. The Faraday shield may extend only part way around the coil in order to adjust capacitive sensitivity.

Alternatively, field concentration and electrostatic shielding can be achieved using two field concentration plates. One placed in front of the coil, the other behind the coil as shown in Figure 12.

Figure 13 shows a cross-section of this arrangement. Connection between the front and back plates can be made anywhere along the plates, however, preferably this is done on the inside and/or the outside of the coil. See Figures 14A, B and C.