EP1485933B1

EP1485933B1 - Magnetron

Info

Publication number: EP1485933B1
Application number: EP03708352A
Authority: EP
Inventors: Michael Barry Clive Brady
Original assignee: e2v Technologies UK Ltd
Current assignee: Teledyne UK Ltd
Priority date: 2002-03-16
Filing date: 2003-03-17
Publication date: 2009-08-26
Anticipated expiration: 2023-03-17
Also published as: CN1643637A; JP2005521201A; GB0206242D0; JP4301958B2; AU2003212532A1; KR20040102044A; DE60328975D1; CN100342478C; WO2003079394A1; GB2386749B; GB2386749A; EP1485933A1; ATE441201T1

Abstract

A magnetron comprises a cathode 5, an anode 2 having at least one vane 3, which defines a plurality of cavities. A dielectric resonator 7 is located between the vane(s) 3 and magnetic pole-piece 6a, such that it is in communication with said vane(s). The di-electric resonator 7 comprises a portion 8 of dielectric material, preferably a ceramic such as alumina, and a portion 9 of lossy material such as a carbon-loaded ceramic. A further conductive portion (15, figure 3) may be interposed between the two sections 8 and 9 of the resonator 7. In use the dielectric resonator 7 at least partially absorbs spurious radiation generated in a predetermined mode of operation of the magnetron, preferably the pi-1 mode, which if transmitted may interfere with other electronic devices.

Description

This invention relates to magnetrons.
In one known strapped anode vane magnetron design, a central cylindrical cathode is surrounded by an anode structure that typically comprises a conductive cylinder supporting a plurality of anode vanes extending inwardly from its interior surface. During operation, a magnetic field is applied in a direction parallel to the longitudinal axis of the cylindrical structure and, in combination with the electrical field between the cathode and anode, acts on electrons emitted by the cathode, resulting in resonances occurring and the generation of r.f. energy. A magnetron is capable of supporting several modes of oscillation depending on coupling between the cavities defined by the anode vanes, giving variations in the output frequency and power. The mode of operation that is usually required is the so-called pi mode of operation.
It is desirable to be able to suppress the transmission of power generated in certain modes, for example, the so-called pi-1 mode. It has been found that power generated in this mode, if transmitted, may interfere with other electronic devices such as mobile phones, satellite links and other communication systems. Various methods have been proposed to suppress this mode of operation but these have generally been found to be costly, complicated, and also to suppress radiation in desired modes of operation, for example the p mode. The invention arose from work relating to magnetrons for marine radar applications. Such magnetrons are small, simple and low cost devices and therefore a low cost and straightforward solution to the problem of pi-1 radiation was sought.
It is known to provide absorbers including lossy and non-lossy ceramic rings in communication with an external cavity resonator of a co-axial magnetron ( GB-B-1134 734 ). The invention is defined in the claims to which reference is directed.
The provision of partly lossy dielectric resonator in communication with the vane or vanes results in the absorption of spurious radiation.
Preferably, the predetermined mode is the pi-1 mode. The absorption of radiation generated in this mode prevents interference with other electronic devices.
Preferably, the lossy portion of the resonator is located further from the anode vane than the other portion. This arrangement is advantageous because electric fields associated with the pi mode do not penetrate into the resonator as deeply as those fields associated with the pi-1 mode. Thus, electrical energy generated in the pi-1 mode is attenuated more than energy generated in the pi mode by virtue of the distal lossy portion.
Advantageously, the lossy portion of the resonator is thinner than the other portion, for example one quarter or less of the thickness of the other portion.
Improved performance of the invention can be achieved by the introduction of an electrically conductive region interposed between the lossy portion and the other portion.
The resonator may comprise two annular members, one of which is lossy. The annuli may be coaxial. A further annulus of electrically conductive material may be interposed between the lossy and non-lossy members in order to achieve the improved performance mentioned above.
The dielectric resonator may include ceramics material, for example alumina. The lossy portion may be of ceramics material loaded with carbon.
The resonator may be annular and co-axial with the vanes of the anodc.
The invention will now be described, by way of example, with reference to the accompanying drawings, in which: -

Figure 1 is a cross-sectional view of a magnetron constructed according to the invention;
Figure 2 is a graph of experimental data, showing the change in electric field with position, of the pi and pi-1 modes of the magnetron of Figure 1;
Figure 3 is a cross-sectional view of an alternative magnetron constructed according to the invention;
Figure 4a is a sectional view and Figure 4b a plan view of the resonator of Figure 3; and
Figure 5 is a graph of experimental data, showing the change in electric field with position, of the pi and pi-1 modes of the magnetron of Figure 3.

Like reference numerals have been given to like parts throughout the specification.
With reference to Figure 1, the basic features of a conventional magnetron, indicated generally by the reference numeral 1, are shown. The main basic features include an anode 2 having a plurality 3 of vanes, two of which 3a, 3b, are visible in this drawing. When viewed from above, the vanes are evenly spaced around the inner circumference of the cylindrical portion 4 of the anode 2, and extend inwardly from it, such that a plurality of resonant cavities are formed. The magnetron also includes a central cathode 5, which is surrounded by the anode 2. The magnetron 1 also comprises pole pieces 6a, 6b arranged to produce magnetic fields required for operation of the magnetron. The anode vanes are strapped, according to the invention as claimed, but straps are not shown in this drawing.
In accordance with the invention, the magnetron further comprises a dielectric resonator 7, a portion of which is lossy. The resonator 7 is located in a space in the magnetron between an end portion of the anode vanes 3 and one of the pole pieces 6a, such that it is in communication with the plurality of vanes, including the vanes 3a, 3b. The resonator is also shown in communication with one of the pole pieces 6a, but it need not be so. The invention has been found to work even when the pole piece is spaced from the resonator 7.
In this embodiment, the resonator 7 is realised in the form of two annular members 8 and 9. The annular members 8, 9 are substantially coaxial and are in intimate contact, although a small degree of separation is allowable. Annulus 8 is of a substantially lossless plain ceramic material; annulus 9 is of lossy material, such as ceramic loaded with carbon powder. The annuli 8,9 are arranged so that the loss-free annulus 8 is interposed between the anode vanes 3a, 3b and the lossy annulus 9. The anode vanes 3a, 3b, and the annuli 8, 9 are also substantially coaxial.
The dimensions of the annuli 8, 9 are predetermined so that the annuli resonate in the so-called TM110 mode as a dielectric resonator. The resonator 7 is arranged to attenuate radiation generated in an unwanted mode of operation of the magnetron, such as the pi-1 mode, by magnetically coupling into the anode and thereby suppressing transmission of power in this mode.
Referring now to Figure 2, this is a graph showing field strength plotted against position along the thickness of the resonator. The vertical axis 10 represents the position at which the anode vane meets the resonator and the vertical axis 11 represents the position at which the resonator meets the pole piece. Vertical axis 12 represents the junction of the lossy and non-lossy portions of the resonator.
The upper line 13 represents the penetration of the TM110 field in the pi-1 mode into the resonator. The electric field is high throughout the depth of the resonator, even into the lossy portion. Therefore, the lossy ceramic acts on almost the entire field of the pi-1 mode. The diameters of the annuli are chosen such that a resonance is set up in the resonator in the TM110 mode, which coincides in frequency with the pi-1 resonance of the anode. These two resonances are strongly coupled together by a common azimuthal magnetic field at the outer diameter, so that the resistive losses in the ceramic resonance are transformed into a comparatively large series resistance in the pi-1 resonance, giving a low Q. In this manner the pi-1 mode is attenuated.
The other line 14 on this chart represents the penetration of the fringing field in the pi mode. Very little of the field enters the lossy portion of the resonator, and so only a portion of the field is suppressed in the pi mode, typically less than 20%. However, it is preferable to minimise reduction of the fields generated in the pi mode: hence a magnetron according to Figure 3 may be employed.
The magnetron illustrated in Figure 3 has the same features as does the magnetron of Figure 1, but with the inclusion of a thin metal annulus 15, interposed between the loss-free annulus 8 and the lossy annulus 9.
Figure 4a illustrates the resonator of Figure 3 in section and also shows the electric fields and currents (I) set up in the resonator in the TM 110 mode. Figure 4b is a plan view of the resonator. The TM110 mode is set up in the lossy annulus 9. The loss-free annulus couples the pi-1 mode to the lossy annulus. The metal ring 15 has a smaller outer diameter than the ceramic annuli and so allows improved magnetic coupling between the resonances in the lossy annulus and the plain annulus so that the pi-1 mode is attenuated as before. The TM110 currents will flow around the outside diameter of the ceramics where the metal ring does not interrupt them. The pi mode residual field is substantially reduced, and may be brought to zero by the metallic ring. The effect of this metal washer 15 is also shown in the graph of Figure 5.
Figure 5 shows the depth into the resonator of the fringing field of the pi mode and the TM110 field of the pi-1 mode. The vertical axis 16 represents the position at which the anode vane meets the resonator and the vertical axis 17 represents the position at which the resonator meets the pole piece. The vertical line 18 represents the position of the metal annulus 15. The horizontal line 19 on this chart plots the strength of fields generated in the pi-1 mode and illustrates that these fields enter the resonator up to and including the lossy portion. Thus, the lossy ceramic is able to act on the residual field and attenuate it. The line 20 plots the strength of field generated in the pi mode. The field strength dips sharply when the field encounters the metal annulus, so that only a minute portion of the field enters the lossy portion of the resonator.
Employing the magnetron arrangement of Figure 3, it is possible to reduce the Q₀ of the pi-1 mode from 1000 to a figure in the region of 50. However, the change in the pi mode is negligible - a change in Q₀ from 1000 to approximately 950. This can be accommodated for by slight adjustments to the operating system of the magnetron, and is within the capabilities of the skilled person.
Preferably, the metal washer has an external diameter less than those of the annuli 8, 9. This feature allows magnetic coupling between the lossy annulus and the loss-free annulus. The metallic annulus may be realised in the form of a metal layer on a surface of one of the annuli 8, 9 or may be formed by metalising both the upper annulus 9 and lower annulus 8. Although the invention has been described in relation to a resonator comprising a plurality of pieces, the resonator may comprise a single piece having different lossy characteristics in different regions of the resonator.
A suitable ceramic for the resonator is alumina, although any vacuum-compatible insulator may be employed. As ceramic washers may be manufactured cheaply in bulk, the inventor's solution to the problem of spurious radiation is both low-cost and simple. The cost of the resonator is typically a few pence, and the fitting of the resonator in the magnetron is uncomplicated, so that there is no appreciable increase in manufacturing and labour costs.
Although the invention was devised in relation to low power magnetrons, it is thought that it could readily apply to high power magnetrons. A conventional strapped anode vane magnetron has been described, but the resonator could be used in conjunction with a rising sun-type magnetron, for example. Further variations may be made without departing from the scope of the invention. For example, the dielectric resonator need not be an annulus and need not be of a closed shape. Furthermore, the dielectric resonator need not contact all of the vanes.

Claims

A strapped magnetron comprising an anode (2) having a plurality of vanes (3) defining a plurality of cavities and a dielectric resonator (7), a portion (9) of which is lossy and a portion (8) of which is substantially lossless, in communication with at least one of the vanes, the resonator being arranged, in use, to at least partially attenuate radiation generated in a predetermined mode of operation of the magnetron.
A magnetron as claimed in any preceding claim, in which the predetermined mode is the pi-1 mode.
A magnetron as claimed in any preceding claim, in which the lossy portion of the resonator is located further from the anode vane than the substantially lossless portion.
A magnetron as claimed in any preceding claim, in which the lossy portion of the resonator is thinner than the substantially lossless portion.
A magnetron as claimed in claim 4, in which the lossy portion of the resonator has a thickness less than one quarter of that of the substantially lossless portion.
A magnetron as claimed in any preceding claim, further comprising an electrically conductive region interposed between the lossy portion and the substantially lossless portion.
A magnetron as claimed in any one of claims 1 to 6, in which the lossy portion of the resonator comprises a first annular member and the substantially lossless portion comprises a second annular member, the first and second annular members being substantially coaxial.
A magnetron as claimed in claim 7, further comprising a third member of electrically conductive material sandwiched between the first and second annular members.
A magnetron as claimed in claim 8, in which the third member is annular and substantially coaxial with the first and second members.
A magnetron as claimed in any preceding claim, in which the dielectric resonator includes ceramics material.
A magnetron as claimed in claim 10, in which the ceramics material is alumina.
A magnetron as claimed in claims 10, or 11, in which the lossy portion comprises ceramics material loaded with carbon.
A magnetron as claimed in any preceding claim, in which the vanes are disposed about a common axis and the resonator is substantially co-axial with the vanes.
A radar system incorporating a magnetron as claimed in any preceding claim.