The invention relates to a magnetron, and in particular to a magnetron having an improved output arrangement.

Conventionally, the output from a magnetron is taken by means of a probe attached to one of the walls separating adjacent resonant cavities in the anode. This can distort the R.F. field pattern and so cause a degradation in performance of the device. It is difficult to couple heavily into the anode structure, because of weak R.F. coupling between adjacent cavities, with the result that with known magnetrons much of the potentially available microwave energy cannot be emitted.

This invention provides a magnetron comprising a cathode, an anode having a plurality of resonant cavities circumferentially spaced about an axis, means for producing an axial magnetic field; and an output arrangement comprising a generally cylindrical resonant cavity disposed co-axially at one end of the plurality of resonant cavities within the magnetic field, means for coupling energy from the plurality of resonant cavities into the cylindrical

cavity, and means for coupling microwave energy from the cylindrical cavity out of the magnetron, the resonant frequency of the cylindrical cavity being selected to be substantially the desired output frequency of the magnetron.

Because of the cylindrical cavity, energy can be coupled symmetrically from each adjacent pair of resonant cavities, without the distortions induced when energy is taken from one pair of cavities only. Furthermore, the cylindrical cavity will tend to suppress the emission of harmonics of the desired signal.

In one embodiment, the means for coupling energy from the plurality of resonant cavities comprises one of a plurality of circular straps connected to circumferentially alternate walls of the anode separating adjacent resonant cavities, which one strap projects axially into the cylindrical cavity further than another of the plurality of the straps.

An axially extensive electrostatic field is generated by the axially projecting strap from which energy is coupled into the cylindrical cavity from each adjacent cavity pair.

In an alternative embodiment posts extend axially across the cylindrical cavity from circumferentially alternate walls separating adjacent resonant cavities. In such a case electric current would travel around adjacent posts to generate a magnetic field by which energy can be coupled into the cylindrical cavity.

The output from the cylindrical cavity may comprise a conventional loop or an elongate probe which couples energy into a coaxial line.

In another embodiment the output may include a movable short circuit for tuning the resonant frequency of the cylindrical cavity.

In a yet further embodiment cylindrical resonant cavities may be disposed about each end of the plurality of circumferentially spaced cavities, which may be of use under especially high load conditions. In such a case, axially projecting straps may be present on each end of the walls of the anode. Output loops or probes may be located within each cylindrical cavity and connected to a common co-axial or other line.

In order that the invention may be well understood, various embodiments thereof will be described with reference to the accompanying diagrammatic drawings, in which;

Figure 1 is a partly schematic, partly longitudinal sectional view through a magnetron according to on embodiment of the invention;

Figure 2 is the same view as Figure 1, but shows a modification of that embodiment;

Figure 3 is a view across arrow A-A of Figure 2; and

Figure 4 is the same view as Figures 1 and 2 but showing a yet further embodiment of the invention.

As seen in Figure 1, a magnetron comprises a substantially cylindrical anode 1 having an axis la within which is located a co-axially mounted cathode 2. Vanes 3 extend inwardly from the inner walls of the anode ,2 to define a plurality of circumferentially spaced resonant cavities 4 (see Figure 3) . Magnetic pole pieces 5 provide an axially extending magnetic field at right angles to the electric field produced between the cathode 2 and anode vanes 3.

A generally cylindrical resonant cavity 6 is disposed co-axially at one end of the plurality of resonant cavities 4 in between the magnetic pole pieces 5. The resonant frequency of the cavity 6 is selected to be substantially equal to the desired output frequency of the magnetron and in this way tends to suppress the emission of harmonics of

the main signal. As is well known, the resonant frequency of such a cylindrical resonator, in the E01 mode, is@@

FF proportional to the radius of that cavity.-

Circular straps 7, 8, 9 are connected to the anode vanes 3 in generally known manner. The radially innermost and outermost straps 7, 9 respectively are each connected to the same circumferentially alternate vanes 3 as each other, while the middle strap 8 is connected to the intervening vanes 3. The innermost strap 7 projects axially further into the resonant cavity 6 than the other straps and in this way._is able, in use, to generate an electrostatic field by which energy can be electrically coupled into the cylindrical cavity 6 from each pair of adjacent resonant cavities 4.

An electric field probe and co-axial line 10 projects co-axially into the cylindrical cavity 6 in between the uppermost, as shown, pole pieces 5. The end of the line 10 remote from the magnetron feeds a co-axial to waveguide transition member 11 for coupling energy into a waveguide 12.

The embodiment shown in Figures 2 and 3 is generally similar to that shown in Figure 1 and the same parts have been identified by the same reference numerals. Instead of electrically coupling into the cylindrical cavity 6, the

innermost strap 7b is the same height as the other straps 8 and 9. Posts 20 extend between circumferentially alternate vanes 3 and the roof 21 of the cavity 6. In use, electric currents travel around the posts 6 and generate a magnetic field by which energy can be coupled into the cavity 6. Instead of the electric field probe, a loop output line 22 extends from one side for magnetically coupling energy from the cavity 6. A slidable short circuit 23 is located within a waveguide 24 for adjusting or tuning the resonant frequency of the cavity 6 so that the output frequency of the magnetron may be pulled over a short range.

In the embodiment shown in Figure 4, as compared to the Figure 1 embodiment, an extra cylindrical resonant cavity 6a is disposed co-axially at the other end, the bottom as shown., of the plurality of resonant cavities 4. A further arrangement of straps 7a, 8a, 9a, a mirror image of straps 7 _r 8, 9, is provided on the lower end of the plurality of vanes 3. A further co-axial electric field probe 10a projects into the resonant cavity 6a and, although not shown in this view, each probe 10, 10a is connected to a common co-axial line for emission, e.g. into an appropriate waveguide. Such an arrangement is particularly suitable if especially heavy coupling is required with the anode.

Each of the magnetrons shown has the advantage that microwave energy can be coupled into the cylindrical resonant cavity symmetrically from each pair of adjacent

resonant cavities 4, which ensures low distortion in the output signal and ensures the efficient transfer of energy. The cylindrical resonant cavity can also be selected to suppress harmonics of the output signal. Normally, in the known magnetrons, resonances in the space above and/or below the vanes can be a problem requiring careful design or the use of extra filters.

Variations may be made to the embodiments shown. For example the anode structure may include integrally formed separating walls which separate the circumferential resonant cavities. The three straps shown coupling together the anode vanes may be replaced by a pair of straps. Magnetic loop outputs may be used as an alternative to the electric field probes shown in the Figures 1 and 4 embodiments. Similarly, in the Figure 4 embodiment one or both cavities 6, 6a may include a tuning arrangement for varying the resonant frequency and energy need not be coupled out from each of the cavities.