BACKGROUND OF THE INVENTION Magnetrons are well-known vacuum electron discharge devices used to generate electromagnetic filed in the microwave frequency range. A typical magnetron comprises a cylindrical cathode, a cylindrical anode surrounding the cathode and a plurality of resonant cavities formed in the anode by either slots or vanes. An electric field is established between the cathode and the anode, and a magnetic field is applied perpendicular to the electric field in the so-called interaction region, which is the evacuated space between the cathode and the resonant cavities in the anode. When the value of E/H is suitable, electrons emitted from the cathode interact with the electric and magnetic fields to generate microwave energy at a frequency determined by the parameters and the resonance characteristics of the cavities.

A known problem with magnetrons is that of unwanted emissions. These generally take two forms: the first is amplification of a harmonic of the desired mode. The other form of unwanted emission is called moding; that is, significant cavity responses occurring at frequencies other than the frequency for which the magnetron is designed to operate.

Conventionally, harmonic emissions are reduced by means of external microwave filters or by employing complex output systems embodying integral filters.

The concept of strapping was devised as a solution to the problem of moding. Strapping, as conventionally understood, is the joining of alternate anode vanes by means of a closed ring. Each pair of vanes forms a resonant cavity there between, and all such cavities are maintained at the same electrical potential. Thus, alternate anode vanes are locked into operation in the desired mode, known as the n mode. The X mode has node points of the same distance apart as the distance between alternate vanes.

A problem has been encountered with conventional straps in that the currents that circulate in the straps, to equalise the voltage on alternate vanes, may also generate an electromagnetic field. This field may interfere with the operation and performance of the magnetron owing to its proximity to the cathode.

A proposed solution to this problem is that of providing a break in the strap. This prohibits the flow of current between the vanes that the strap connects.

However, a drawback of this approach is that it increases the frequency separation of the modes of operation and may also cause oscillations at undesired frequencies.

SUMMARY FO THE INVENTION The invention is defined in the claims to which reference is directed.

An embodiment of the invention provides a magnetron comprising a plurality of anode vanes and a first strap in contact with alternate ones of the vanes, the strap having a protruding region. The provision of protruding regions around the ring provides variations in capacitance between the strap and another strap or between the strap and the

others of the anode vanes. The change in capacitance increases the stability of the magnetron in the wanted mode, limits oscillations at undesired frequencies and reduces the coupling of harmonics into the cathode structure.

Preferably, a plurality of protruding regions is provided, the protrusions being spaced regularly around a portion of the strap.

Advantageously, the strap is adjustable, to allow the capacitance to be adjusted as required. This feature allows the user to retune the pi-1 mode to a different frequency range.

The principle operation is that distortions or protuberances of one strap ring cause localised variations in capacitance. The capacitance between strap rings at the localised portions or between a strap ring and anode vanes is increased. The effect can thus be accomplished either between rings, or between a strap and an anode vane, which is attached to a different strap.

Typically, the protrusions are arranged periodically, but other arrangements are possible as will be described.

The arrangements can affect the distributions of RF voltages for the desired and undesired modes of oscillation differently and/or can reduce harmonic levels and/or RF coupling into the cathode structure.

A plurality of straps may be provided, some or all of which may include at least one protrusion.

BRIEF DESCRIPTION OF THE FIGURES An embodiment of the invention will now be described, by way of example, with reference to the accompanying drawings, in which:- Fig. 1 is a plan view of the anode and strap arrangement of a conventional magnetron; Fig. 2 is a perspective view of vanes of the anode arrangement of Figure 1 ; Fig. 3 is a graph illustrating magnetron output power against frequency, for a conventional magnetron; Fig. 4 is a plan view of the anode and strap arrangement of a magnetron constructed according to the invention; Fig. 5 is a graph illustrating magnetron output power against frequency, for the magnetron of Figure 4; Fig. 6 is a plan view of the anode and strap arrangement of an alternative magnetron constructed according to the invention; Fig. 6A is a plan view of the anode and strap arrangement of a further alternative magnetron constructed according to the invention; Fig. 7 is a perspective view of anode vanes and a strap arrangement of a magnetron constructed according to an alternative embodiment of the invention; and Fig. 8 is a perspective view of anode vanes and a strap arrangement of a magnetron constructed according to a further alternative embodiment of the invention.

DESCRIPTION OF EMBODIMENTS OF THE INVENTION Like reference numerals have been given to like parts throughout the specification.

With reference to Figures 1 and 2, a conventional typical anode arrangement is illustrated. The anode arrangement comprises a cylindrical anode wall, indicated generally by the reference numeral 1, and a plurality of anode vanes, four of which 2-5 are illustrated in both figures 1 and 2. The anode vanes extend radially inwardly from the inner wall of the cylindrical anode 1. A plurality of straps 6,7 and 8 for the vanes are also illustrated. Strap 6 contacts alternate anode vanes, such as vanes 3 and 5, and is spaced from the other vanes, such as vanes 2 and 4. Cut-out regions 2', 2" and 4', 4" in the vanes 2 and 4 respectively help to ensure that the strap 6 does not come into contact with these vanes. Strap 7 contacts those anode vanes that are not contacted by the strap 6 i. e. it contacts vanes 2 and 4. This strap is also spaced from the other alternate vanes, such as vanes 3 and 5. Similar cutouts 3'and 5'in vanes 3 and 5 respectively help to ensure that the strap 7 does not come into contact with them. The third strap 8 is in contact with the same vanes as is the strap 6. The purpose of this extra strap is to balance the n mode r. f. field at the cathode. This is described in our UK Patent No. 2054256.

The straps 6,7, 8 comprise concentric rings, typically made of silver-plated copper. The rings shown in figures 1 and 2 are open rings; ie. They each have a so- called strap break 9 to 11 respectively. Typically, the breaks 9 to 11 are a little longer than the width of a vane, although this need not be the case. In this drawing, the strap breaks 9 to 11 are located over anode vanes, but other arrangements are possible. Although the strap breaks 9 to 11 are necessary to prohibit flow of current between the vanes, they tend to cause greater frequency separation between the s mode of operation and the s-l mode.

Furthermore, the strap breaks may cause oscillations at undesired frequencies.

The graph of figure 3 represents the output of a conventional magnetron, such as that shown in figures 1 and 2, across a range of frequencies (the X-band). The main output peak occurs at approximately 9.4GHz, which represents the X mode i. e. the desired mode of operation.

However, there is a secondary peak occurring at approximately 9.9GHz. This represents the undesired w-1 mode of operation. Power is also emitted at higher frequencies.

Figure 4 illustrates components of a magnetron constructed according to the invention. The anode arrangement is shown, which arrangement comprises a cylindrical anode 12 and a plurality of anode vanes 13 to 30 extending radially inwardly from the inner wall of the cylindrical anode. Two straps 31,32 are also provided.

The inner strap 31 comprises a ring attached to alternate ones of the vanes, for example vanes 13,15, 17, 19, 21, 23,25, 27 and 29. The outer strap 32, concentric with strap 31, is attached to the other vanes, in this instance vanes 14,16, 18,20, 22,24, 26,28 and 30.

In accordance with the invention, the outer strap 32 includes a plurality of protrusions 34 to 40. In this embodiment, the protrusions 33 to 40 project inwardly from the ring and are also periodically spaced around the ring.

An exception to this is the portion that includes the strap break 41, which portion does not have a protruding region. The protrusions are arranged between the parts of the ring that are attached to the anode vanes. In other words, the protrusions are located adjacent the vanes that are not attached to the outer strap.

The protrusions 33 to 40 introduce a variation in capacitance between the inner strap 31 and the outer strap

32. Owing to the periodic spacing of the protrusions, there is a periodic variation in inter-strap capacitance.

The protrusions create a"lumped"variation in capacitance, which has the effect of increasing stability of the pi mode. The invention also reduces the harmonic content of the pi mode, as well as limiting oscillations in the pi-1 mode.

The plot of Figure 5 illustrates the output power achievable with the present invention, across the same range of frequencies as that shown in Figure 3. The invention gives a cleaner spectrum of the desired pi mode at approximately 9.4GHz and the secondary peak in Figure 3 representing the pi-1 mode is no longer present. It is also noticeable that the output at higher frequencies is markedly smaller than that shown in Figure 3.

In the embodiment shown in Figure 4, the protrusions take the form of inwardly indented regions of a ring-type strap. The diameter of the strap itself remains substantially constant around the ring, and so the outer edge of the strap protrudes inwardly to correspond with the inward indentations of the strap. Alternatively, as shown in Figure 6, the outer diameter of the ring can remain substantially constant, so that the outer edge of the strap 42 describes a circle. In this embodiment, the inward protrusions 43 to 50 cause a thickening of the diameter of the strap at the protruding regions. The provision of protruding regions causes a variation in separation between the inner and outer straps, thereby causing a variation in capacitance that has a stabilising effect on the magnetron.

The embodiments shown in Figures 4 and 6 show protrusions in the plane of the strap. In addition, or alternatively, protrusions may be arranged out of the

plane of the strap. Examples are shown in Figures 7 and 8, which both illustrate a group of vanes 51 to 55 and an inner strap 56, but having different outer straps.

A further embodiment is shown in Figure 6A in which protrusions are provided asymmetrically in the sense that the distribution of the protrusions is not even around the strap ring. In this case, the protrusions are arranged only on one half of the strap ring. In this arrangement, cavities numbered 6 to 16 couple the n mode, cavities 7 to 15 control coupling the tri-1 mode.

In Figure 7, the protrusions take the form of tabs 57, 58 attached to the inner edge of the outer strap 59.

The tabs 57 and 58 are arranged to hang downwards, in a plane transverse to the plane of the strap 59. The tabs 57 and 58 are located close to the vanes 52 and 54, to which the strap is not attached. The tabs 57, 58 need not be located intermediate the attachment points; the tabs may also be located at the vanes to which the strap 59 is attached or at any other desired position.

Figure 8 illustrates an embodiment in which the protrusions extend in more than one plane. In this drawing, the protrusions take the form of indented regions 60,61 and 62. Protrusions 60 and 61 occupy portions of the strap 63 intermediate the attachment points at vanes 51,53 and 55. The strap 63 is arranged so that the protrusions 60 and 61 extend downwardly towards cutout portions of the vanes 52 and 54. Protrusion 62, which is located close to the vane 53 to which the strap 63 is attached, is arranged to extend inwardly, i. e. towards the cathode.

In any of the examples shown, the capacitance change has the same sense as the direction of protrusion.

Specifically, the capacitance is increased at the protruding portions, which protrude so as to be nearer either another strap or anode vane.

In these drawings, the straps have been shown with protrusions extending inwardly or downwardly from the main body of the strap. The protrusions may be arranged to extend outwardly or upwardly from the strap. The protrusions need not be only in the plane of, or perpendicular to, the strap. The protrusions may extend obliquely from the strap.

Alternatively, or additionally, the inner strap could have protrusions, and these need not extend in the same direction, or even in the same plane, as the protrusions on the outer strap. A plurality of straps may be employed, some or all of which may have protruding regions.

The straps need not be rigid; flexible straps may be employed, so that the extent of one or all of the protrusions may be adjusted. This arrangement allows for adjustment of the inter-strap or strap-to-vane capacitance. The provision of a variable capacitance permits the undesired pi-1 mode to be retuned to a different frequency range.