GB2151070A

GB2151070A - Improvements relating to magnetrons

Info

Publication number: GB2151070A
Application number: GB08332085A
Authority: GB
Inventors: Alan Butler Cutting; John Edwin Baker
Original assignee: EMI Varian Ltd; Thorn EMI Varian Ltd
Current assignee: EMI Varian Ltd; Thorn EMI Varian Ltd
Priority date: 1983-12-01
Filing date: 1983-12-01
Publication date: 1985-07-10
Also published as: GB2151070B

Abstract

A magnetron includes a number of anode vanes (21) spaced apart at regular intervals around the axis (XX) of a central cathode (40). A tuning member (30), positioned outwardly of the anode vanes, consists of a ribbon-like metal strip (32) anchored to a support ring (31) at regular intervals around the axis to form respective, generally arcuate tuning elements (T1 ... TN). The coefficient of expansion of the strip material is greater than that of the support material, the radius of curvature of each tuning element being caused respectively to increase or decrease in response to a rise or fall of temperature. The resulting radial displacement ensures that the resonant frequency of the magnetron is substantially independent of the temperature change. <IMAGE>

Description

SPECIFICATION Improvements relating to magnetrons This invention relates to magnetrons.

A magnetron typically comprises a microwave cavity including a cathode lying on the longitudinal axis of the cavity and an anode in the form of a cylindrical anode block supporting a plurality of anode vanes which surround, and project radially inwards towards the cathode.

A problem associated with a magnetron constructed in this manner arises when a change in the temperature of the magnetron occurs since this results in a change in the linear dimensions, and so the resonant frequency, of the cavity.

It is an object of the present invention to provide a magnetron wherein the above-described problem is substantially alleviated.

Accordingly there is provided a magnetron comprising a cathode; a plurality of anode vanes spaced apart at regular intervals around the longitudinal axis of the cathode and a tuning member which includes a plurality of tuning elements arranged outwardly of said anode vanes and spaced apart, at regular intervals, around said axis, each tuning element being capable of undergoing a radial displacement relative to an anode vane, in response to a change of temperature of the magnetron, the extent and sense of said radial displacement being such that the resonant frequency of the magnetron, is substantially independent of a said change of temperature.

A rise in temperature of the magnetron causes a reduction in the radial separation of the anode vanes and the tuning elements which tends to offset the fall in resonant frequency of the magnetron due to the temperature change. Conversely a fall in temperature of the magnetron results in an increase in the radial separation of the anode vanes and tuning elements.

In an embodiment of the invention said tuning member comprises a strip of an electrically conductive material which is anchored to a support member at regular intervals around said axis thereby to define a respective, arcuate tuning element between each pair of adjacent anchorage positions, the coefficient of expansion of said strip being greater than that of the support member so that the radius of curvature of each said arcuate tuning element is caused respectively to increase or decrease in response to a rise or a fall of temperature thereby resulting in said radial displacement.

In order that the invention may be carried into effect an embodiment thereof is now described, by way of example only, by reference to the Figures of the drawings of which, Figure 1 shows, schematically a vertical, cross-sectional view through a magnetron and, Figure 2 shows schematically a horizontal, cross-sectional view on line AA in Fig. 1.

For reasons of clarity Figs. 1 and 2 show only those parts of the magnetron which have direct relevance to the present invention; persons skilled in the art will appreciate readily how the illustrated arrangement may be incorporated in a magnetron of conventional construction.

Referring to Figs. 1 and 2, the anode block and anode vanes of a magnetron are shown at 20 and 21 respectively and a tuning member, in accordance with the invention and referenced generally at 30, is centred on the longitudinal axis XX of cathode 40.

In this example, the tuning member 30 includes two distinct components, namely a mounting ring 31, formed of molybdenum, say, and a ribbon-like strip 32 of a metal, such as copper, having a coefficient of thermal expansion greater than that of the mounting ring. The strip is anchored to the mounting ring at discrete positions Po. .. Pn spaced apart at regular intervals around axis XX so as to create a respective tuning element T,...

TN, arcuate in form, between each pair of adjacent anchorage positions. The tuning member has, in effect, a scalloped configuration and is mounted fixedly in the magnetron cavity so that the central portion of each tuning element is positioned immediately adjacent to the outer edge of a respective anode vane.

A tuning member constructed in this way can be dimensioned so that, in accordance with the invention, the resonant frequency of the magnetron cavity is substantially independent of a change in the temperature of the magnetron.

If, in operation, the temperature of the magnetron shouid rise the resulting expansion of strip 32, which has a relatively high coefficient of thermal expansion, exceeds that of the mounting ring causing a decrease in the radius of curvature of each tuning element.

Thus, the central portion of each tuning element is displaced inwardly towards an anode vane tending to offset a fall in the resonant frequency of the magnetron occasioned by the rise in temperature. Conversely, if the temperature of the magnetron should fall the radius of curvature of each tuning member will increase and the central portion thereof will be displaced outwardly, away from an anode vane.

In a typical example, the magnetron may be dimensioned to operate at a frequency of around 9200 MHz and, in these circumstances, the temperature coefficient of the magnetron, reflecting changes in the linear dimensions of the resonant cavity caused by a change in operating temperature, may be of the order of - 0.15 MHz/"C. A change in the radial separation of the vanes and tuning elements of about 0.24jL*m/4C is then needed to compensate for a change of temperature.

The outer edges of the anode vanes move radially outwards or inwards due to expansion or contraction of the anode block by an amount greater than the expansion or contraction of the molybdenum ring. This gives partial compensation. Full compensation is achieved, in accordance with the present invention, by suitable displacement of the tuning elements. If the radius of the mounting ring 31 is about 10 mm and the free arc of each tuning element subtends an angle of about 16 at the centre of the ring the central portion of the element is displaced radially, respectively inwards or outwards, by about 0.14 ym per 'C rise or fall of temperature. By suitably tailoring the tuning elements (e.g. by adjusting the width, the radius of curvature and the distance between anchorage positions) it is possible to optimise the degree of compensation achieved.

The compensating ring is preferably mounted on the anode block by welding using a good thermal conductor with some radial flexibility to avoid distortion of the molybdenum ring. Since both the molybdenum ring and copper ribbon can be made of thin material (e.g. 0.2mm) their heat capacity can be very small so that they will follow the temperature of the anode block very closely and compensation can be achieved even when rapid changes of temperature occur.

Claims

1. A magnetron comprising a cathode; a plurality of anode vanes spaced apart at regu lar intervals around the longitudinal axis of the cathode and a tuning member which includes a plurality of tuning elements arranged outwardly of said anode vanes and spaced apart at regular intervals around said axis, each tuning element being capable of undergoing a radial displacement relative to an anode vane, in response to a change of temperature of the magnetron, the extent and sense of said radial displacement being such that the resonant frequency of the magnetron, is substantially independent of a said change of temperature.

2. A magnetron according to Claim 1 wherein said tuning member comprises a strip of an electrically conductive material which is anchored to a support member at regular intervals around said axis thereby to define a respective, arcuate tuning element between each pair of adjacent anchorage positions, the coefficient of expansion of said strip being greater than that of the support member so that the radius of curvature of each said arcuate tuning element is caused respectively to increase or decrease in response to a rise or a fall of temperature thereby resulting in said radial displacement.

3. A magnetron according to Claim 2 wherein said support member is of molybdenum and said strip of an electrically conductive material is of copper.

4. A magnetron substantially as hereinbefore described by reference to and as illustrated in the accompanying drawings.