GB2191889A - Improvements relating to spin- tuned magnetrons - Google Patents

Abstract

In a spin-tuned magnetron, Fig. 1, having an anode structure in the form of anode wall (1) and a plurality of anode vanes (2), the cylindrical tuning member (3) is modified, as shown in Fig. 4(a) so as to be formed with a plurality of open- ended, circumferentially spaced notches (8, 9), with an electrically conductive sleeve (10) is provided radially outwards of the notches to inhibit propagation of microwave radiation in the space between the tuning member (3) and the anode wall (1). The conductive sleeve 10 may, as shown, be spaced from the notches, or may, Fig. 3(a) not shown, be brazed directly to the castellated end of the tuning member 3. The notches may be of regular or irregular shape and in the form of elongated slots. <IMAGE>

Description

SPECIFICATION Improvements relating to spin-tuned magnetrons This invention relates to magnetrons and it relates particularly to variable frequence magnetrons, of the kind known as spin-tuned magnetrons, wherein the magnetron frequency is varied by means of a rotatable tuning member. U.S. Patent No.3,365,609 describes a known spin-tuned magnetron. As is shown schematically in Fig. 1 of the accompanying drawings, the magnetron has anode structure in the form of a cylindrical anode wall 1 coupled to a plurality of anode vanes 2 (of which only two are shown in the drawings) arranged radially inwards with respect to the anode wall at regular intervals around a longitudinal axis XX of the magnetron. Adjacent ones of the anode vanes define respective anode cavities.

The magnetron has a cylindrical tuning member 3 which can rotate on axis XX and has a number of evenly spaced circumferential holes (eg 4) which move past successive anode cavities in a gap 5 between the anode vanes and the anode wall.

The magnetron resonant frequency is determined mainly by the dimensions of the anode structure, whereas a change of resonant frequency is determined by the relative positions of the holes and the anode cavities.

It is found that two conflicting tuning mechamisms can operate. One tuning mechanism arises from a change in the effective volume of the anode cavities when the hole centres move past the anode vanes. Since, in general, the holes, considered as cylindrical waveguides, present a resonant frequency well below the magnetron cut-off frequency, r.f. field penetration is low and so the tuning effect is negligible. The other tuning mechanism is associated with a variation of the coupling of r.f fields in adjacent anode cavities as a function of the effective spacing of the tuning member and the anode vanes. The effective spacing (and so the coupling) is at a maximum when the holes in the tuning member pass behind the anode vanes. Fig. 2 of the drawings illustrates how the excursion in magnetron frequency depends on the clearance between the anode vanes and the tuning member (c in Fig.

1), and it will be apparent that for operating frequencies in the region of X-band a reasonable bandwidth, in excess of 5-% say, can be achieved provided the clearance is less than about 0.1 mum. This restriction imposes severe constraints on tolerancing both in regard to the machining of component parts and to positioning of the tuning member, and this can lead to increased costs in manufacture and reduced yields in production. Furthermore, the region between the anode wall and the tuning member, referenced at 6 in Fig. 1, can, in certain operational conditions, act as a coaxial transmission line. If the r.f fringeing fields in the vicinity of the anode vanes exhibit an assymetry, region 6 can support a number of modes, notably the TE,1 and TE2, modes.In these circumstances, r.f energy is lost from the magnetron reducing its performance and operating life. In order to reduce such assymetry to an acceptable !ever the clearance between the tuning member and the anode vanes should be reasonably uniform around the longitudinal axis, and typically for frequencies in the region of X-band any variation in the clearance should be less than about 0.1 mum. Bearing in mind that the tuning member must rotate at high speed with respect to the anode vanes this restriction also imposes severe constraints on tdlerancing.

It is an object of the present invention to provide a magnetron which substantially alleviates the above-described problems.

Accordingly there is provided a spin-tuned magnetron including an anode structure comprising an anode wall and a plurality of anode vanes arranged radially inwards with respect to the anode wall at regular intervals around a longitudinal axis of the magnetron, adjacent ones of the anode vanes defining respective anode cavities, and a cylindrical tuning member rotatable on said longitudinal axis and being formed at one end with open-ended, circumferentially spaced notches disposed between said anode vanes and said anode wall, the tuning member being provided with an electrically conductive sleeve adjacent to, and radially outwards of, the notches to inhibit propagation of microwave radiation in the space between the tuning member and the anode wall.

The notches may be of regular or irregular shape and may comprise elongated slots.

The inventor has appreciated that with a notched tuning member, the tuning mechanism arises mainly from a variation in the effective volume of the anode cavities due to movement of the notches past the anode vanes and that, in these circumstances, a relatively large clearance between the anode vanes and the tuning member can be tolerated, the tuning mechanism due to coupling being relatively unimportant. The inventor has discovered that use of open-ended notches, instead of holes, is made possible by provision of said electrically conductive sleeve which inhibits penetration of microwave energy into the space between the tuning member and the anode wall and so reduces serious losses which would otherwise arise.

The inventor finds that a magnetron in accordance with the present invention can achieve a relatively large bandwidth, and since it is possible to provide a relatively large clearance between the anode vanes and the tuning member the constraints on tolerancing can be relaxed thereby reducing manufacturing costs significantly.

In order that the invention may be carried readily into effect an embodiment thereof is now described, by way of example only, by reference to the accompanying drawings of which, Figure 1 shows a schematic, cross-sectional view through a known magnetron, Figure 2 illustrates how the excursion in magnetron frequency, achieved by the magnetron of Fig. 1, depends on the clearance between the anode vanes and the tuning member, Figure 3a and 3b show respectively a crosssectional side view through, and an end view of, a tuning member used in a magnetron in accordance with the present invention and Figures 4a and 46 show respectively a cross-sectional side view through, and an end view of, a different tuning member used in a magnetron in accordance with the present invention.

In this example of the invention, the magnetron has an anode structure similar to that described by reference to Fig. 1 of the drawings; however, the tuning member is modified significantly.

Referring initially to Figs. 3a and 3b, a cylindrical tuning member 3, typically made of copper, has a number of evenly-spaced, circumferential notches in the form of open-ended elongated slots 8, adjacent ones of the slots defining therebetween respective castellations 9.

A cylindrical sleeve 10, also made of copper, is attached directly to the castellations by brazing. The sleeve which is positioned radially outwards of the castellations inhibits penetration of r.f radiation, via the slots, into space 6 between the anode wall and the tuning member and so substantially prevents losses which would otherwise occur due to propagation of radiation in that space.

Some losses may be associated with the braze material adjacent to the castellations and so, in an alternative embodiment shown in Figs. 4a and 4b, the sleeve may be spaced apart from the castellations and attached to, or formed integrally with, the tuner wall at a position above the castellations.

In a typical example, the slots formed in the tuning member are each 4mm long and 2mm wide, the wall of the tuning member being 2mm thick. As described hereinbefore, use of open-ended slots permits a relatively large clearance between the anode vanes and the tuning member, and in this example a clearance of 0.48mm is used. The bandwidth achieved with this construction is found to be relatively large-about 9%. Moreover, since a relatively large clearance can be used, and since variations in this clearance have a relatively insignificant effect of r.f fringing fields in the vicinity of the anode cavities, constraints on mechanical tolerancing can be relaxed considerably, leading to a significant reduction in manufacturing costs and an improved yield in production.

Claims (4)

1. A spin-tuned magnetron including an anode structure comprising an anode wall and a plurality of anode vanes arranged radially inwards with respect to the anode wall at regular intervals around a longitudinal axis of the magnetron, adjacent ones of the anode vanes defining respective anode cavities, and a cylindrical tuning member rotatable on said longitudinal axis and being formed at one end with open-ended, circumferentially spaced notches disposed between said anode vanes and said anode wall, the tuning member being provided with an electrically conductive sleeve adjacent to, and radially outwards of, the notches to inhibit propagation of microwave radiation in the space between the tuning member and the anode wall.

2. A spin-tuned magnetron according to Claim 1 wherein said notches comprises elongated slots.

3. A spin-tuned magnetron according to Claim 1 or Claim 2 wherein said electrically conductive sleeve is spaced apart from said notches.

4. A spin-tuned magnetron substantially as herein described by reference to the accompanying drawings.