MICROWAVE METALLIC CAVITY

Abstract: It is described a microwave metallic cavity, the resonating frequency of which is stabilized versus operating temperature variations. The said stabilization is achieved by implementing the cavity with a conical base (3) having a thickness and a coefficient of linear expansion smaller than the ones of the cavity cylidrical body (1). In this way the volume enclosed by the conical base (3) varies in inverse ratio versus operat­ ing temperature variations, so as to compensate the variation in volume of the cavity cylindrical body (1), which results in a stabilization of the resonating frequency.

Description: The present invention refers to a temperature compensated microwave metallic cavity. It is known that the resonating frequency of a microwave resonating cavity depends on the volume of the same cavity and, more precisely, it is known that an increase in the volume of the cavity results in a decrease of the resonating frequency, whereas a decrease in the volume of the cavity results in an increase of the resonating frequency. It is also known that in a metallic resonating cavity a temperature variation results in a variation of the volume and therefore of the resonating frequency. Precisely the resonating frequency varies in inverse ratio with respect to the operating temperature and to the coefficient of linear expansion of the material used to implement the cavity. This is to say that as higher are the operating temperature and the coefficient of linear expansion of the material of the cavity, as lower is the resonating frequency of the metallic cavity. It is also known that the material which is most commonly used in manufacturing the waveguide components is brass, which features a coefficient of linear expansion of 18 x 10⁻⁶ [°C]⁻¹. By using such material at resonating frequencies of 15 to 20 GHz, a temperature increase of 25 °C results in a decrease of the resonating frequency of about 7 to 9 MHz. It is also known that, in order to compensate for the resonating frequency variations versus the operating temperature, materials featuring low values of coefficient of linear expansion, for instance invar whose coefficient of linear expansion is 1,5 x 10⁻⁶ [°C] ⁻¹, are used in the construction of microwave cavities, so as to reduce the variations in volume of the cavity versus temperature variations. However, the use of invar results in production costs considerably higher, as a matter of fact the material is intrinsically more expensive and the working times are considerably longer, because of the higher difficulties encountered in machining invar with machine tools. A first temperature compensated microwave metallic cavity is disclosed in the US 3,202,944 patent in which the cavity is composed by a hollow hemisphere connected to a cylindrical side wall closed by a flat end wall. This particular shape is the consequence of the main purpose of the invention, i.e. to obtain an extremely high Q resonator useful, for example, as a stabilizing or reference resonator for high frequency oscillators. The main drawback of this cavity is that a single cavity cannot be both tunable and temperature compesated simultaneously. This restriction results by the fact that either the tuning means or the temperature compensation means are located at the cylindrical end of the cavity, and both vary the volume of the cavity by means of two opposite modalities. A second temperature compensated microwave metallic cavity is disclosed in the CA 1,152,169 patent in which the cavity includes two bimetallic, or trimetallic, bases soldered respectively to the upper and lower lips of a cilindrical wall. The two bases are made by two or three layers of different materials with different thermal expansion coefficients. These layers are superimposed one upon the other, and are connected together very tightly involving a large connection area. The cavity volume is substantially the volume of the cylindrical body, the bases only flex with the temperature variations increasing or decreasing the volume of the cylindrical body in order to mantain the volume of the cavity substantially constant at any temperature. The tuning is obtained by means of screws which do not vary the volume of the cavity, but they act on the electromagnetic field lines. A first disadvantage of said second cavity is that, because of its particular configuration, the resonance frequency compensation with the temperature is poor. A second disadvantage is that the bimetallic motion originates strong tensions in the bimetallic layers, because of the large connection area between the layers. Consequently they can break or disconnect each other; in addition the bases can unsolder from the cylindrical body. In any case the fatigue of the materials reduces the lifetime of the bimetallic bases. A third disadvantage is that this cavity does not allow the resonant frequency adjustment, but just a sloght tuning. That means that the position of the screws depends on the particular propagation mode of the electromagnetic wave inside the cavity. In fact if the propagation mode is changed, also the electromagnetic field lines change, and the screws have to be positioned in new places. So this cavity needs as many particular constructions as propagation modes. A third temperature compensated microwave metallic cavity is disclosed in the US 3,873,949 patent. This patent discloses a rectangular cavity having a hollow compensation inwardly cone inserted into a cavity wall. The cone changes the resonant frequency by changing the electrical features of the cavity; at any temperature increasing (or decreasing) the height of the cone decreases (or increases) and the cone operates as an "automatic lenght controlled" screw. No compensation of the total volume of the caity is obtained. As said for the second known cavity, the change in the kind of cavity or propagation mode brings to a different electromagnetic field lines configuration. So the frequency compensation disclosed in the third patent can only be used with a rectangualr cavity and only for TEO1 propagation mode. Yet, this cavity does not have any means to adjust the resonant frequency. Therefore, purpose of the present invention is to overcome the said drawbacks and to indicate a microwave metallic cavity implemented with materials having high values of coefficient of linear expansion, easy and economical to machine with machine tools, and which presents a volume and consequently a resonating frequency stabilized versus operating temperature. To achieve these purposes, the present invention refers to a microwave metallic cavity characterized in that it comprises: a first hollow body (1) defined by a cylindrical metallic wall having at first end an integral first metallic wall (2), the second end (8) of the body being open, said first hollow body enclosing a first volume (V1); a second hollow body (3) defined by a concave second metallic wall, said second wall enclosing a second volume (V2); said second hollow body being rigidly connected to said second end of the first hollow body, to form a cavity whose volume is the sum of said first and second volumes; a metallic mobile sheet (6) placed within the first hollow body closing the cavity and allowing a tuning adjustment of its resonant frequency, said adjustment being produced by translations of said sheet inside the microwave cavity varying the volume of the cavity; whereby the wall thickness and the thermal coefficient of expansion (α) of said first hollow body are greater than the wall thickness and the thermal coefficient of expansion (β) respectively of said second hollow body, and whereby an increase in the cavity temperature produces a prevalent expansion of said first hollow body which mechanically deforms the second wall making it less concave, the first hollow body expansion and the second wall deformation respectively increasing said first volume and decreasing said second volume so as to keep the microwave cavity volume constant. Further purposes and advantages of the present invention shall be clear from the following detailed description of a preferred embodiment and from the attached drawings given only as explicating not limiting example, where: Fig. 1 shows a cross section of a cylindrical metallic cavity according to the present invention; Fig. 2 shows a cross section of a constructive detail of the cylindrical metallic cavity of Fig. 1; and Fig. 3 shows a schematic detail of the cylindrical metallic cavity of Fig. 1. With reference to Figs. 1, 2 and 3, the metallic cavity shown therein is formed of a hollow cylindrical body 1, an upper base 2 and a lower base 3. The upper base 2 has a flat circular shape. The cylindrical body 1 and the upper base 2 of the cavity are made of brass, copper or aluminium having a thickness of 2 to 5 mm, and feature a coefficient of linear expansion α . The upper base 2 has a threaded hole 4 in which an adjusting screw 5 is screwed in. A mobile sheet 6 also made of brass, copper or aluminium having a thickness of 1 to 2 mm is firmly connected to the end of the adjusting screw 5 which is inside the cavity. The lower base 3 of the cavity, according to the invention, has a conical shape, the vertex being faced to outside the cavity, is made of an iron-nickel alloy, for example invar, having a thickness of 0,1 to 0,4 mm and features a coefficient of linear expansion β, much less than α . More precisely, the lower internal section of the cylindrical body 1 has a cylindrical groove 7, in which the conical base 3 is inserted, so as to identify a circular surface 8 which is common to the cylindrical body 1 and to the conical base 3. A retaining ring 9 is located above the peripheral section of the conical base 3. The retaining ring 9 and the peripheral section of the conical base 3 are then soldered onto the internal section of the cylindrical body 1 so as to form one body. The cylindrical body 1, the mobile base 6 and the circular surface 8 enclose a first volume "V1", whereas the circular surface 8 and the conical base 3 enclose a second volume "V2". The total volume of the cavity, therefore, results formed by the first volume "V1" due to the cylindrical body 1 of the cavity and from the second volume "V2" due to the comical base 3 of the cavity. The required resonating frequency is obtained by moving the mobile sheet 6 by means of the adjusting screw 5 in order to obtain the right volume "V1+V2" of the cavity. At the reference temperature "To" the cylindrical body 1 of the cavity has a volume "V1o", whilst the conical base 3 has a radius "Ro" and a height "ho" and, therefore, a volume "V2o". The total volume of the cavity at the ambient temperature "To" is consequently "V1o+V2o". Any increase in operating temperature results in a thermal expansion of the cylindrical body 1 of the cavity and therefore in an increase in its volume, which becomes "V1". The conical base 3, as already said, has the following characteristics: is soldered to the cylindrical body 1, has a thickness much smaller that the thickness of the cylindrical body 1 and features a coefficient of linear expansion β, which is much lower than the coefficient of linear expansion, α , of the cylindrical body 1 and consequently undergoes a mechanical expansion much higher than the thermal expansion which would be caused by that determined temperature increase, and a variation of its geometrical dimensions. As a matter of fact, under the said conditions the conical base 3 has a radius "R" (greater than "Ro") and a height "h" (lower than "ho") and therefore a volume "V2". It can be demonstrated that the volume "V2" of the conical base 3 of the cavity is lower than the volume "V2o" of the same at the reference temperature "To". It is obvious that an appropriate selection of the material used to implement the conical base 3 and an appropriate dimensioning of the same conical base 3, permit to make the decrease in the volume "V2" of the conical base 3 of the cavity equal to the increase in the volume "V1" of the cylindrical body 1 of the cavity, so as to obtain a microwave cavity whose volume, and consequently resonating frequency, is stabilized versus operating temperature variations. A relationship to be used for the said dimensioning is given hereunder where: "ho-h" is the variation in height of the conical base 3, "T-To" is the temperature variation, "Ro" is the radius of the conical base 3 at the reference temperature "To", "α" is the coefficient of linear expansion of the cylindrical body 1 "β" is the coefficient of linear expansion of the conical base 3, "γ o " is arctg ho/Ro and "γ" is arctg h/R By using the equation hereabove, comical bases 3 have been selected having a height "ho" ranging between 0.5 and 2 mm to implement cylindrical cavities whose resonating frequencies range between 15 and 20 GHz. It has been seen from experimental tests that a temperature variation of 25 °C with respect to the reference temperature "To" has resulted in a resonating frequency variation between 0,5 and 1 MHz. It is obvious that any geometrical shape whose volume decreases while temperature increases, for instance a spherical bowl, can be selected as a basis for compensating the volume variations of the body of the cylindrical cavity. It is also obvious that the principle of compensating volume variations, and consequently resonating frequencies, versus temperature variations can be used with any type of metallic cavity, for instance rectangular or elliptical cavities. From the description given so far, the advantages of the microwave metallic cavity object of the present invention are clear. In particular they result: from the fact whereby a metallic cavity has been achieved whose resonating frequency is stabilized versus operating temperature variations; from the fact whereby materials having high values of coefficient of linear expansion can be used for its implementation, for instance aluminium, which is specially suited for that equipment in which weight plays a very important role, for instance equipment to be installed on board of satellites, thanks to its reduced specific weight; from the fact whereby an improving factor of 10 is achieved in the stabilization of the resonating frequency with respect to the techniques known so far, the material used and the temperature variations been equal; from the fact whereby materials like brass, copper or aluminium are much cheaper than invar, which results in cost reduction; from the fact whereby such materials, being easy to machine with machine tools, result in a further reduction in the production costs. It is clear that many other modifications are possible to the described microwave metallic cavity object of the present invention by a skilled in the art without departing from the scope of the present invention.

Claims: Microwave metallic cavity comprising a first hollow body (1) defined by a cylindrical metallic wall having at a first end an integral first metallic wall (2), the second end (8) of the body being open, said first hollow body enclosing a first volume (V1); characterized by a second hollow body (3) defined by a concave second metallic wall, said second wall enclosing a second volume (V2); said second hollow body being rigidly connected to said second end of the first hollow body, to form a cavity whose volume is the sum of the first and second volumes; a metallic mobile sheet (6) placed within the first hollow body closing the cavity and allowing a tuning adjustment of its resonant frequency, said adjustment being produced by translations of said sheet inside the microwave cavity varying the volume of the cavity; whereby the wall thickness and the thermal coefficient of expansion (α) of said first hollow body are greater than the wall thickness and the thermal coefficient of expansion (β) respectively of said second hollow body, and whereby an increase in the cavity temperature produces a prevalent expansion of said first hollow body which mechanically deforms the second wall making it less concave, the first hollow body expansion and the second wall deformation respectively increasing said first volume and decreasing said second volume so as to keep the microwave cavity volume constant. Microwave metallic cavity according to claim 1, characterized in that the metallic mobile sheet (6) during said translations inside of the microwave metallic cavity, remains in contact with said cylindrical wall to close the cavity; an in that the metallic mobile sheet (6) has a thermal coefficient of expansion equal to the thermal coefficient of expansion (α) of said cylindrical wall so that the microwave metallic cavity is closed at any temperature. Microwave metallic cavity according to claims 1 or 2, characterized in that said first hollow body (1) is a cylindrical hollow body, said metallic mobile sheet (6) is a circular plate, and said second hollow body (3) is a conical hollow body whose vertex is faced to outside said first hollow body; the conical hollow body being given by the following relationship: where: "T o " is the cavity reference temperature, "T" is the cavity effective temperature, "h o " is the height of said conical hollow body (3) at the reference temperature, "h" is the height of the conical hollow body (3) at the effective temperature, "R o " is the radius of a circular lip of the conical hollow body (3), at the reference temperature, "R" is the radius of a circular lip of the conical hollow body (3) at the effective temperature, "β" is the thermal coefficient of expansion (β) of said second wall, "γo" is arctg h o /R o , and "γ" is arctg h/R. Microwave metallic cavity according to the claim 3, characterized in that said rigid connection between the first and second hollow body to form a unitary hollow body is obtained by inserting said lip of the conical hollow body (3) into a cylindrical groove (7) of the cylindrical wall and soldering the lip and the second wall together.

Inventors: GIAVARINI, ANDREA
Applicants: SIEMENS TELECOMUNICAZIONI S.P.A.
Application year: 1986
Main classification: H01P7
IPC codes: H01P7, H01P1