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(11) | EP 2 083 471 A1 |
| (12) | EUROPEAN PATENT APPLICATION |
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| (54) | Temperature compensated coaxial cavity resonator using anisotropic material |
| (57) The invention relates to cavity resonators for usage in the field of telecommunications,
notably radio frequency and microwave radio communications. A cavity resonator operable
to exhibit a resonance frequency is disclosed, comprising a housing (41) made of a
material with a first thermal expansion coefficient in the first direction and a first
plate (42) made of a material with a first thermal expansion coefficient in a second
direction, essentially perpendicular to the first direction. The cavity resonator
further comprises an inner conductor (44) made of a material with a second thermal
expansion coefficient in the first direction and a second plate (40) made of the same
material as the inner conductor (44) with a second thermal expansion coefficient in
the second direction. The cavity resonator is characterized in that the first and the second thermal expansion coefficient in the second direction are
such that the mechanical stress at the joint between the first plate (42) and the
housing (41) and the mechanical stress at the joint between the second plate (40)
and the housing (41) caused by a significant temperature change is essentially zero.
Furthermore the first and the second thermal expansion coefficient in the first direction
are such that the resonance frequency remains within a preset bound, which is essentially
zero, over the significant temperature change. Furthermore, as method for making such
a cavity resonator is disclosed.
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[0001] The invention relates to cavity resonators for usage in the field of telecommunications, notably radio frequency and microwave radio communications.
[0002] Cavity resonators are resonators where the radio frequency electromagnetic energy resonates in an empty volume, typically air or vacuum, this volume being surrounded by a conductor, e.g. metal. By combining several resonators, cavity resonator filters with sophisticated frequency selective behavior can be obtained.
[0003] In general, the size of a cavity resonator depends on the desired frequency of operation. At microwave frequencies (0.3 GHz - 30 GHz), size and weight of cavity resonators are significant. They are typically milled in or cast from metal. Figure 1 illustrates the cross-section of a prior-art coaxial cavity resonator, built completely from metal. The outer - mostly cylindrical - conductor 10 and the inner - mostly cylindrical - conductor 12 are made from metal and form a cavity 11 filled with a gas, e.g. air. Since the geometrical shape determines the frequency of resonance, high mechanical accuracy is required and/or post-production tuning is applied. Post-production tuning is usually achieved by placing a metallic tuning screw through the resonator wall, and turning it, causing suitable field distortion and thereby resonance frequency variation.
[0004] It should be noted, that for manufacturing purposes cavity resonators are frequently built of two parts as shown in Figure 4. One part comprises the inner conductor 44 and a cover plate 40 to which the inner conductor 44 is attached, and a second part comprises the, mostly cylindrical, housing 41 and a ground plate 42. Usually, but not necessarily, each part is made of only one material. The two parts are then joined using different techniques, such as soldering, gluing, welding, snap-in and others.
[0005] In order to save weight and/or cost, cavity resonators can be made of metalized plastics. In such cases, the structural elements of the cavity resonator, notably the outer and the inner conductors 10, 12, shown in Figure 1 are made of molded or cast plastic which will subsequently be covered with a thin layer of metal. Just as in the traditional full metal constructions, using this process, cavity air volume 11 surrounded by conductive metal elements 10, 12 is formed.
[0006] Since the geometrical shape determines the frequency of resonance, thermal expansion of the constituent parts of the resonator, notably its inner and outer conductors 10, 12, causes the resonance frequency to vary as a result of varying ambient temperatures. Notably for outdoor operations for which microwave resonators are often used, such ambient temperature variants may be quite significant. As in the majority of applications a constant resonance frequency is required, this effect is not welcome.
[0007] As all materials are subject to the effect of thermal expansion, one approach is to select those materials, which have a relatively low thermal expansion coefficient α, measured in ppm/K, wherein ppm stands for parts per million and K for Kelvin. Due to the impact of thermal expansion of a cavity resonator on its resonance frequency, the measure ppm/K can also be used here for the relative negative frequency variation. By employing materials with a low thermal expansion coefficient, the frequency variations over temperature can be reduced. Standard metals such as aluminum or brass exhibit a thermal expansion coefficient α in the order of magnitude of 20 to 25 ppm/K. By way of example, the cavity resonator of Figure 1 may be made from aluminum with a thermal expansion coefficient α = 23 ppm/K. Consequently, a temperature increase would result in a lowering of the resonance frequency of the cavity resonator by 23ppm/K.
[0008] Another possibility is the use of Invar, a FeNi alloy, having thermal expansion coefficient α in the range of 1 to 4 ppm/K. However, Invar has other drawbacks as it is expensive, difficult to machine and rather heavy.
[0009] If metalized plastics are used, one has to overcome the relatively large thermal expansion coefficients of polymers, which are typically larger than 50 ppm/K. This is usually done by blending the polymer with glass, which may be milled or in a fiber form, and/or mineral powder. By substitution of about 30 to 65 volume percent of the polymer with these inorganic blends, thermal expansion coefficients can be reduced down to the ones which are known for metals such as aluminum or brass. However, these highly blended polymers are much more difficult to mold than their unfilled counterparts due to their high viscosity and relative abrasiveness.
[0010] Other than using materials with low thermal expansion coefficients, temperature-dependent frequency variations of a coaxial cavity resonator can be compensated or reduced by the suitable use of two different materials, having different thermal expansion coefficients, for the different parts of the cavity resonator. Notably, by fabricating the inner conductor 22, shown in Figure 2, from a material with a smaller thermal expansion coefficient αi than the thermal expansion coefficient αo of the material used for the outer conductor 20, the temperature-dependent frequency variations can be reduced. By way of example, the thermal expansion coefficient of the material used for the outer conductor 20, e.g. aluminum with αo = 23 ppm/K, is larger than the one of the material used for the inner conductor 22, e.g. steel with αi = 11 ppm/K. As the temperature rises the outer resonator 20 expands, causing the resonance frequency to be reduced. At the same time, the inner conductor 22 expands as well, but less than the outer resonator 20. This leads to an enlargement of the capacitive gap 23 above the inner conductor 22 and by consequence to an increase of the resonance frequency. By selecting the right materials with the right set of thermal expansion coefficients, a cavity resonator can be built, for which both frequency shifting effects cancel each other out, such that a constant resonance frequency can be maintained. This overall method will be referred to as frequency shift compensation.
[0011] This technique, however, exhibits some difficulties, notably when used in conjunction with cavity resonator components made from metalized plastics. Due to the fact that the two cavity resonator components, i.e. the outer conductor 20 and the inner conductor 22, have different thermal expansion coefficients, high stress zones build up at the joint or joints 21 between the different components. Depending on the method of joining the components, e.g. soldering, gluing, welding, snap-in and others, such stress zones may be critical. Notably for metalized plastics such stress zones are critical, as the adhesion of the metal on the plastic is often limited and as the joints are usually located in areas with high electrical currents, where a good contact between the cavity resonator components is required.
[0012] The present invention discloses a method for reducing and possibly removing the stress zones between the joints of multiple components of a cavity resonator. By reducing the stress zones, cavity resonators with high temperature tolerance to thermal expansion can be built that use frequency shift compensation as explained above. Using the disclosed method, it is possible to build cavity resonators with reduced or no frequency drift over large temperature intervals. Additionally, the method allows for significant cost reduction, as it will be possible to manufacture cavity resonators from inexpensive polymer filter structures as well as to apply less costly metallization processes to coat the cavity resonator components.
[0013] According to an aspect of the invention, a cavity resonator operable to exhibit a resonance frequency is disclosed. The cavity resonator comprises a housing made of a material or a coated material with a first thermal expansion coefficient in a first direction. Preferably, the housing has a cylindrical form and is made of metal or a, possibly blended, polymer, coated with a layer of metal. For reasons of conciseness, in the following and notably in the claims, the word "material" is to be understood as "material" or "coated material" and shall cover in particular metals or metalized plastics. Furthermore, the cavity resonator comprises a first plate made of a material with a first thermal expansion coefficient in a second direction, essentially perpendicular to the first direction. By way of example, the first plate may be the ground plate of a cavity resonator, but it may also be the cover of the cavity resonator. The first plate may be made from the same material or metalized material as the housing. However, it may also be beneficial to select a different material that exhibits a particular thermal expansion coefficient in the second direction. Furthermore, by way of example, if the second direction is the vertical direction, then the first direction is the horizontal direction. In order to close the cavity, the resonator further comprises a second plate made of a material with a second thermal expansion coefficient in the second direction. Preferably this second plate is the cover plate, but it may also be the ground plate. Within the cavity, the resonator comprises an inner conductor made of the same material as the second plate with a second thermal expansion coefficient in the first direction. Preferably, the inner conductor is attached to the second plate essentially at a right angle and extends essentially parallel to the walls of the housing. It may also be beneficial to build the second plate and the inner conductor in one component, as is general practice when manufacturing cavity resonators.
[0014] The cavity resonator has the characteristic that the first and the second thermal expansion coefficients in the second direction are set such that the mechanical stress at the joint or the joints between the first plate and the housing and the mechanical stress at the joint or the joints between the second plate and the housing caused by a temperature change does not exceed a preset value. It should be noted that depending on the cavity design, notably the geometry of the housing, there may be one or more joints between the components. For reasons of conciseness, in the following, notably within the claims, the word "joint" shall be understood as possibly comprising several joints. Furthermore, the first and the second thermal expansion coefficient in the first direction are set such that the resonance frequency remains within a preset bound for the temperature change. By way of example, for a cavity resonator with a resonance frequency in vertical direction, it could be said that the second plate and the first plate should have essentially the same thermal expansion coefficients in horizontal direction in order to avoid stress. At the same time the housing and the inner conductor should have different vertical thermal expansion coefficients that allow for frequency shift compensation.
[0015] According to another aspect of the invention, the preset value and the preset bound are essentially zero for a significant temperature range. It should be noted that in practice the value zero can only be approximated. As explained above, using materials such as Invar, relatively low frequency shifts in the range of 1 to 4 ppm/K may be achieved. This range of bounds for the frequency shift may be targeted by the disclosed cavity resonator. Similarly, it will not be possible to completely remove stress resulting from temperature change. As explained above, the stress that is acceptable also depends on the method by which the components have been joined. Consequently, the preset value for an acceptable stress level has to take into account several factors, such as the used materials, the used metallization techniques and the method used for joining, etc. The preset value can be determined by a person skilled in the art.
[0016] The first condition related to stress compensation can be achieved by selecting the same material for the second plate and the first plate, as in such cases the thermal expansion coefficients in the second direction are equal and stresses at the joint or joints could be avoided. However, as the material of the second plate is the same material as the material of the inner conductor, the second thermal coefficient in the first direction would be preset. If a material for the housing is identified that has acceptable mechanical and cost characteristics and that has an appropriate thermal expansion coefficient in the first direction, then frequency shift compensation could be implemented. However, if the first plate and the housing are made from the same material and in one component, frequency shift compensation will not be possible.
[0017] Consequently, due to a limited number of degrees of freedom in selecting material and related thermal expansion coefficients, special measures may be taken in order to build a cavity resonator that fulfills both the conditions stated above. Therefore, according to another aspect of the invention, at least one of the materials used for the cavity resonator is an anisotropic material, i.e. a material for which the thermal expansion coefficient α is direction-dependent. In particular, some polymer materials show different coefficients of thermal expansion α within the direction of the mold flow compared to the perpendicular direction of the mold flow. This effect may be enlarged by blending the polymer with fibered materials. Depending on the type of plastics and blending used, the anisotropy effect may become quite large, typically within a ratio of 1:5 up to 1:8. According to an aspect of the invention, such anisotropy is used to build a cavity resonator that fulfills the conditions with respect to its thermal expansion coefficients stated above.
[0018] According to another aspect of the invention, a method for building a cavity resonator component, the resonator component comprising the inner conductor and the second plate, as well as a cavity resonator as described above is disclosed. The inner conductor and the second plate are made integrally within one component of the same anisotropic material. This material is appropriate for casting or molding and exhibits a third thermal expansion coefficient in flow direction and a fourth thermal expansion coefficient in a direction essentially perpendicular or normal to the flow. The method comprises the step of providing a mold for the combined inner conductor and second plate component and pouring the material in its liquid state into the mold and assuring that for the inner conductor the material flow direction corresponds to the first direction and that for the second plate the material is mixed in flow and perpendicular to flow direction within the second direction.
It should be noted that the used material in its liquid state could be melted material or a plastic material before curing or another anisotropic material appropriate for casting or molding known to the person skilled in the art.
[0019] The described pouring and mixing step is configured such that the inner conductor exhibits the second thermal expansion coefficient in the first direction corresponding to the third thermal expansion coefficient and that the second plate exhibits the second thermal expansion coefficient in the second direction corresponding to the designed mixture of the third and fourth thermal expansion coefficient. By way of example, the first direction is the vertical direction and the second direction is the horizontal direction. In such a case, it may be beneficial to build a mold with the second plate on the bottom and the inner conductor sticking out like an obelisk. By pouring the liquid material from the top into the mold, the material could run down along the walls of the inner conductor, thus assuring that the material for the inner conductor part is orientated in flow direction. When reaching the second plate part at the bottom, the mold would force the material to change its flow direction, in order to achieve a predetermined mixture between the third and the forth thermal expansion coefficients. Such a predetermined mixture can be obtained by providing means for directing and mixing, such as ribs, within the bottom part of the mold. This way, a second thermal expansion coefficient in the second direction, i.e. vertical direction, could be designed that meets the stress condition of the cavity resonator.
[0020] It should be noted that the invention also relates to the cavity resonator component and cavity resonator obtainable from the method described above.
[0021] A cavity resonator exhibiting the above mentioned stress and frequency shift compensation characteristics could be built as follows. First materials for the housing and the first plate are selected, preferably mainly according to their mechanical, notably weight, and cost characteristics. Secondly an anisotropic material is selected for the inner conductor and the second plate. This material is selected such that in flow direction its thermal expansion coefficient is such that frequency shift compensation can be implemented in conjunction with the material selected for the housing. The mold for making the inner conductor and second plate component is designed such that a mixture of the in-flow and perpendicular-to-flow thermal expansion coefficient of the anisotropic material is achieved that allows to implement the low stress condition of the cavity resonator.
[0022] According to another aspect of the invention, a metalized, blended plastic material may be used for the housing, also referred to as the outer conductor, and the first plate. Such metalized, blended plastic materials are readily used in prior art cavity resonators. Preferably, the cavity resonator and consequently also the housing has a coaxial form. Alternatively, metals could be used. The inner conductor together with the second plate is preferably formed from another metalized plastic part, of which the plastic material exhibits a significant anisotropy in thermal expansion depending on the direction of the mold flow. Using the anisotropy and the mixing method described above, a thermal expansion coefficient can be designed according to the needs of the cavity resonator.
[0023] It should be noted that basically the above mentioned aspects of the invention may be combined in many useful ways. Furthermore, it should be noted that the disclosure of the invention also covers other claim combinations than the claim combinations which are explicitly given by the back references in the dependent claims, i.e., the claims may be basically combined in any order.
[0024] The advantages and features of the invention will become apparent from the description of preferred embodiments. The present invention is described in the following by referring to exemplary embodiments illustrated schematically in the accompanying figures, wherein
Fig. 1 illustrates the cross-section of a cavity resonator built from one piece;
Fig. 2 illustrates the cross-section of a frequency shift compensated cavity resonator;
Fig. 3 illustrates the cross-section of a cavity resonator with a combined second plate and inner conductor component; and
Fig. 4 illustrates the critical dimensions of a cavity resonator.
[0026] Figure 3 illustrates the cross-section of a coaxial cavity resonator. The injection gate 31 is placed along the rotational axis of the inner connector 33. It should be noted, that the inner conductor 33 could have other forms deviating from the circular cylindrical shape. Figure 3 shows a cavity resonator with a combined first plate and housing 30 and a combined inner conductor 33 and second plate 34. Furthermore, the two directions, i.e. the first direction z allowing for the resonance frequency and the second direction r are indicated. In the shown embodiment, the cavity 32 is filled with air, but also vacuum or other fillings such as gases are possible.
[0027] When applying the casting method disclosed above, the mold will flow along the inner conductor 33 in a highly directed manner towards the second plate 34. In the second plate 34, the mold will flow radially outwards, although means such as ribs could be added at the outside to force some flow away from the radial direction towards a circumferential direction.
[0028] The described mold flow will cause anisotropic thermal expansion as follows. At the inner conductor 33 in z direction, the thermal expansion coefficient will equal the in-flow thermal expansion coefficient of the anisotropic material. This expansion will have a first-order influence on the resonance frequency, as it will affect the capacitive gap (refer to 23, Fig. 2). At the inner conductor 33 perpendicular of the direction of the inner conductor 33, i.e. the r direction, the thermal expansion coefficient will equal the normal-to-flow thermal expansion coefficient of the anisotropic material. This expansion will only have lower-order influence on the resonance frequency.
[0029] At the second plate 34, the thermal expansion coefficient will end up between the lower and upper bounds defined by the in-flow and normal-to-flow thermal expansion coefficients of the anisotropic material. It should also be noted that due to the circular geometry radial and circumferential thermal expansion will be equal, as residual stress will be absorbed in the material. By adding ribs in radial or circumferential direction within the second plate 34 section of the mold, the outcome of the thermal expansion coefficient of the mixed material can be engineered.
[0030] As explained above, an aspect of the invention is to adjust the resulting thermal expansion coefficient of the second plate 34 to the thermal expansion coefficient of the housing and first plate 30 material such that a stress-free joint between the two parts, i.e. the combined housing and first plate part 30 and the combined inner conductor 33 and second plate 34 part, can be realized. In this context it is to be noted, that a perfect match for the thermal expansion coefficients is not necessarily required, depending on the method used to join both parts, e.g. soldering, gluing, welding, snap-in, etc.
[0031] Figure 4 is used to provide a quantitative example obtained from simulations done using a finite element solver. A housing 41 and a first plate 42 is shown. Furthermore, the inner conductor 44 and the second plate 40 are pictured. Additionally, some key dimensions of the cavity resonator are shown, notably dimension 46, affected by the second thermal expansion coefficient in the second direction, and dimension 48. For the first direction, i.e. the vertical direction in the illustrated embodiment, dimension 47 is relevant and is affected by the second thermal expansion coefficient in the first direction. Furthermore, dimension 45 is relevant and is affected by the first thermal expansion coefficient in the first direction.
| reference geometry & simulation | Ultem 2312 PEI 30% glass | Zenite 7244 LCP 40% glass/mineral | Zytel HTN52G45 polyamide 45% glass | Zytel HTN54G50 polyamide 50% glass | |
| Dim. 45 | 35 mm | +33 ppm/K | |||
| Dim. 48 | 36 mm | +33 ppm/K | |||
| Dim. 46 | 10 mm | +33 ppm/K | +45 ppm/K | +66 ppm/K | +50 ppm/K |
| Dim. 47 | 31 mm | +33 ppm/K | +6.5 ppm/K | +18 ppm/K | +14 ppm/K |
| Δfres | 1852.5 MHz | -32.9 ppm/K | +11.9 ppm/K | -10.2 ppm/K | -1.6 ppm/K |
[0032] Table 1 shows concrete values and simulation results for the model cavity resonator. The second column indicates the concrete dimensions of the model resonator, notably the dimensions 45, 46, 47, 48 and the resonance frequency fres = 1852.5 MHz. For the housing 41 and first plate 42 an isotropic material is assumed as for example Ultem 2312 of GE Plastics with an isotropic thermal expansion coefficient of α = 33ppm/K.
[0033] In the third column, the simulation results are shown, if for the inner conductor 44 and the second plate 40 the same material, i.e. Ultem 2312, is used. In such a case, there is no stress at the joints between the second plate 40 and the housing 41 as well as between the first plate 42 and the housing 41, as the horizontal thermal expansion coefficients for the second plate 40 and the first plate 42 are equal at α = 33ppm/K. However, no frequency shift compensation can be achieved with a Δfres = -32,9 ppm/K.
[0034] Column 4 and 5 show simulation results with two anisotropic materials, i.e. Zenite and Zytel grades of DuPont. However, for the shown materials the in-flow thermal expansion coefficients are either too small (α = 6.5 ppm/K) or too big (α = 18 ppm/K) in order to provide a good frequency shift compensation. Using the disclosed mixing method for the manufacturing of the second plate 40, a horizontal thermal expansion coefficient could be designed in order to avoid stress at the joints.
[0035] A suitable material for frequency shift compensation could be for example another Zytel grade of DuPont, for which the simulation results are shown in column 6. It realizes almost perfect compensation with a Δfres = -1.6 ppm/K. Again, by applying the disclosed mixing method, the horizontal thermal expansion coefficient of the second plate 40 can be designed to match α = 33 ppm/K, in order to avoid stress at the joints. This has been taken into account within the shown simulations.
[0036] The disclosed invention allows the use of cheaper and easier to mold polymers for the housing filter structure, because the detrimental effects of the thermal expansion of these materials can now easily be compensated by the described frequency shift compensation method. Depending on the materials involved and the shape of the resonator parts, the frequency shift compensation can go up to zero frequency drift over temperature. Furthermore, it allows a cheaper metallization process, i.e. a process yielding a lower adhesion, for all the involved resonator parts, notably the combined housing and first plate part and the combined second plate and inner conductor part, because the mechanical stress at the connection between the parts due to thermal expansion is reduced or even eliminated.
[0037] As shown in the present invention, by using the anisotropy, with respect to their thermal expansion, of molded polymer blends and the by using the disclosed mixing method, it is possible to manufacture a combined inner conductor and second plate component, that can yield frequency shift compensation of the entire resonator over a large temperature range due to low expansion in one important geometrical direction. Furthermore, the expansion of the component can be matched to the other resonator part at their joining location, which will make for an easy, stress-free joint between them.
[0038] This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to make and use the invention. While the invention has been described in terms of various specific embodiments, those skilled in the art will recognise that the invention can be practiced with modification within the spirit and scope of the claims. Especially, mutually non-exclusive features of the embodiments described above may be combined with each other. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art.
- the inner conductor (44) is attached to the second plate, the inner conductor (44) expanding along the first direction;
- the second plate (40) expands in the second direction, essentially perpendicular to the first direction; and
- the inner conductor (44) and the second plate (40) are made from the same anisotropic material, appropriate for casting and exhibiting a third thermal expansion coefficient in-flow direction and a fourth thermal expansion coefficient in a normal-to-flow direction; comprising the steps of
- providing a mold for the combined inner conductor (44) and second plate (40) component;
- pouring the material in its liquid state into the mold;
- assuring that for the inner conductor (44) the material flow direction corresponds to the first direction, such that the second thermal expansion coefficient in the first direction is essentially equal to the third thermal expansion coefficient in-flow direction; and
- mixing within the second plate (40) the material in in-flow and in normal-to-flow direction with respect to the second direction so that the mixed material essentially exhibits the second thermal expansion coefficient in the second direction.
- providing a housing (41) from a material with a first thermal expansion coefficient in a first direction;
- providing a first plate (42) from a material with a first thermal expansion coefficient in a second direction, essentially perpendicular to the first direction;
- making an inner conductor (44) and a second plate (40) component according to the method described in claim 1; and
- joining the housing (41), the first plate (42) and the combined second plate (40) and inner conductor (44) component to form a cavity enclosing the inner conductor (44),
wherein- the first and the second thermal expansion coefficient in the second direction are such that
* the mechanical stress at the joint between the first plate (42) and the housing (41); and
* the mechanical stress at the joint between the second plate (40) and the housing (41) caused by a temperature change do not exceed a preset value; and
- the first and the second thermal expansion coefficient in the first direction are such that the resonance frequency remains within a preset bound over the temperature change.
- the material used for the housing (41) and the first plate (42) is Ultem 2312; and
- the material used for the inner conductor (44) and the second plate (40) is Zytel HTN54G50.
- a housing (41) made of a material with a first thermal expansion coefficient in a first direction;
- a first plate (42) made of a material with a first thermal expansion coefficient in a second direction essentially perpendicular to the first direction;
- an inner conductor (44) made of a material with a second thermal expansion coefficient in the first direction; and
- a second plate (40) made of the same material as the inner conductor (44) with a second thermal expansion coefficient in the second direction;
wherein- the first and the second thermal expansion coefficient in the second direction are such that
* the mechanical stress at the joint between the first plate (42) and the housing (41); and
* the mechanical stress at the joint between the second plate (40) and the housing (41) caused by a significant temperature change do not exceed a preset value, which is essentially zero; and
- the first and the second thermal expansion coefficient in the first direction are such that the resonance frequency remains within a preset bound, which is essentially zero, over the significant temperature change.
Amended claims in accordance with Rule 137(2) EPC.
- the inner conductor (44) is attached to the second plate, the inner conductor (44) expanding along the first direction;
- the second plate (40) expands in the second direction, essentially perpendicular to the first direction; and
- the inner conductor (44) and the second plate (40) are made from the same anisotropic material, appropriate for casting and exhibiting a third thermal expansion coefficient in-flow direction and a fourth thermal expansion coefficient in a normal-to-flow direction;
comprising the steps of- providing a mold for the combined inner conductor (44) and second plate (40) component;
- pouring the material in its liquid state into the mold;
- assuring that for the inner conductor (44) the material flow direction corresponds to the first direction, such that the second thermal expansion coefficient in the first direction is essentially equal to the third thermal expansion coefficient in-flow direction; and
- mixing within the second plate (40) the material in in-flow and in normal-to-flow direction with respect to the second direction so that the mixed material essentially exhibits the second thermal expansion coefficient in the second direction.
- providing a housing (41) from a material with a first thermal expansion coefficient in a first direction;
- providing a first plate (42) from a material with a first thermal expansion coefficient in a second direction, essentially perpendicular to the first direction;
- making an inner conductor (44) and a second plate (40) component according to the method described in claim 1; and
- joining the housing (41), the first plate (42) and the combined second plate (40) and inner conductor (44) component to form a cavity enclosing the inner conductor (44),
wherein- the first and the second thermal expansion coefficient in the second direction are essentially the same, such that
* the mechanical stress at the joint between the first plate (42) and the housing (41); and
* the mechanical stress at the joint between the second plate (40) and the housing (41) caused by a temperature change
is essentially zero; and- the first and the second thermal expansion coefficient in the first direction are
such that the resonance frequency remains within a preset bound over the temperature
change.
the material used for the inner conductor (44) and the second plate (40) is Zytel
HTN54G50.
- a housing (41) made of a material with a first thermal expansion coefficient in a first direction;
- a first plate (42) made of a material with a first thermal expansion coefficient in a second direction essentially perpendicular to the first direction;
- an inner conductor (44) made of a material with a second thermal expansion coefficient in the first direction; and
- a second plate (40) made of the same material as the inner conductor (44) with a second thermal expansion coefficient in the second direction;
wherein- the first and the second thermal expansion coefficient in the second direction are essentially the same, such that
* the mechanical stress at the joint between the first plate (42) and the housing (41); and
* the mechanical stress at the joint between the second plate (40) and the housing (41) caused by a significant temperature change
do not exceed a preset value, which is essentially zero; and- the first and the second thermal expansion coefficient in the first direction are such that the resonance frequency remains within a preset bound over the significant temperature change.
- the material used for the housing (41) and the first plate (42) is Ultem 2312; and
- the material used for the inner conductor (44) and the second plate (40) is Zytel HTN54G50.