TITLE "MICROWAVE RESONATOR" DESCRIPTION The present invention relates to a microwave resonator. FIELD OF THE INVENTION Modern radar and telecommunications systems require high frequency signal sources and signal processing systems with stringent performance requirements and extremely good spectral purity. Thus, there is a need for signal processing systems and signal sources with ever increasing spectral purity, stability and power-handling requirements. Resonators by their nature provide discrimination of wanted signals from unwanted signals. The purity and stability of the signals produced is directly linked to the resonator used as the frequency determining device and is dependent upon its Q-factor, power handling ability and its immunity to vibrational and temperature related effects. It is known that a piece of dielectric material has self-resonant modes in the electromagnetic spectrum that are determined by its dielectric constant and physical dimensions. The spectral properties of a given mode in a piece of dielectric material are determined by the intrinsic properties of the dielectric material, its geometric shape, the radiation pattern of the mode and the properties and dimensions of the materials surrounding or near the dielectric. Prior art resonators have traditionally relied on metallic cavities containing no dielectric material, or on metallic

cavities containing a dielectric material which were limited in Q-factor by the properties of the metallic cavity and hence were operated at cryogenic temperatures in order to obtain a better Q-factor. However, to maintain cryogenic temperatures requires equipment which is cumbersome and difficult to incorporate into a portable or compact apparatus. SUMMARY OF THE INVENTION The present invention provides a microwave resonator operable at or near ambient temperatures whilst offering improved Q-factor over existing prior art resonators. In accordance with one aspect of the present invention there is provided a method of producing a microwave resonator operating in a desired mode and at a desired frequency at a particular temperature comprising a cavity defined, at least in part, by a generally cylindrical wall having an electrically conductive inner surface and containing a generally cylindrical piece of low loss dielectric material, characterised by (1) producing a first generally cylindrical piece of low loss dielectric material of predetermined size and placing same in a cavity to produce a microwave resonator; (2) passing microwave radiation into the cavity; (3) searching for and measuring an initial output frequency from the first piece corresponding to the desired operating mode at the particular temperature; (4) producing a second generally cylindrical piece of dielectric material by scaling from the first piece of dielectric material according to the ratio between the

initial and desired output frequencies; and (5) then producing a further cavity of substantially the same size as the first mentioned cavity but varying the diameter and/or height of the further cavity to compensate for manufacturing inaccuracies in the second piece so as to obtain an output frequency closer to the desired output frequency; and (6) placing the second piece in the further cavity to produce the microwave resonator operating in the desired mode and at the desired frequency. BRIEF DESCRIPTION OF THE DRAWINGS The present invention will now be described, by way of example, with reference to the accompanying drawings, in which:- Figure 1 is an underneath and sectional side view along the line A-A of a microwave resonant cavity in accordance with a first embodiment of the present invention; Figure 2 is an underneath and sectional side view along the line A-A of microwave resonant cavity in accordance with a second embodiment of the present invention; Figure 3 is an underneath and sectional side view along the line A-A of a microwave resonant cavity in accordance with a third embodiment of the present invention; Figure 4 is an underneath and sectional side view along the line A-A of a microwave resonant cavity in accordance with a fourth embodiment of the present invention; Figure 5 is a side view of a microwave resonant cavity in accordance with a fifth embodiment of the present invention;

Figure 6 is a side view of a microwave resonator in accordance with the present invention; Figure 7 is a plan view of the microwave resonator shown in Figure 6; Figure 8 is a schematic block diagram of a temperature controller for use in microwave resonators; Figure 9 is a schematic block diagram of an alternative temperature control for use in microwave resonators; Figure 10 is a graph showing the losses within a microwave resonator operating in various TM(N,l,d) modes for N between 1 and 5 as the ratio of the radii of the piece of dielectric material and the cavity walls changes; Figure 11 is a graph showing the losses within a microwave resonator operating in various TE(N,l,d) modes for N between 2 and 6 as the ratio of the radii of the piece of dielectric material and the cavity walls changes; Figure 12 shows a plot of the electromagnetic field strengths of a dielectrically loaded microwave resonant cavity operating in TM(5,l,d) mode; Figure 13 shows a plot of the electromagnetic field strengths of a dielectrically loaded microwave resonant cavity operating in TE(6, 1,d) mode; Figure 14 is a graph showing the variation of frequency of a sapphire loaded cavity microwave resonator (TM(5,l,d)) operating at 10GHz versus the operating temperature of the resonator in degrees Celsius; Figure 15 is a graph showing the relationship between the ratio of the radii of the cavity and the dielectric material to the operating frequency of the resonator and

the loss factor of the resonator system for a resonator operating in TM(5,l,d) mode; Figure 16 is a graph showing the relationship between the ratio of the height of the cavity and the dielectric material to the loss factor of the resonator system for a resonant cavity operating in TM(5,l,d) mode; Figure 17 is a graph showing the relationship between the ratio of the heights of the cavity and the dielectric material to the operating frequency of the resonator and the loss factor of the resonator system for a resonant cavity operating in TM(8,l,d) mode; Figure 18 is a graph showing the relationship between the ratio of the radii of the cavity and the dielectric material to the operating frequency of the resonator and the loss factor of the resonator system for a resonator operating in TM(5,1,d) mode; Figure 19 is a graph showing the relationship between the ratio of the radii of the cavity and the dielectric material to the operating frequency of the resonator and the loss factor of the resonator system for a resonator operating in TM(7,l,d) mode; and Figure 20 is a graph showing the relationship between the ratio of the radii of the cavity and the dielectric material to the operating frequency of the resonator and the loss factor of the resonator system for a resonator operating in TE(7,l,d) mode;

DESCRIPTION OF THE INVENTION In Figure 1 of the accompanying drawings, there is shown a

microwave resonant cavity 10 in accordance with the present invention. The microwave resonant cavity 10 comprises a cylindrical wall 12, a circular base 14 and a circular lid 16. Within the cylindrical wall 12 there are a number of microwave ports 18. The number of ports 18 depends upon the application for which the microwave resonant cavity 10 is intended to be used. The microwave ports 18 provide means for delivering the microwave into the cavity 10 and for receiving microwaves from the cavity 10. The cylindrical wall 12 has formed therein holes 26 to provide means for mounting the cavity 10. Each of the base 14 and lid 16 contains an axial recess 20 and an annular groove 21. The axial recess 20 and the cylindrical wall 12 are aligned co-axially. The annular grooves 21 accommodate a gasket, such as an indium gasket, to improve thermal conductivity between the cylindrical wall 12 and the base 14 and the lid 16. Shown in the upper diagram of Figure 1 is an underneath view of the base 14. However, it is to be appreciated that the diagram is equally applicable to the lid 16. The base 14 is provided with a plurality of holes 27 arranged in a circle and radial slots 28. The holes 27 are for mounting the base 14 to the cylindrical wall 12 by any convenient means, such as bolting. The radial slots 28 inhibit unwanted modes within the cavity 10. The number of radial slots 28 is dependent upon the resonant mode in which the cavity 10 is intended to operate. The cylindrical wall 12 has a surface 25 for mounting the

cavity 10 to a cooling means. There is also a flat surface 23 for each port 18 to facilitate mounting a microwave probe into the port 18. The resonant cavity 10 contains a generally cylindrical piece of dielectric material 22. The piece of dielectric material 22 is provided with an integral axial spindle 24 at each flat end of the cylinder. The spindles 24 are also formed of the dielectric material 22. The spindles 24 are designed to be accommodated within the recesses 20 of the lid 16 and base 14. Thus, the piece of dielectric material 22 is held between the lid 16 and the base 14 co-axially with the cylindrical wall 12. Figures 2,3 and 4 show alternative embodiments to the microwave resonant cavity shown in Figure 1, with like reference numerals denoting like parts. Shown in Figure 2 is a second embodiment of a microwave resonant cavity 30 in accordance with the present invention comprising a left section 32 and a right section 34. Each of the sections 32 and 34 contains an inner half cylindrical surface 31. A rod 36 of semicircular cross-section extends from each flat end of the section 32 inwards into the cavity 10 to terminate in a free end. A rod 38 of semicircular cross-section extends from each flat end of the section 34 inwards into the cavity 10 to terminate in a free end. The rods 36 are formed integrally with the section 32 and the rods 38 are formed integrally with the section 34. The rods 36 and 38 are aligned co-axially with the cylindrical surface 31 and each rod 36 is contiguous with the

corresponding rod 38. The free end of each pair of rods 36 and 38 has an axial recess 40 formed therein. The spindles 24 of the piece of dielectric material 22 are accommodated within the recesses 40 of the rods 36 and 38. Hence, the dielectric material 22 is held between the rods 36 and 38 co-axially with the cylindrical surface 31. The use of the sections 32 and 34 instead of the lid 16, base 14 and cylindrical wall 12 of the embodiment shown in Figure 1 provides increased suppression of unwanted modes within the cavity 30, as well as providing improved thermal conduction from the piece of dielectric material 22 to a cooling means. Shown in Figure 3 is a third embodiment of a microwave resonant cavity 50 in accordance with the present invention comprising a lid 52 and a base 54. The base 54 has formed integrally therewith a cylindrical wall 64. Coaxial rods 56 and 58 of circular cross-section extend from the lid 52 and the base 54 respectively into the cavity 50 to terminate in free ends. The rod 56 is formed integrally with the lid 52 and the rod 58 is formed integrally with the base 54. The piece of dielectric material 22 has formed therein axial recesses 60 at the top and bottom of the piece of dielectric material 22. The rods 56 and 58 are accommodated within the axial recesses 60 of the piece of dielectric material 22, holding the piece of dielectric material 22 co-axial with the cylindrical wall 64. Each of the rods 56 and 58 has formed therein an axial vent 62. The axial vent prevents any air being trapped in the axial recesses 60 when the cavity 50 is evacuated.

The cylindrical wall 64 has an annular projection 68 to provide a good contact with the lid 52. A space 66 is formed between the projection 68, the lid 52 and the cylindrical wall 64. The space 66 is designed to accommodate a gasket, ensuring a good thermal contact between the cylindrical wall 64 and the lid 52. Figure 4 shows a fourth embodiment of a microwave resonant cavity 70 in accordance with the present invention comprising a lid 72 and a base 74 having a flat end. The base 74 has formed integrally therewith a cylindrical wall 82. Extending from the flat end of the base 74 into the cavity 70 is a co-axial cylindrical rod 76. The rod 76 is long enough to extend through to the lid 72 and, as shown, to be integrally formed with the lid 72. Extending through the rod 76 is a hole 80. The hole 80 allows a temperature probe to be placed within the rod 76 close to the piece of dielectric material 22. The piece of dielectric material 22 has an axial cylindrical hole 78 formed therein. The piece of dielectric material 22 is designed to be suspended on the rod 76 as shown in Figure 4. The suspension of the piece of dielectric material 22 on the cylindrical rod 76 is achieved by one of the following means. Firstly, the axial cylindrical hole 78 formed in the piece of dielectric material 22 may be of a slightly smaller diameter than the cylindrical rod 76. By cooling the cylindrical rod 76 to a low temperature, the thermal contraction of the cylindrical rod 76 allows the dielectric material 22 to be placed in position over the cylindrical

rod 76. As the cylindrical rod 76 returns to ambient temperature, it will expand due to thermal effects, thus holding the piece of dielectric material 22 along its length. Alternatively, the hole 78 in the piece of dielectric material 22 may be plated with a metallic material. It is then possible to weld or solder the piece of dielectric material 22 to the rod 76. The slots 28 in the cavities 10, 30, 50 and 70 of the present invention are designed to supress unwanted modes within the cavity. The slots 28 are placed at positions around the lid of the cavity which do not interfere with the desired operating mode. This corresponds to positions at which there is a low concentration of electromagnetic energy in the desired operating mode. Many of the undesirable modes will have a considerable amount of energy at these positions, thus the slots 28 will act as suppressors for these modes. The effect of the slots 28 is to make the cavity non-radiating with respect to the desired operating mode and radiating with respect to most undesired modes. Hence the slots 28 help reduce the density of unwanted modes in the resonator. One of the losses in a microwave resonant cavity is due to dissipation of the electromagnetic field within the dielectric material. This dissipation causes heat build up within the dielectric material. Most dielectric materials have a resonating frequency dependent upon temperature. That is, the resonant frequency of the dielectric material will change as temperature changes. Hence, it is

undesirable to have the dielectric material change in temperature during operation. For this reason, it is necessary to dissipate the heat built up in the dielectric material as a result of dissipation of the electromagnetic field within the dielectric material. Therefore, it is desirable to have the lid, the cylindrical wall and the base of the microwave resonant cavities of the present invention formed of a material having good thermal conductivity. Having the lid base and walls of the resonant cavities of the present invention made of material with high thermal conductivity allows cooling of the cavity by any convenient means. However, there remains the inherent problem that the transfer of heat between the dielectric material and the base and lid of the cavity may take a considerable period of time. Hence, it is desirable to ensure that the design of the cavity allows the heat to be transferred as efficiently as possible. The microwave resonant cavity 10 shown in Figure 1, while offering excellent immunity to mechanical vibrations since the piece of dielectric material 22 is held securely between the lid 16 and the base 14, offers relatively poor thermal properties. This is because the spindles 24 are relatively long and thin compared to the cylindrical portion of the piece of dielectric material 22. The spindles 24 are thus effectively a very high thermal impedance, slowing the transfer of heat from the cylindrical portion of the piece of dielectric material 22 to the lid 16 and base 14.

The microwave resonant cavity 30 shown in Figure 2 offers an improvement in thermal properties in that the rods 36 and 38, made of the same material as the lid 32 and base 34, replace most of the spindles 24 of Figure 1. Thus, the spindles 24 are relatively small and are retained mainly for the purpose of holding the piece of dielectric material 22 co-axial with the cylindrical wall 12. A further improvement may be achieved by the microwave resonant cavity 50 shown in Figure 3. Here, the rods 56 and 58 extend into the piece of dielectric material 22, thus eliminating the need for spindles. Further, the thermal conductivity between the rods 56 and 58 and the dielectric material 22 is improved since the rods extend into the piece of dielectric material 22 and are thus closer to the heat to be dissipated. The microwave resonant cavity 50 still offers good resistance to mechanical vibration since the dielectric material 22 is held between the rods 56 and 58. The microwave resonant cavity 70 shown in Figure 4 offers the best thermal dissipation of the four embodiments illustrated in Figures 1 to 4. This is due to the presence of the rod 76 extending entirely through the piece of dielectric material 22. Thus, heat from the dielectric material is transferred directly into the rod 76 allowing the maximum possible dissipation of heat. However, since the dielectric material is suspended on the rod 76 purely by thermal expansion, the microwave resonant cavity 70 does not offer the same resistance to mechanical vibration as do the microwave resonant cavities shown in Figures 1,2 and 3.

Shown in Figure 5 is a fifth embodiment of a microwave resonant cavity 90 in accordance with the present invention comprising a cylindrical wall 92, a base 94 and a lid 96. The lid 96 has internal and external concentric annular sections 98 removed as shown. Also, the cylindrical wall 92 has external sections 100 removed from its upper and lower ends. The sections 98 are removed to allow for thermal contraction and expansion if the resonant cavity 90 is operated at cyrogenic temperatures. The sections 100 are removed to help provide good electrical contact, via a knife edge effect, between the cylindrical wall 92 and the lid 96 and the base 94. The resonant cavity 90 further comprises a locking means 102, a first circular projection 104, a second circular projection 106 and inner and outer concentric cylindrical pieces of dielectric material 108 and 110, respectively. The locking means 102 is designed to pass axially through the lid 96 and to engage the base 94 by any convenient means, such as threadedly. The locking means 102 holds the base 94 and the lid 96 in place between the cylindrical wall 92 and also holds the pieces of dielectric material 108 and 100 between the projections 104 and 106. The projection 104 extends into the resonant cavity 90 and has an annular form with a largely rectangular cross-section. The corners of the projection 104 extending innermost into the resonant cavity are removed to accommodate the pieces of dielectric material. The projection 104 is formed integrally with the lid 96 and is co-axial therewith. The projection 106 is formed

integrally with the base 94 and in all other respects is the same as the projection 104. The pieces of dielectric material 108 and 110 have a substantially constant thickness throughout their length. However, at each end of the cylinder, the thickness of the dielectric material 108 and 106 is decreased. When the pieces of dielectric material 108 and 100 are placed within the cavity and held between the projections 104 and 106, there is formed a gap 112 between the two pieces of dielectric material 108 and 110. At the ends close to the projections 104 and 106 where the thickness of the pieces of dielectric material 108 and 110 is decreased there is formed a broader gap 114. The function of the gap 114 is to present a substantially increased electromagnetic impedance to the microwave energy, by appearing as a waveguide operating below the cut-off frequency, to confine it between the gaps 114. The function of the gap 112 is to reduce the effects of losses within the dielectric material from which the pieces of dielectric material 108 and 110 are formed. Figure 12 of the accompanying diagrams show pictorially the distribution of the electromagnetic field within a dielectric material operating in TM(5,l,d) mode. Dark areasindicate a high concentration of electromagnetic radiation and light areas indicate a low concentration of electromagnetic radiation. The boundary of the cavity is shown by the black lines labelled "C". The boundary of the dielectric material is shown by the black lines labelled "D". The upper diagram shows a plan view of the dielectric

material The lower diagram shows a side view of the dielectric material. As can be seen in Figure 12, the majority of the electromagnetic radiation is contained with in the dielectric material. It is also to be noted that there is negligible electromagnetic radiation within the centre of the dielectric material. Hence, it is possible to remove the central dielectric material without impeding the operation of the resonator. Figure 13 is a pictorial representation of the electromagnetic field distribution within a dielectric material operating in TE(6,l,d) mode. The boundary of the dielectric material is shown by the black lines labelled "D". As can be seen to accommodate the increased number of modes, the piece of dielectric material has had to be increased in size for the same frequency of electromagnetic radiation. Also, more of the electromagnetic radiation is contained within the dielectric material. Examining Figures 12 and 13 it becomes apparent that most of the electromagnetic radiation is contained within a relatively narrow annulus. Thus, it is possible to form two concentric cylinders of dielectric material to contain the electromagnetic radiation whilst allowing the space between to be free space. It is well known that free space is a lossless media for electromagnetic radiation. Hence, the pieces of dielectric material 108 and 110 serve to confine the electromagnetic radiation in a similar manner to the other cavities in the other embodiments of the present invention, however, the gap 112 also allows for a

substantial decrease in the losses associates with these cavities. This is because the majority of the electromagnetic radiation is confined within the gap which is a lossless media. Hence, the Q factor of the resonator cavity 90 is better than that of the other embodiments of the present invention. It is also envisaged that the gaps 112 and 114 could be filled with a suitable material to allow the functioning of a MASER. Such suitable material would be, for example, Rubidium gas, or excited hydrogen gas. The performance of a microwave resonant cavity is largely determined by the geometries of the microwave resonant cavity and the piece of dielectric material 22. Specifically, the following measurements have been found to be relevant to resonator performance: a) the diameter of the piece of dielectric material 22, b) the height of the piece of dielectric material 22, c) the ratio of the diameter of the piece of dielectric material 22 and the diameter of the inner face of the cylindrical wall 12, and d) the ratio of the height of the piece of dielectric material 22 and the height of the cylindrical wall 12. Further, the Q-factor of a dielectric resonator is determined by losses due to dissipation of the electromagnetic field in the dielectric material, radiation of the electromagnetic field into the surrounding space and dissipation in the cavity walls. It is known that radiation losses are reduced for certain resonant modes. Of the multitude of electromagnetic modes

one of the most favoured for the reduction of radiation losses is a group known as "whispering gallery" modes. For these modes most of the electromagnetic field is contained within the dielectric material, reducing radiation losses. In particular, the modes preferred for use in the present invention are Quasi Transverse Electric modes, TE(N,l,d), Quasi Transverse Magnetic Modes, TM(N,l,d) and Quasi Transverse Hybrid Modes, N=3 to infinity, preferably 3 to 20, more preferably 4 to 7. The value of N chosen, and hence the resonant mode chosen, and the frequency of operation of the resonator, affect the determination of the dielectric material geometry. Figure 10 shows for TM(l,l,d) to TM(5,l,d) the normalised maximum Q factor obtainable for a cavity for various ratios of the radii of the cavity to the diameter of the piece of dielectric material. The curves in Figure 10 are for a sapphire dielectric material in a cavity with copper walls. As can be seen for low values of N, especially N less than or equal to 3 there are appreciable losses due to the interaction of the electromagnetic mode with the cavity walls, or radiation of the electromagnetic field into free space. Further, it is also apparent that N=5 is the only mode shown on the graph for which the normalised Q factor is greater than or equal to 1. Hence, it is preferable that the mode chosen for transverse magnetic modes is at least TM(5,l,d). This choice allows the maximum Q factor obtainable from the dielectric material to be achieved within the cavity, allowing for other limitations. However, as the mode number increases so does the size of

the dielectric material needed to accommodate it, for the same resonant frequency. Thus, it is optimal to choose, for transverse magnetic modes, N equal to five to give the maximum Q factor obtainable from the piece of dielectric material whilst having the cavity the minimum possible size. Figure 11 shows a normalised graph of the maximum Q factor obtainable within a cavity for transverse electrode modes TE(2,l,d) to TE(6,l,d) for various ratios of radii of the cavity and the piece of dielectric material. The curves in Figure 11 are for a sapphire dielectric material in a cavity with copper walls. The vertical axis represents the normalised Q factor obtainable and the horizontal axis is the ratio between the radius of the cavity and the radius of the dielectric material. As can be seen from this graph, it is necessary to operate in TE(6,l,d) to obtain the maximum Q factor available for the dielectric material within the resonant cavity. Hence, for a microwave resonant cavity operating in transverse electric mode, it is preferable to operate in TE(6,l,d) mode to obtain the maximum Q factor available for the dielectric material whilst minimising the cavity size. Another consideration is the fact that as the mode of the cavity increases, more of the electromagnetic radiation is contained within the dielectric material. Effectively, this results in a decreased ability to tune the operating frequency by varying the size of the cavity. The modes TM(5,l,d) and TE(6,l,d) are considered to provide an excellent compromise between the tunability of the cavity

and the loss within the cavity. Further, in accordance with the present invention the effects of the radiation losses from the dielectric material are reduced by placing the dielectric material within an electrically conductive cavity. This can be achieved by making the base lid and cylindrical wall of the resonant cavity from a highly electrically conductive material such as copper or silver. Alternatively , the base, lid and cylindrical wall of the resonant cavity may be plated with highly conductive material such as copper, silver or gold to an appropriate thickness. It has been found that 20 microns is sufficient for most applications. Silver is generally preferred as it exhibits the lowest resistivity. Still further, reduction of the radiation losses in the dielectric material can be achieved by choosing a low loss dielectric material with one or more of the following desirable properties: low loss tangent, moderate or high dielectric constant, small temperature coefficient of expansion, small temperature coefficient of dielectric constant, high Youngs modulus and high dielectric strength. Whilst the preferred form of dielectric material is pure sapphire, other materials may be used in the construction of such a resonator. Some other suitable materials are barium titanate, quartz, doped quartz, YIG (Yittrium Indium Garnate), YAG (Yittrium Aluminium Garnate), lithium niobate and lanthinate. Further, it may be preferable to dope the dielectric material with selected atomic species to alter certain

characteristics of the dielectric material to improve the resonator performance. As an example, it may be advantageous to introduce selected paramagnetic species of atom into the sapphire lattice to a determined doping level. This paramagnetic species will interact with the microwave resonance of the resonator and result in the resonator having a generally reduced frequency dependence on temperature. It has been found that a diameter of 21.68 mm and height 20.58 mm is desirable for a sapphire dielectric material operating in TM(5,l,d) mode at 10 GHz. This value was determined by solving Maxwell's equations in known manner. Having obtained a first value by solution of Maxwell's equations, it is possible to obtain diameters for pieces of dielectric material operating in other modes by the following process. Firstly, a resonator using the piece of dielectric material of 21.68mm diameter is built. The resonator is operated a a temperature close to the desired operating temperature of the cavity to be made. The resonator should have the same ratios for the heights and diameters at the desired cavity, and should be within the tunable range for the desired operating mode, for example between 1.65 and 2.00 for the ratio of diameters of a cavity desired to operate in TM(5,l,d) mode. Next, the resonant frequency of the resonator for the desired operating mode is measured using known means. By measuring this frequency, it is possible to determine to within machining tolerances, the diameter of a piece of dielectric material which will operate in th

desired mode at the desired frequency. This is possible since the diameter of the dielectric material is proportional to resonant frequency, thus calculation of the necessary diameter of the dielectric material is by a simple ratio. That is, by dividing the calculated resonant frequency of the sample dielectric material by the desired operating frequency and multiplying the result by the diameter of the sample dielectric material, it is possible to arrive at an approximate diameter for the desired microwave resonator. However, since machining of dielectric materials has inherent inaccuracies, the desired operating frequency and the actual operating frequency of the dielectric material will be somewhat different. To overcome this problem, it is possible to tune the resonant frequency of the microwave resonator by altering the ratio of the radius of the cavity walls to the radius of the dielectric material. Figure 15 is a graph representing the variation of resonant frequency with variation of the abovementioned ratio and the loss in Q-factor associated with this change for a cavity operating in TM(5,l,d) mode. In this graph, the horizontal axis represents the ratio between the radius of the cavity and the radius of the dielectric material. The left vertical axis represents the normalised Q-factor obtainable. The right vertical graph represents the operating frequency, in MHz, of the cavity. It is considered preferable to operate within the range of 1.65 to 2.00 for the ratio of the radii of the cavity to the piece of dielectric material for TM(5,l,d) mode. This

gives a tuning range of approximately 15 MHz at a resonant frequency of 10 GHz but only sacrifices 10% of the Q-factor. This is considered an acceptable loss in Q-factor in order to achieve greater tunability of the microwave resonator. Thus, once the dielectric material has been machined to the diameter calculated above, the resonant frequency of the dielectric material is measured. By calculating the discrepancy in the actual resonant frequency and the desired resonant frequency, it is possible to adjust the radius of the cavity walls to compensate for the machining discrepancy in the dielectric material by referring to Figure 15. For example, by making the initial measurement with the ratio of the radii being equal to 2.0 and by machining the sapphire so that the resonant frequency is slightly below that which is desired is possible simply by decreasing the ratio of the radii to increase the resonant frequency by up to 15 megahertz. One final piece of tuning is achieved by adjusting the operating temperate of the resonant cavity. Shown in Figure 14 is a graph of the change in resonant frequency for a sapphire dielectric material for various temperatures. The horizontal axis has units degrees Celcius. The vertical axis is the operating frequency of the cavity, in GHz. It can be seen from the graph that sapphire has a temperature co-efficient of approximately 671 KHz per degree Celsius. By maintaining the temperature of the resonant cavity to within 1/lOOOth of a degree Celsius, it is possible to tune the resonant cavity to have

a resonant frequency that is accurate to within one part per million. As the above process is carried out for multiple modes, a library of information can be made to simplify the design of similar cavities. Figure 16 is a graph showing how the losses within the cavity is related to the ratio of the height of the metal cavity to the height of the piece of dielectric material for a cavity operating in TM(5,l,d) mode. The horizontal axis is the ratio of the height of the cavity to the height of the dielectric material. The vertical axis represents the maximum normalised Q-factor obtainable for the cavity. To ensure that the ratio of the heights has a minimal effect on the losses within the resonant cavity, it is desirable to operate in the region of Figure 16 where the graph is close to 1.0. For example, where the ratio of the heights is well above 1.2, preferably approximately 1.6. It is possible to tune a cavity by altering the ratio of the heights of the cavity and the dielectric material. Figure 17 shows the effect on resonant frequency and cavity losses of altering the ratio of the heights for a resonator operating in TM(8,l,d) mode for various conditions. The horizontal axis represents the ratio of the height of the cavity to the height of the dielectric material. The left vertical axis is the normalised Q-factor obtainable within the cavity. The right vertical axis show the relative frequency shift of the operating frequency in percent. The curve labelled 1 is for a cavity operating at a temperature of 20 degrees Celcius. The ratio of the radii

was 1.7 and the resonator had a copper shield. The curve labelled 2 is for a cavity operating at a temperature of 4.2 Kelvin. The ratio of the radii was 1.9 and the resonator had a niobium shield. The curve labelled 3 is for a cavity operating at a temperature of 4.2 Kelvin. The ratio of the radii was 2.2 and the resonator had a copper shield. The curve labelled 4 shows how the operating frequency changes with the ratio of the heights. Curve 4 is equally applicable to curves 1, 2 and 3. From Figure 17 it can be seen that the tunable range achieved by altering the heights in a resonator operating in a transverse magnetic mode is less than that achieved by altering the diameter for the same cavity loss. Thus, it is preferred to alter the ratio of the diameters in a TM mode cavity. In a TE mode cavity, the ratio of the heights will give the greatest tuning range for the same cavity loss. Figures 18, 19 and 20 show the effect on resonant frequency and cavity losses of altering the ratio of the heights for a resonator operating in various modes. The horizontal axes represents the ratio of the height of the cavity to the height of the dielectric material. The left vertical axes is the normalised Q-factor obtainable within the cavity. The right vertical axes show the operating frequency of the cavity in GHz. Figure 18 shows this relationship for a cavity operating in TM(5,l,d) mode, Figure 19 shows a cavity in TM(7,l,d) mode and Figure 20 shows a cavity operating in TE(7, 1,d) mode. The information for Figures 18, 19 and 20 was derived at a

temperature of 20 degrees Celcius, with a piece of sapphire dielectric material of 21.67mm diameter and 20.58mm height, and the ratio of the heights of the cavity to the sapphire was 1.2. To obtain the maximum performance from the cavity it is neccessary to rotate the piece of dielectric material with respect to the ports 18. This is because the piece of dielectric material is not a perfect cylinder, or the dielectric material axis is not exactly aligned with the cylinder axis, or the dielectric material may have defects in its crystal structure due to manufacturing limitations. Thus there may be some positions for which the performance of the resonator is better due to the orientation of the piece of dielectric material. This adjustment is made by having the cavity in operation and observing the effect of rotating the piece of dielectric material with respect to the ports. In Figures 6 and 7 of the accompanying drawings, there is shown a microwave resonator 200 incorporating the microwave resonant cavity 50 of Figure 3, with like numerals denoting like parts. It is to be appreciated that any of the microwave resonant cavities 10,30,50,70, or 90 could be used. To reduce the effects of temperature variations on the frequency of operation, there is provided a cooling means 202 and a vacuum canister 204 mounted onto an enclosure 212. To allow the evacuation of the vacuum canister 204 there is provided a vacuum pump-out port 206. There is also provided a hermetic feed through 208 in the vacuum

canister 204 to allow cabling to pass through the vacuum canister 204. By placing the cavity 50 within the vacuum canister 204, the cavity 50 is evacuated, effectively insulating the cavity 50 against variations in ambient temperature. Cooling means 202 is preferably a compact device, such as a Peltier heat pump. The cooling means 202 is held between the cavity 50 and the enclosure 212 to allow heat transfer therebetween. In this embodiment of the present invention, the enclosure 212 acts as a heat sink. Cooling the cavity 50 gives an increase in resonator performance. The cooling means 202 is controlled by a thermal stabiliser circuit 214, allowing the temperature of the cavity 50 to be maintained, within acceptable tolerances, at a constant temperature, further improving the temperature stability of the resonator 200. To provide still further insulation, it is possible to wrap the cavity 50 in a multi-layer super insulation, of known type. To facilitate the transfer of microwave radiation between the dielectric material 22 and the ports 18, the ports 18 are terminated within the cavity 50 by known microwave field probes 220. Access to the ports 18 is provided by external connectors 222 attached to the enclosure 212. There is a hermetic port 216 for each external connector 222 to ensure there is no loss of the vacuum within the vacuum cannister 204. Each connector 222 is linked to a ports 18 by a suitable microwave conductor 224, such as co-axial cable or a microwave waveguide. Shown in Figure 8 of the accompanying drawings is a block

diagram of a temperature stabiliser circuit 214. The temperature stabiliser circuit 214 comprises a temperature sensor 150, a bridge 152, lock-in amplifier 154 and a proportional, integral and differential controller 156 and servo amplifier 158. There is also shown a cavity 160. The cavity 160 could be any of the cavities 10, 30, 50, 70 or 90 of the present invention. The temperature sensor 150, bridge 152, lock-in amplifier 154, controller 156 and servo amplifier 158 form a single stage closed loop controller of well known type. As shown in Figure 9 of the accompanying drawings is a block diagram of an alternative embodiment of a temperature stabiliser circuit 214. Again, there is shown a cavity 160 which may correspond to any of the cavities 10, 30, 50, 70 or 90 of the present invention. In this embodiment, which is a dual stage controller, there are two separate single stage closed loop controllers, a coarse controller 176 and a fine controller 188. The coarse controller 176 comprises a temperature sensor 170, a lock-in amplifier 172 and a PID and servo amplifier 174. The coarse controller 176 maintains the temperature of the microwave cavity to within a reletively narrow range, for example 0.1°C. The fine controller 188 comprises a temperature sensor 180, a lock-in amplifier 182, a PID and servo amplifier 184 and a fine heater or thermoelectric module 186. The temperature sensor 180 is used to sense the temperature of the piece of dielectric material 22 directly. The heater or thermoelectric module 186 is used to directly control the temperature of the piece of dielectric material 22.

Because the coarse controller 176 maintains a temperature of the microwave cavity to within a relatively small range, the fine controller 188 is thus made immune to changes in the ambient temperature. Hence the fine controller 188 can be made far more sensitive to small variations in temperature. Hence, the fine controller 188 is used to control far more accurately the temperature of the dielectric material 22. Thus the coarse controller 176 maintains an approximately constant temperature against variations in ambient temperature, while the fine controller 188 maintains the temperature of the piece of dielectric material to within a very narrow range. It is possible with the dual stage controller to control the temperature of the piece of dielectric material to within a few microdegrees Celcius. In use, the microwave resonator 200 is attached to a signal source via connectors 222 as shown in Figures 6 and 7. The signal travels along the microwave conductor 224 and is emitted into the cavity 50. Any component of the signal whose frequency and mode does not correspond to a resonant frequency of the cavity 50 will be reflected at the field probe 220. Thus, the only components of the signal which are present within the cavity 50 are those which correspond to a resonant frequency of the cavity 50. Most of the signal within the cavity 50 is contained within the dielectric material 22. Any leakages from the dielectric material 22 are either reflected from the wall 12 back into the dielectric material 22 or are absorbed by a field probe 220 and transmitted along a microwave

conductor 224. The signal which is sent along a microwave conductor 224 is used by the device to which the microwave resonator 200 is attached. Such devices include oscillators at microwave frequencies and filters. The losses within the cavity 50 are reduced to losses within the dielectric material 22 and losses within the walls of the cavity 50. By making the walls of the cavity 50 from a low electrical resistance metal, such as copper or silver, losses within the walls become negligible. Thus, the losses are largely defined by the type of dielectric material 22. It has been found that sapphire is an extremely suitable material for this purpose, having a low loss tangent. Further, the losses in both the metals and the dielectric are decreased at lower temperatures. The cooling means 202 is designed to provide cooling which is still near ambient temperature, between -80°C and 50°C, compared with the cryogenic temperatures of prior art devices. Whilst cooling the present invention to cryogenic temperatures would yield still further improvements in performance, the performance of the resonator 200 is currently well in excess of existing devices.