Login| Sign Up| Help| Contact|

Patent Searching and Data


Title:
STRUCTURES WITH MAGNETIC PROPERTIES
Document Type and Number:
WIPO Patent Application WO/2003/032438
Kind Code:
A1
Abstract:
A microstructured material consists of an array of Swiss roll resonant elements having capacitance and inductance such that the structure exhibits a predetermined magnetic permeability at r.f. frequencies, wherein at least one of the elongate means consisting for example of a roll of conducting material (3) wound interspersed with insulating material on a mandrel (4) is tuned to a desired frequency by applying a sleeve wound into a roll, to an end of the Swiss roll at a given overhang (x), and of an appropriate width, which can be determined for example from reference measurements showing the width necessary to make a desired adjustment. The adjustment could also be made by selecting a suitable overhang for a given width of sleeve.

Inventors:
Wiltshire, Michael Charles Keogh (3 The Brackens, High Wycombe, Bucks. HP11 1EB, GB)
Steele, Timothy Charles (3 Fylingdale, Brampton Park, Northampton NN2 8WR, GB)
Application Number:
PCT/GB2002/004376
Publication Date:
April 17, 2003
Filing Date:
September 27, 2002
Export Citation:
Click for automatic bibliography generation   Help
Assignee:
MARCONI UK INTELLECTUAL PROPERTY LTD (New Century Park, P.O. Box 53, Coventry CV3 1HJ, GB)
Wiltshire, Michael Charles Keogh (3 The Brackens, High Wycombe, Bucks. HP11 1EB, GB)
Steele, Timothy Charles (3 Fylingdale, Brampton Park, Northampton NN2 8WR, GB)
International Classes:
H01Q15/00; H01Q17/00; (IPC1-7): H01Q17/00
Domestic Patent References:
WO2000041270A1
Foreign References:
EP0439337A2
US5812080A
Other References:
SASAKI T ET AL: "Generation of homogeneous high magnetic fields within superconducting Ð?Swiss rollÐ?" CRYOGENICS, IPC SCIENCE AND TECHNOLOGY PRESS LTD. GUILDFORD, GB, vol. 35, no. 5, 1 May 1995 (1995-05-01), pages 339-343, XP004038164 ISSN: 0011-2275
Attorney, Agent or Firm:
Waters, Jeffrey (Marconi Intellectual Property, Crompton Close Basildon, Essex SS14 3BA, GB)
Download PDF:
Claims:
CLAIMS
1. A structure with magnetic properties comprising an array of elongate means having capacitance and inductance, the structure exhibiting magnetic permeability at r. f. frequencies, at least one elongate means having a sleeve extending from an end of the elongate means for tuning the elongate means to a desired r. f. frequency.
2. A structure as claimed in Claim 1, in which the sleeve comprises a roll of conducting sheet, the turns of which are separated by an insulating material.
3. A structure as claimed in Claim 1 or Claim 2, in which the sleeve is wound around the outside of the elongate means.
4. A structure as claimed in any one of Claims 1 to 3, in which the elongate means comprises a roll of conducting sheet, the turns of which are separated by an insulating material.
5. A structure as claimed in Claim 4, in which the elongate means and the sleeve are made of the same material.
6. A structure as claimed in Claim 5, in which the width of the sleeve in the peripheral direction is chosen to produce a desired adjustment in resonant frequency.
7. A structure with magnetic properties substantially as herein described with reference to the accompanying drawings.
8. An elongate means to which a sleeve has been applied for tuning purposes for use in an array in a structure as claimed in any one of Claims 1 to 7.
9. A method of tuning an elongate means having capacitance and inductance to be arranged in an array to form a structure exhibiting magnetic permeability at r. f. frequencies, which comprises applying a sleeve extending from an end of the elongate means in order to tune that elongate means to the desired frequency.
10. A method of tuning elongate means of a structure to a desired r. f. frequency substantially as herein described with reference to the accompanying drawings.
Description:
STRUCTURES WITH MAGNETIC PROPERTIES This invention relates to structures with magnetic properties at radio frequencies.

The invention particularly relates to structures comprising an array of elements having capacitance and inductance, magnetic permeability being exhibited at wavelengths greater than the spacing of the elements (Magnetism From Conductors and Enhanced Non-Linear Phenomena, J B Pendry, A J Holden, D J Robbins and W J Stewart, IEEE Transactions on Microwave Theory and Techniques, 1999, 47,2075-2084). These microstructures can be designed to show quite large positive or negative permeability in the r. f. range, for example, at MHz or GHz. Typically the elements are spaced at less than a fifth of the wavelength of the radiation at which the microstructure is resonant, but they could be spaced by greater amounts (less than one half of the resonant wavelength for example), or lesser amounts (less than one tenth, or less than one hundredth), of the resonant wavelength, for example.

Typically, the magnetic permeability varies with the frequency of the incoming radiation, and the use to which the structure is put often requires a predetermined magnetic permeability. For example, the structures can be used as an r. f. flux guide (WO 01/67750, WO 01/67125) which requires a positive enhanced magnetic permeability. The structures can also be used as an r. f. screen (WO 01/67553, WO 01/67126), for which the magnetic permeability should be zero or negative at the frequency of interest. The structures have been proposed for focussing (British Patent Application No. 0015067.2) which requires a magnetic permeability of-1. Equally, for

some applications, particularly when the structures are used as flux guides or screens in magnetic resonance imaging, a use to which they are particularly suited since the structures do not exhibit any magnetic properties in static magnetic fields, the structure must be tuned to a precise value, for example, 21.3 MHz.

One form which the elements of such a microstructure can take is a roll of conducting sheet, the turns of which are separated by insulating material (a so-called"Swiss roll" structure). Inductance is provided by currents circulating around the curved wall of the Swiss rolls, and capacitance is provided by the self-capacitance between the inner and outer ends of the roll.

The r. f. frequency to which the microstructure is tuned is the frequency to which each element is tuned.

Figure 1 shows a Swiss roll element 1, together with a typical arrangement of the rolls in an array 2. The material is designed to produce a magnetic. permeability jUeff to incoming radiation K normal to the axes of the rolls, the magnetic vector H of which is parallel to the axes of the rolls, according to the following formula:

Here, r is the core radius, a the lattice spacing, N the number of turns of the spiral. The sheet resistance of the conductive layer is p, while the permittivity of the dielectric interlayer is between the conducting sheets of the roll e. The layer spacing of the conducting sheets of the roll is d. co is the speed of light in vacuo and, go is the permeability of free space. The frequency of the incoming r. f. radiation is.

The formula applies to the ideal situation where all the rolls have the same layer spacing d. If each roll of the array had a narrow resonance, but all differing slightly from each other, both the real and imaginary part of the magnetic permeability of the microstructure would have broad resonant peaks, and a broad peak in the imaginary part of the magnetic permeability implies loss, which would make the microstructure unsuitable for some applications such as the focussing one referred to earlier.

It is therefore desirable that each individual element of the array should have a narrow resonant peak in the permeability, and that all these should lie at the same frequency.

Of particular concern in the manufacture is the tightness of the winding: even a small variation in the layer spacing, caused by differences in the winding, can have a significant effect on the resonant frequency. For example, when the proprietary material Espanex (Trade Mark) (which comprises 18pm of copper deposited on a substrate of 12, um of Kapton (Trade Mark) ) is used to construct Swiss rolls for use in the region of 30 MHz, a spread of resonant frequency of about 10% is observed.

Other forms of resonant element which, when arranged in an array, are suitable for forming microstructures with magnetic properties at r. f. frequency are disclosed in WO

00/41270, for example, a stack of planar loops, and the stack operates in a similar manner to the Swiss rolls. Because the planar loops can be printed, there should be less variation between the resonant peaks corresponding to each individual stack of such planar loops.

The invention provides a structure with magnetic properties comprising an array of elongate means having capacitance and inductance, the structure exhibiting magnetic permeability at r. f. frequencies, at least one elongate means having a sleeve extending from an end of the elongate means for tuning the elongate means to a desired r. f. frequency.

The sleeve or sleeves enable the elongate means to be tuned all to the same r. f. frequency or to be adjusted from one r. f. frequency to another.

The sleeve advantageously comprises a roll of conducting sheet, the turns of which are separated by an insulating material. This adds to the capacitance of the elongate means to facilitate tuning.

The elongate means may comprise a roll of conducting sheet, the turns of which are separated by an insulating material, and such a sleeve may extend inside the roll or may surround the roll, with or without a spacer between the exterior of the roll and the interior of the sleeve, or the exterior of the sleeve and the interior of the roll. The sleeve is preferably, but does not have to be, of the same material as the roll, which is usually selected for inherently desirable properties for the application in question, and the width

of sleeve in the winding direction may be chosen to produce a desired adjustment in the resonant frequency. The resonant frequency of an unadjusted roll can be measured by inserting it into a coil which is driven at a suitable r. f. frequency and measuring the variation of inductance and resistance with frequency. The frequency at which the resistance peaks corresponds to the resonant frequency of the roll. A particular width of sleeve may then be chosen using previous measurements to obtain the desired offset in the resonant frequency. Alternatively, the length of overlap of a given sleeve could be adjusted.

The invention is equally applicable to a sleeve extending down the hollow axis of a column of planar loops forming an elongate means having capacitance and inductance.

Ways of carrying out the invention will now be described in detail, by way of example, with reference to the accompanying drawings, in which: Figure 1 shows a known Swiss roll resonant elongate means together with an array of such Swiss rolls; Figure 2 (not to scale) shows part of a Swiss roll resonant element in axial cross-section together with a tuning sleeve which is also shown projected into an unwound position; Figure 3 is a graph of relative frequency shift against sleeve width for a particular length of Swiss roll resonant element for three different overhangs of the sleeve relative to the roll; and

Figure 4 is a plot of the frequency distribution of one batch of Swiss rolls before and after tuning.

Referring to Figure 1, a structure with magnetic properties according to the invention comprises an array of Swiss roll resonant elements such as is shown in Figure 1. Each Swiss roll 1 of the array 2 comprises a roll of conducting sheet, the turns of which are separated by means of an insulating material (not shown) such that adjacent turns are separated by a radial distance d. Such a microstructured material exhibits non-unity magnetic permeability at wavelengths much greater than the spacing of the element to incoming radiation K incident normal to the axes of the rolls, the magnetic vector H of which is parallel to the axes of the rolls. The microstructured material can take any of the forms described in WO 00/41270 or WO 01/67549.

In one example, it was desired to tune 91 rolls, each formed from a sheet 50mm long and 750mm in width of Espanex (Trade Mark), rolled onto an 8mm diameter mandrel, to a common frequency in the region of 32 MHz. Espanex (Trade Mark) consists of zum of copper deposited on a substrate of 12u. m of Kapton (Trade Mark).

Tests were made to establish the resonant frequency of each individual Swiss roll. To do this, r. f. measurements were made using an HP4195A network analyser to record the series inductance and resistance of the Swiss roll as a function of the frequency of a solenoid, which was 160mm long with a diameter of 60mm, wound with eight turns of <BR> multi-strand wire (M C K Wiltshire et al, Science 291,849-851 (2001) ). Readings were taken from the network analyser both when the solenoid was empty and when the Swiss

roll was inserted along its axis, which changes the inductive loading on the analyser. At resonance, the load is purely resistive, and the resonant frequency of the Swiss roll can be determined by noting the frequency at which the resistance of the solenoid shows a peak.

The dimensions of the 91 rolls thus measured were chosen to produce a desired magnetic permeability at a desired resonant frequency using the formula for the magnetic permeability given before but, referring to Figure 4, it will be seen that there is a spread of resonant frequencies between 29.6 and 33.3 MHz.

In accordance with the invention, a sleeve is applied if required to an end of each Swiss roll, in order to bring its resonant frequency to a target resonant frequency, in this example, 32.3 MHz. Thus, referring to Figure 2, one Swiss roll is shown comprising Espanex (Trade Mark) 3 rolled onto a mandrel 4, and to one end of the roll is applied a sleeve 5 of the same material. The sleeve 5 is also in the form of a roll of Espanex (Trade Mark), the sheet of Espanex (Trade Mark) having length 1, width w and overhang over the end of the Swiss roll 1-x. The sleeve 5 is co-extensive with the roll 3 over a length x. The width of the sleeve is selected to bring the resonant frequency to the target frequency.

The width necessary is established by experiments carried out using a variety of sleeves of different length, width and overhang to establish their impact on the resonant frequency. The results of one such experiment as shown in Figure 3, which demonstrates that the frequency shift can be controlled by varying the width of the

sleeve for a selected length, the length 1 being 40mm in these experiments. The experiments were carried out using 3 different overhangs of the sleeve relative to the end of the Swiss roll i. e. the overhang 1-x being successively 15mm, 10mm and 5mm for the plots on the graph shown in Figure 3 from top to bottom, respectively.

The central graph corresponding to overhang 1-x = 10mm indicates how different widths of material enable the relative frequency shift i. e. the percentage frequency shift compared to the resonant frequency can be either positive or negative, depending on the width of the sleeve.

Thus, from data such as is shown in Figure 3, each roll is fitted with a sleeve of appropriate width, if necessary, in order to tune them to the desired target frequency.

Figure 4 shows the frequency distribution of the 50mm rolls before and after tuning to the target frequency of 32.3 MHz. It will be noted that the spread of frequencies has been greatly reduced.

Adhesive is used to secure the sleeve in position.

It is believed that the reason that the sleeve tunes the Swiss roll to which it is attached is that the capacitance of the combined structure is altered. It follows that it is not necessary for the material of the sleeve to be the same as that of the Swiss roll. Other thicknesses and types of rolls of conductor could be applied to the end, successive turns being spaced by an electrical insulator, although it will usually be found desirable to use the same material as that of the Swiss roll, because that has usually been selected for

advantageous properties for the use to which the structure is to be put. There is no need for the sleeve to surround the outside of the Swiss roll, and it could in fact be inserted into the end of the Swiss roll, provided the mandrel was suitably cut away to allow this.

Further, there is no necessity for the sleeve to be in contact with the end of the Swiss roll. Insulating packing material could be wrapped around the end of the Swiss roll and the sleeve could be wound around this. Equally, if the sleeve was inserted into the end of the Swiss roll, it could be spaced from the inside thereof by a suitable spacer.

Instead of adjusting the width of the sleeve, the length of the overhang could be adjusted instead for sleeves of a given width to produce a desired tuning adjustment.

The sense of winding of the sleeve, whether of the same or of the opposite sense to that of the roll, has no significant effect on the tuning. Further, a second sleeve may be added to the opposite end of the roll if desired. This has an additive effect. For example, two 30mm sleeves, arranged on the two ends of the roll have the same effect as a single 60mm sleeve, arranged to have the same total overhang distance.

The procedure for tuning the Swiss rolls could be readily automated.

While the invention has been described in relation to Swiss rolls, other forms of resonant elongate means having capacitance and inductance, arranged in an array to form a microstructured material, may be tuned by the sleeves described. For example, sleeves could be used with split cylinders or with columns of printed loops, both as described in WO 00/41270, although in the latter case it would probably be simpler for

the sleeves to extend into the ends of the spirals, the substrate being suitably bored through.

Although the resonant elements described have been for use in the MHz region, the invention is applicable to any radio frequency including microwave, for example in the GHz region as well.