EP0505040A1

EP0505040A1 - Microwave devices for controlling the direction of a beam

Info

Publication number: EP0505040A1
Application number: EP92301523A
Authority: EP
Inventors: Peter Miles Brigginshaw
Original assignee: GEC Marconi Ltd; Marconi Co Ltd
Current assignee: BAE Systems Electronics Ltd
Priority date: 1991-03-22
Filing date: 1992-02-24
Publication date: 1992-09-23
Also published as: GB2253947A; GB9106106D0

Abstract

A microwave beam steering device comprises a block (1) of ferromagnetic material across which a magnetic field gradient is applied, using coils (9N, 9S, 11N, 11S) within the block. Microwave radiation incident on the block in a direction perpendicular to the field gradient is deflected, the direction (ϑ) and extent of deflection being dependent on the magnetic field.

Description

This invention relates to microwave devices.
More particularly the invention relates to microwave devices adapted to be positioned in the path of a beam of electromagnetic microwave energy for controlling the direction of the beam leaving the device.
In a known microwave beam controlling device, the microwave beam passes through a rectangular block of dielectric material formed by two wedge-shaped members, one of ferrite and one of non-ferrite material with their sloping faces in juxtaposition. An external magnetic field is applied to the block in a direction perpendicular to the direction of propagation of the microwave beam. Since the permeability of the ferrite material varies with the magnetic field applied, a beam travelling through the ferrite will travel more quickly when a magnetic field is applied. If a microwave beam is directed through the block so as to travel in turn through the ferrite and non-ferrite material, opposite edges of the beam will travel through different lengths of ferrite material causing a differential phase shift. Thus the beam is deflected since the phase at one edge lags that at the other edge.
Devices of this kind are difficult to construct and prone to errors due to beam reflection at the junction between the ferrite, and non-ferrite wedge-shaped members. They produce beam deflection in one plane only; two devices in series would be required to produce conical steering. Only an external solenoid can be used to control such devices.
According to the present invention there is provided a device adapted to be positioned in the path of a beam of electromagnetic microwave energy for controlling the direction of the beam leaving the device comprising a body of dielectric material having a permeability which varies with the strength of an applied magnetic field and means for applying to said body a magnetic field characterised in that said field has a gradient in a direction perpendicular to the direction of propagation of said beam.
Preferably the body is in the form of a rectangular block of said material.
Preferably the magnetic field is applied in a direction substantially parallel to the direction of propagation of the beam.
It is preferred that the magnetic field be in opposite directions on opposite sides of said beam. The resultant flux along the direction of propagation of the beam will then be zero. The means for applying said magnetic field suitably then comprises sources of magnetic flux positioned on opposite sides of the axis defined by the direction of propagation of said beam, said sources being arranged to produce magnetic fields in opposite directions parallel to the direction of propagation of the beam.
To achieve conical beam steering the magnetic field may have a gradient in each of two respective directions perpendicular to one another and to the direction of propagation of the beam. This enables the direction of the beam to be controlled in two orthogonal directions and may suitably be achieved by embedding a pair of coils in proximity to each of the four faces of the dielectric block through which the incident beam does not pass.
The gradient or gradients are preferably substantially linear.
Ferrite material has proved particularly suitable for the dielectric material of a device according to the invention since it combines high permeability with low conductivity and thus has low losses. Due to this low conductivity, ferrites are easily penetrated by microwave fields.
One device in accordance with the invention will now be described by way of example with reference to the accompanying drawing which is a diagram illustrating the device.
Referring to the drawing, the device comprises a square block 1 of ferrite material having pairs of identical circular coils 9N, 9S and 11N, 11S embedded in recesses provided in opposite faces 3 and 5 of the block 1. Each coil is positioned with its axis normal to the faces 3, 5 with the axes of the coils 9N and 9S in parallel spaced relationship and the coils 11S and 11N respectively coaxial with the coils 9N and 9S.
In use of the device the coils 9S, 9N, 11S, 11N are energised by the same current source so that the magnetic field produced by the two coils 9N and 9S in the central region of the block is in a direction generally normal to faces 7 and 13 of the block and in the opposite direction to the magnetic field produced in the central region of the block 1 by the coils 11N and 11S, as indicated by lines F. As a result the magnetic field produced in the central region of the block 1 when the coils are energised has a gradient in the direction parallel to the axes of the coils, i.e. to the faces 7 and 13, with zero magnetic field in a plane parallel to and central between the faces 3 and 5.
When a beam 21 of circularly polarised microwave energy is directed centrally onto the face 7 of the block 1 in a direction normal to the plane of the face 7 by means of a suitable lens arrangement 23 e.g. a dielectric lens, and no current is supplied to the coils, the beam 21 emerges from the block 1 via the face 13 opposite the face 7 in the same direction as the beam 21 is incident on the face 7. However, when a current is supplied to the coils the beam 21 emerges from the block 1 in a direction at an angle ϑ to the normal to the face 13, where ϑ lies in a plane parallel to the resultant field and increases as the current applied to the coils increases.
The deflection of the beam arises as a result of differential phase shift across the beam in the direction of the magnetic field gradient on the magnetic field directed parallel to the direction of the propagation of the beam. This differential phase shift is caused by changes in the permeability of the ferrite along the direction parallel to the axes of the coils. Between the central plane and face 3 the magnetic field is in one direction, whilst it is in the opposite direction between the central plane and face 5. Since the permeability of the ferrite depends on the direction and magnitude of the magnetic field, the phase at the top of the beam will lag that at the bottom of the beam and the beam will be deflected upwards. To deflect the beam downwards, the direction of current flow in the coils is reversed to switch the direction of the magnetic fields.
The degree of deflection is controlled by varying the curent supplied to coils to alter the magnitude of the magnetic fields.
In one particular embodiment of the device shown in the figure, the ferrite block has dimensions of 30 mm. The surfaces of the block are drilled to provide circular recesses into which coils of diameter 0.8 cm, each having 4 turns of 0.6mm diameter wire, may be inserted. To complete the magnetic circuit for the flux produced within the block, each recessed face of the block is provided with a cover (not shown) of the same ferrite material as the block which is approximately 5mm thick. The effect of these covers is to reduce flux leakage to the atmosphere. The ferrite material is a magnesium ferrite material of type TT1-3,000 sold by Trans-Tech Inc. which has a dielectric constant of 12.5 and a saturation magnetisation value of 0.3 Tesla.
A beam of circular cross-section of diameter 20 mms and frequency 95 GHz is deflected by the device through 8° when a current of 1.6 A passes through all the coils.
The device is suitably matched to free space at its input and output ends by means of a coating (not shown) of dielectric material on faces 7 and 13.
To establish the required magnetic field gradient in the block material, the windings may be energised continuously.
However, if the material used is a ferrite which has a 'square' B-H characteristic, pulses may be applied to the windings to latch the remanent magnetisation in the ferrite to desired points on the B-H characteristic in a manner well known with other ferrite devices. No demagnetizing fields are present and no holding current is required.
It will be appreciated that the dielectric material chosen should exhibit low loss at the microwave frequencies concerned, satisfactory power handling capability, good temperature stability and high saturation magnetisation value, the latter so that the largest possible maximum beam deflection is obtained.
One particular application envisaged for a device in accordance with the invention is in a rapid-scanning antenna e.g. in radar equipment, the device having the advantage over conventional such antennae that no mechanical motion is involved.
In general the device may find application in any equipment wherein quasi-optical transmission of radio waves between components of the system is employed.
Whilst the coils which produce the magnetic field within the block of dielectric material are shown as having a circular cross-section, they may alternatively have an elliptical cross-section to produce a more uniform magnetic field in proximity to the sides of the block.
To reduce edge effects the edges and corners of the block may be bevelled.
To achieve conical beam steering, two further pairs of coils, similar to coils 9N, 9S, 11N, 11S, are embedded in recesses provided in opposite faces 25 and 27 of block 1. These further pairs of coils produce a second gradient in the magnetic field, perpendicular to the gradient produced by coils 9N, 9S, 11S, 11N and the direction of propagation of the beam 21.

Claims

A device adapted to be positioned in the path of a beam (21) of electromagnetic microwave energy for controlling the direction (ϑ) of the beam leaving the device comprising a body (1) of dielectric material having a permeability which varies with the strength of an applied magnetic field and means (9N,9S,11N,11S) for applying to said body (1) a magnetic field, characterised in that said field has a gradient in a direction perpendicular to the direction of propagation of said beam (21).
A device as claimed in Claim 1 wherein said magnetic field is applied in a direction substantially parallel to the direction of propagation of the beam (21).
A device as claimed in Claim 1 or Claim 2 wherein the magnetic field is in opposite directions on opposite sides of the axis defined by the direction of propagation of said beam (21).
A device as claimed in Claim 3 wherein said means for applying to said body (1) a magnetic field comprises sources (9N,9S,11N,11S) of magnetic flux (F) positioned on opposite sides of the axis defined by the direction of propagation of said beam (21) , said sources (9N,9S,11N,11S) being arranged to produce magnetic fields in opposite directions parallel to the direction of propagation of the beam (21).
A device as claimed in Claim 4 wherein each said source comprises a pair of coils (9N,9S,11N,11S) embedded in said body with their axes substantially perpendicular to the direction of propagation of said beam (21) and spaced in the direction of propagation of said beam (21).
A device as claimed in any preceding claim wherein said magnetic field has a gradient in each of two respective directions perpendicular to one another and to the direction of propagation of said beam (21).
A device as claimed in any preceding claim wherein said gradient or gradients are substantially linear.
A device as claimed in any preceding claim wherein said dielectric material is a ferrite.
A device as claimed in any preceding claim wherein said body is in the form of a rectangular block of said material.