EP0209219A1

EP0209219A1 - Improvements relating to coaxial magnetrons

Info

Publication number: EP0209219A1
Application number: EP86303537A
Authority: EP
Inventors: Alan Butler Cutting; Melvin Gerrard England; Roger John Gates
Original assignee: Thorn EMI PLC
Current assignee: Thorn EMI PLC
Priority date: 1985-07-15
Filing date: 1986-05-09
Publication date: 1987-01-21

Abstract

A coaxial magnetron has a coaxial stabilising cavity (4) and a tuner comprising a tuning member (12) and a tuning member (9) rotatable, on a longitudinal axis of the magnetron, relative to tuning member (12). Respective surfaces (13, 10) of the tuning members (12, 9) define part of the stabilising cavity wall and tuning elements (14, 11), made of an electrically conductive material and formed integrally with members (12, 9), are positioned adjacent to the wall where the electric field is relatively small.

Description

This invention relates to coaxial magnetrons and it relates particularly to coaxial magnetrons which have a rotatable tuning member, for example a spin tuned coaxial magnetron. Spin tuned coaxial magnetrons produce a microwave output of varying frequency and are useful in radar applications requiring frequency agility.
Various designs of spin-tuned frequency-agile coaxial magnetrons are known. For example, USP 3,412,285 (W.A.Gerard, Westinghouse Electric Corp.) discloses a tuning mechanism comprising a fixed tuning member and a rotatable tuning member which extend from opposite end faces of the annular stabilizing cavity to overlap near the centre of the cavity, where the electric field is high. Each tuning member has at least two arcuate portions or teeth, with gaps between them, the rotatable teeth passing close to the stationary teeth. Both tuning members are made of a "dielectric or any other suitable material that will concentrate the electric field". A different design is disclosed in B.P 1485949 (A.B Cutting, EMI Varian Limited), in which several arcuate electrically conducting tuning elements are mounted in a stationary annular dielectric channel extending from one end face into the stabilizing cavity and similar elements are mounted on dielectric fingers extending from the same end of the cavity and within the annular dielectric channel. These fingers are rotatable about the magnetron axis. It is arranged that the rotatable conducting tuning elements pass close to the stationary ones in the central region of the cavity where the electric field is high.
In the known designs, tuning is achieved by disturbing the electric field in the central region of the cavity, where the electric field strength is high. It is therefore necessary to construct those parts of the stationary and rotatable tuning members which are in the cavity mainly or entirely of dielectric material. However, such materials are expensive to machine to required shapes, not very robust and sensitive to thermal shock. Furthermore, the dielectric loss reduces the Q-factor & the efficiency of the magnetron.
An object of the invention is to minimise the use of dielectric material within the cavity so as to alleviate the above disadvantages.
Accordingly, there is provided a coaxial magnetron comprising a resonator cavity, a stabilising cavity coupled to the resonator cavity, and a tuning arrangement to influence the frequency of electromagnetic radiation generated, wherein said tuning arrangement includes a plurality of first tuning elements rotatable, in the stabilising cavity, on a longitudinal axis of the magnetron relative to a plurality of second tuning elements in the stabilising cavity, the said first and second tuning elements being made of an electrically conductive material and being positioned adjacent to a wall of the stabilising cavity where the electric field is relatively small.
The inventors have discovered that it is possible to achieve frequency agility by disturbing the magnetic field close to the cavity wall where the electric field is low. The tuning members can then be constructed of conducting material (e.g copper) and manufactured at low cost to form a mechanically and thermally robust structure.
In a preferred embodiment, said tuning arrangement includes first and second tuning members defining part of the stabilising cavity wall, said first and second tuning elements being formed integrally with said first and second tuning members respectively. Said part of the stabilising cavity wall and said first and second tuning elements may be substantially orthogonal to one another.
In order that the invention may be more readily understood and carried into effect, a specific embodiment thereof is now described by reference to, and as illustrated in, the accompanying drawings of which:

Figure 1 shows a cross-sectional side view through part of a coaxial magnetron constructed in accordance with the present invention.
Figure 2 shows the end view of the tuning members forming part of the end wall of the coaxial cavity.
Figure 3a and 3b show two examples of tuning element profiles.
Figure 4a and 4b show isometric views of the tuning elements having the profile shown in Figure 3a.
Figure 1 shows a coaxial magnetron comprising the usual known elements, namely a cathode 1 surrounded by a coaxial cylindrical anode 2, having radial vanes such as 3, and a surrounding coaxial stabilizing cavity 4. Coupling slots (not shown) are provided in the anode 2 between alternative pairs of vanes. Microwave energy is coupled out of the stabilizing cavity 4 through a slot 6 and an output waveguide 7 having a dielectric window 8.

Referring now to Figures 1 and 2, an electrically conductive tuning member 9 has an annular end surface 10 disposed at an end wall of the stabilizing cavity and is rotatable about the axis of the magnetron. The tuning member 9 has a plurality of arcuate electrically conductive tuning elements such as 11 spaced along the outer circumference of the annular end surface and protruding into the stabilizing cavity 4. In this example, the tuning elements extend over an arc length of 30 and are spaced apart by 30. This fixed tuning member 12 also has an annular end surface 13 disposed concentrically with and around, the annular end surface 10 and has a matching set of tuning elements such as 14 spaced along the inner circumferences of the annular end surface and protruding into the stabilizing cavity 4.
Figures 3a and 3b show profiles of tuning elements shaped to provide respectively sinusoidal and triangular frequency variations as the tuning member 9 is rotated. It will be appreciated that other profiles can be devised in order to obtain other forms of frequency variation.
Figures 4a and 4b show isometric views of tuning elements having the profile shown in Figure 3a. Eight elements are shown on each tuning member. Figure 4a shows the two sets of elements as misaligned and Figure 4b shows the two sets as aligned.
The frequency bandwidth is related to the height of the tuning elements. When they are short, only the magnetic field is significantly affected, giving a negative change of frequency, as the height is increased this effect is enhanced until the electric field is disturbed and introduces a positive frequency variation offsetting that produced by the magnetic field.
An annular choke cavity may be provided at the end of each gap between adjacent tuning elements so that the gap has the effect of a short circuit.

Claims

1. A coaxial magnetron comprising a resonator cavity, a stabilising cavity coupled to the resonator cavity, and a tuning arrangement to influence the frequency of electromagnetic radiation generated, wherein said tuning arrangement includes a plurality of first tuning elements rotatable, in the stabilising cavity, on a longitudinal axis of the magnetron relative to a plurality of second tuning elements in the stabilising cavity, the said first and second tuning elements being made of an electrically conductive material and being positioned adjacent to a wall of the stabilising cavity where the electric field is relatively saall.

2. A coaxial magnetron according to Claim 1 wherein said tuning arrangement includes first and second tuning members defining part of the stabilising cavity wall, said first and second tuning elements being formed integrally with said first and second tuning members respectively.

3. A coaxial magnetron according to Claim 2 wherein said part of the stabilising cavity wall and said first and second tuning elements are substantially orthogonal to one another.

4. A coaxial magnetron according to Claims 1 to 3 wherein said tuning elements are made of copper.