Magnetrons

Abstract: A magnetron having a rising sun anode includes a slot 8 which is extensive in a plane transverse to the axis X-X and in which a tuning member 9 is located. The tuning member 9 may be moved inwardly and outwardly to control the frequency of the magnetron output radiation, the member 9 being arranged such that it can intercept one or more of the larger anode cavities by a variable amount to change their resonant characteristics.

Description: This invention relates to magnetrons and more particularly to magnetrons which are capable of being tuned to vary the frequency of their output radiation. A magnetron includes a cathode and anode arranged coaxially about a longitudinal axis, the anode defining a plurality of resonant cavities, The frequency of radiation generated by the magnetron is principally determined by the dimensions of the resonant anode cavities, especially their length in the axial direction and also, but not to such a significant extent, their radial lengths. One method which is currently employed to proved frequency tuning involves use of an annular plate arranged over the anode cavities. The plate is vibrated so as to change its distance from the anode and hence alter the resonant frequency characteristics. In another method, prongs are inserted by a variable amount into the cavities to produce perturbations causing the output frequency to change. FR-A-2 104 094 discloses method of tuning a magnetron, which may be of a rising sun configuration, in which plungers are slidable in holes to enter one or more anode cavities to vary the frequency of the output radiation. The present invention arose from an attempt to provide a magnetron frequency tuning member mechanism which is relatively inexpensive to implement and which permits rapid, accurate changes in output frequencies to be achieved. According to the present invention there is provided an anode coaxially arranged about a cylindrical cathode, the anode having a plurality of larger and a plurality of smaller anode cavities arranged in a rising sun configuration, and wherein a slot in the anode is extensive in a plane substantially orthogonal to the longitudinal axis of the anode; and a tuning member of dielectric material which is movable in the slot so as to enter at least two of only the larger anode cavities by an amount which is variable to vary the frequency of the output radiation. The longitudinal axis is that about which the magnetron anode and cathode are coaxially arranged. When the tuning member's position in the slot alters, the resonant characteristics of the cavity or cavities it intercepts are changed in dependence of volume which enters them and the material of which the member consists. The change in the resonant characteristics causes the output frequency to be varied. By employing the invention, tuning may be achieved whilst experiencing no, or very small, losses in output power because induced currents in the cylindrical anode wall tend to flow in a generally circumferential direction and therefore do not intercept the slot. The slot may be arranged to guide the tuning member relatively closely, enabling it to be moved quickly and accurately to alter the resonant frequency characteristics. Also, as the slot is arranged orthogonal to the longitudinal axis, the magnetron may be made more compact in the axial direction than would be the case were conventional tuning mechanisms employed for which it is necessary to be able to access the anode from its ends. The invention may be applied to anode structures which are open and include strapping and to closed-end anodes. The invention may be particularly advantageously employed where the anode is of the closed-end type in which conductive end plates are used to define the anode cavities in the radial plane in addition to the usual cavity-defining surfaces in the axial direction so that the only opening in the cavity is that directed towards the cathode. It is sometimes desirable to use such a configuration when the magnetron operates at relatively high frequencies and by using the invention, the advantages of such a design may be obtained whilst still enabling frequency tuning to be implemented. It may be preferred to locate the slot substantially mid-way along the anode in the axial direction. This is particularly suitable for a closed end anode device because the end plates constrain the voltage distribution such that there is a voltage maximum of the anode centre. However, the slot could be located at other positions along the axis. The tuning member is of a dielectric material, such as alumina, which lowers the output frequency of the magnetron as it is gradually inserted into the anode cavity. In this case, the dielectric tuning member alters the capacitance when it is moved to produce the change in resonance frequency. The use of a dielectric material is particularly advantageous as leakage is reduced and hence the tolerances required for the fit of the tuning member in the slot need not be so tight as would be necessary with a metal tuning member. It has been found that the use of a dielectric tuning member gives good results for a closed-end anode magnetron. The number of the larger anode cavities which may be intercepted by the tuning member when it is moved inwardly to its greatest extent may range from two to more cavities. If a larger number of anode cavities are entered by the tuning member, the tuning range is extended compared to that available when only two, for example, can be intercepted. It may be preferred that the slot be arranged in the part of the anode diametrically opposite means for extracting output radiation from the magnetron. The means may comprise a slot in the anode wall extending in an axial direction or might be for example, a loop by which radiation can be coupled out of the device. The tuning member may be such that its leading edge which intercepts the larger anode cavities is straight. Then, for example, one cavity would first of all be intercepted, and then the larger anode cavities to each side of it subsequently entered by the tuning member, as it is moved further inwardly. It may be preferred however to use a tuning member which is curved to give a concave leading edge. This may be arranged to intercept two or more cavities simultaneously at a certain position of travel and to ensure that similar amounts of the tuning member enter each of the intercepted cavities at the same time. The frequency tuning characteristics may be further controlled by varying the radial lengths of the anode cavities around the anode. This may be achieved even in a rising sun configuration by arranging that although the larger cavities may, say, be longer in the radial direction if they are nearer the slot than those further away, that the smaller cavities also change proportionally in size, so as to retain the relationship between the two sizes. It may be preferred to include a plurality of slots, each slot being extensive in a plane parallel to that in which the or each other is extensive and each slot including a tuning member movable therein which is movable to enter at least one of the larger anode cavities by an amount which is variable. The slots may be arranged at the same distance along the anode in the axial direction so that they all lie in the same plane or they could be arranged at different distances, for example, one above another in the axial direction. For example, a closed end anode magnetron may have two slots arranged substantially at the mid-point of the anode in the axial direction with only a small distance between them. The tuning members may be able to intercept the same anode cavities or respective different ones. The tuning members may be moved in synchronism or independently. Some ways in which the invention may be performed are now described by way of example with reference to the accompanying drawings in which: Figure 1 is a schematic longitudinal section through a magnetron in accordance with the invention; Figure 2 is a view along the line II-II of Figure 1; Figure 3 is a schematic longitudinal section of another magnetron in accordance with the invention having a closed-end anode; Figure 4 is a view along the line IV-IV of Figure 3; Figure 5 schematically illustrates in transverse section another magnetron in accordance with the invention; and Figure 6 is a schematic transverse section through a further magnetron in accordance with the invention. With reference to Figures 1 and 2, a magnetron includes a cathode 1 surrounded by a cylindrical anode 2 arranged coaxially about a longitudinal axis X-X. Magnetic pole pieces 3 and 4 produce a magnetic field parallel to the longitudinal axis X-X in the interaction region of the device between the cathode 1 and anode 2. The anode 2 includes a plurality of anode cavities arranged in a rising sun configuration, having larger cavities 5 and cavities 6 between them with a smaller radial dimension, the larger cavities 5 having a radial length approximately three times that of the smaller cavities 6. A longitudinal slot 7 in the anode 2 parallel to the axis X-X enables radiation to be coupled from the magnetron into an output waveguide. A slot 8 in the anode 2 is extensive in a plane orthogonal to the longitudinal axis X-X and parallel to the direction of current flow in the walls of the anode 2 defining the resonant cavities 5 and 6. The slot extends into the anode 2 to such an extent that it opens into the walls of three of the larger cavities 5. A tuning member 9, which comprises a planar plate of dielectric material, is located in the slot 8 and is movable inwardly and outwardly in the direction indicated by the arrows by an actuator mechanism shown at 10. In the position illustrated, the tuning member enters only the central anode cavity 5a of the three it is capable of intercepting. If it is moved inwardly towards the cathode 1, a greater area of the cavity 5a is intercepted and the cavities 5b and 5c are also entered by the metal tuning member 9. This causes the frequency of the generated radiation to be decreased. By moving the tuning member 9 outwardly, the frequency is increased. With reference to Figures 3 and 4, a magnetron includes an anode 11 which is of a rising sun configuration and which is of the closed-end type, having two annular end plates 12 and 13 fixed on either side of the anode 11 to further define the anode cavities. A slot 14 is located in the central transverse plane of the anode 11 orthogonal to the longitudinal axis X-X. A planar dielectric tuning member 15 is located in the slot and movable inwardly and outwardly to vary the frequency of the generated radiation. The leading edge of the tuning member 15 is curved as can be seen in Figure 4. Thus, as the tuning member 15 is moved inwardly a plurality of the larger cavities 16 are entered by the tuning member at substantially the same time. As the magnetron is a closed-end anode device, the central plane in which the slot is located is positioned at a voltage maximum and the current is a minimum, leakage through the tuning member 15 thus being extremely low. The dielectric material acts to lower the frequency as the tuning member 15 is pushed inwards towards the cathode. With reference to Figure 5, another magnetron in accordance with the invention includes three slots 17, 18 and 19 which in this embodiment are located in a common plane which is orthogonal to the longitudinal axis. A tuning member 20, 21 and 22 is associated with each of the slots 17, 18 and 19 respectively and is movable independently from the others so as to give greater control over the frequency changes provided by their positioning. Figure 6 illustrates another magnetron in accordance with the invention in which one tuning member 23 is employed to provide frequency tuning. The anode is of the rising sun type and includes anode cavities having a radial dimension which depends on their positions relative to the slot 24.

Claims: A magnetron comprising: an anode (2, 11) coaxially arranged about a cylindrical cathode (1), the anode (2, 11) having a plurality of larger (5) and a plurality of smaller (6) anode cavities arranged in a rising sun configuration, and wherein a slot (8, 14) in the anode (2, 11) is extensive in a plane substantially orthogonal to the longitudinal axis of the anode; and a tuning member (9) of dielectric material is movable in the slot so as to enter at least two of only the larger anode cavities (5) by an amount which is variable to vary the frequency of the output radiation. A magnetron as claimed in claim 1 wherein the slot (14) is located substantially midway along the anode (11) in its longitudinal axial direction. A magnetron as claimed in claim 1 or 2 wherein the larger anode cavities (5) are approximately three times longer in the radial direction than the smaller anode cavities (6). A magnetron as claimed in any preceding claim wherein the radial lengths of the anode cavities depend on their positions relative to the slot (24). A magnetron as claimed in any preceding claim wherein the slot (8) is located in the part of the anode (2) diametrically opposite means (7) for extracting output radiation from the magnetron. A magnetron as claimed in any preceding claim wherein the tuning member (15) has a concave leading edge. A magnetron as claimed in any preceding claim wherein the anode is of the open-end type. A magnetron as claimed in any of claims 1 to 6 wherein the anode is of the closed-ended type having end plates (12, 13) defining the axial extent of the anode cavities (16). A magnetron as claimed in any preceding claim and including a plurality of slots (17, 18, 19), each being extensive in a plane parallel to that in which the or each other is extensive and each slot (17, 18, 19) including a tuning member (20, 21, 22) of dielectric material therein which is movable to enter at least two of only the larger anode cavities by an amount that is variable. A magnetron as claimed in claim 9 wherein each tuning member (20, 21, 22) is movable independently of the others. A magnetron as claimed in claim 9 or 10 wherein the slots (17, 18, 19) are extensive in substantially the same plane orthogonal to the longitudinal axis.

Inventors: ROBERTSON MARK ANDREW, ROBERTSON, MARK ANDREW
Applicants: EEV LTD, EEV LIMITED
Application year: 1991
Main classification: H01J23
IPC codes: H01J23, H01J25