Dielectric-loaded cavity resonator

Abstract: The cavity resonator is composed of a closed metallic body transversally subdivided into two parts (CE,CS), housing a small dielectric cylinder (RC), which is held in coaxial position inside the cavity by two small quartz plates (RS1,RS2) provided with a centering indentation. Some holes are made in the lateral surface for tuning screws (T) and an access connector (CO) and coupling irises (IR) can be cut in the bases.

Description: The present invention concerns the devices for microwave telecommunications systems and more particularly a dielectric-loaded cavity resonator. In the telecommunications systems for civilian use the problem exists of implementing microwave filters allowing the various transmission channels to be allocated in the desired frequency bands. Usually these filters are implemented with a plurality of cavity resonators mutually-coupled through irises, screws or the like. When such filters are to be used in transponders installed on board a satellite, the resonator size has to be as small as possible. In fact, since some ten filters could be used and each filter is generally composed of 4 to 8 resonators, the encumbrance is considerable. Namely, at a center frequency of 12 GHz, a 6-pole filter implemented with dual-mode cylindrical cavities has, as a whole, a 30 mm diameter and a 60 mm length. A small dielectric cylinder has been recently introduced into each cavity resonator to reduce said filter sizes. This has been rendered possible by the availiability of high-permittivity, low-loss, high temperature-stability dielectric materials. The high permittivity of the material introduced into the resonator renders the electromagnetic field practically completely concentrated inside it, that is why the cavity dimensions, calculated to obtain the resonance at a determined wavelength, result highly reduced. Under the same conditions as those of the preceding example, the total dimensions of an equivalent filter with dielectric-loaded resonators decrease to about 20 mm for the diameter and 30 mm for the length, with an overall reduction to less than a fourth of the original volume. One of the problems encountered while implementing a dielectric-loaded resonator of this kind resides in the way of conveniently supporting the small dielectric cylinder placed inside the resonator. In fact dielectric material cannot completely fill up the metallic cavity both because of the high loss increase due to the contact between metal and dieletric and of the necessity of inserting tuning screws into the lateral resonator surface. Hence the requirement arises of providing a supporting structure for the dielectric material, which is capable of holding it in the correct position without detriment to its electrical characteristics, by keeping losses low, and of assuring the necessary mechanical stability of the structure, chiefly for use on board a satellite. The article entitled "Dielectric-Resonators Design Shrinks Satellite Filters and Resonators" by S. Jerry Fiedziuszko, issued in MSN & CT, August 1985, describes a cylindrical cavity resonator of the same type as those conventionally used in unloaded-filters, whereinto an ultra-low-loss ceramic material cylinder is introduced. The small dielectric cylinder is held in correct position by a plastic material disk or by a more complex support made of silicon foam. Yet this solution presents a number of inconveniences if the filter is to be used for processing signals even with moderate powers. In fact plastic material can tolerate moderate temperatures, usually lower than 100°, and silicon foam presents extremely-low thermal conductivity, that is why the heat produced in the dielectric cylinder is only partly dissipated. In addition, by using a single supporting disk, as it can be seen in Fig. 11 of the cited article, mechanical stability seems rather limited, unless adhesives are used between the disk and the small dielectric cylinder, which considerably increase losses. Other solutions providing the use of supporting disks made of different materials, such as alumina or forsterite, are not considered satisfactory by the author of the article above owing to their poor temperature stability. In document US-A-3 973 226 a filter for electromagnetic microwaves is described, comprising cavities which each contain a dielectric resonator ring disk supported by opposing insulating tubes one of which is erected on the bottom of the cavity and the other one is dependet from the lid which is a screw disk closing the cavity on top. This construction entails the disadvantages that the pressure exerted on the resonator disk can not be exactly determined and is accompanied by a torsion. These facts expose the dielectric resonator disk to the risk of breakings or chippings, particularly since the tubes do not allow for some deformation in case of an undue screwing extent. In document DE-B-1 195 828 a cavity waveguide for electromagnetic waves is disclosed. This waveguide uses an inner lining of dielectric material and coaxially supported therein a gyromagnetic rod which is held in place by circular radial plates. The drawbacks above are overcome by the dielectric-loaded cavity resonator provided by the present invention, which does not present particular limitations to operating temperatures and owns a considerable mechanical stability even without the use of adhesives, keeping thus a very high quality factor. The present invention provides a dielectric-loaded cavity resonator as stated in the prior art portion of claim 1, which is characterized in that said metallic body is subdivided transversally to the axis into said two parts, the first part presenting inside in its side wall face(s) a circumferential step by a slight increase of the cavity inner transverse size; that said dielectrical supports are formed as plates held inside the cavity and maintained at a convenient distance from the bottom thereof by the step in the side wall face(s) of the first part in a plane transversal to said axis, which step has a depth equal to the height of the group of the dielectric cylinder and the support plates; that the second part of the metallic body presents an inner transversal size slightly smaller than the one of said plates; and that the first and the second part of the metallic body are mutually fastened by a number of screws. In such cavity resonator, the pressure which keeps the dielectric cylinder in place does not depend on manual adjustments, but only on the accuracy of the manufacturing. The two plates holding the dielectric cylinder can easily face a slight flexion for compensating for production margins. As a consequence, the risk of breakings is minimized and the pressure can be predetermined in the fabrication phase. These features allow the filter to be used for spatial missions. As concerns the electric behaviour, the cavity parts present a good contact between each other, particularly if flanges are foreseen for mutual fastening. The extraneous material, necessary to the supports, is reduced to the minimum. This allows high Q factors to be obtained. Moreover, the resonator can be coupled to other similar resonators through the bases. This allows filters using the mode HE₁₁ to be built up. These filters reuse each cavity by exploiting two independent orthogonal polarizations and are particularly interesting for spatial use due to the reduced encumbrance. The foregoing and other characteristics of the present invention will be made clearer by the following description of a preferred way of embodiment thereof, given by way of non-limiting example, and by the annexed drawing in which: Fig. 1 is a longitudinal section of the resonator. Fig. 2 is a view from top of the same resonator as in Fig. 1. Fig. 3 is a partial longitudinal section of the resonator. The cavity resonator described in the following has a cylindrical shape and consists of a duly-shaped metallic part and of a pair of duly-shaped supporting plates for a dielectric cylinder, such as to form as a whole a mechanically-stable structure without the use of adhesives. In Fig. 1 RC denotes the cylinder made of dielectric material, i.e. of ceramics, by which the cavity resonator is loaded. It is held in a position coaxial with the cylindrical cavity by two small plates RS1 and RS2 shaped as disks, each with an axial hole, useful to reduce losses, and with a centering indentation apt to house one of the bases of the cylinder RC. The metallic body of the cylindrical resonator is subdivided transversally to the axis into two parts CE, CS, each with a flange for the mutual fastening by screws V. The part denoted by CE houses the group of dielectric elements formed by disks RS1, RS2 and by dielectric cylinder RC. This group is housed in part CE thanks to a slight increase of the inner cavity diameter and is kept at a suitable distance from the bottom by the step due to the diameter difference. The depth of the cavity portion with greater diameter is advantageously made equal to the height of the group of disks and dielectric cylinder. In this way it is enough to realize part CS with a diameter slightly inferior to that of the disks to tightly hold in place the group of dielectric elements. Apart from the coaxiality condition between the dielectric cylinder and the cylindrical cavity, there are no further constraints in the position of the cylinder itself along the cavity axis, provided there is enough space for the insertion of a coaxial access connector CO, equipped with a coupling probe SO. In the base of part CS there is cut a cruciform iris IR for the coupling with other possible resonators forming the filter. A similar iris can be also cut in the base of part CE whenever the resonator is used in an intermediate stage of the filter. Along the lateral surface of CE, in correspondence with the intermediate zone between the disks, threaded holes are made whereinto some screws T can be housed for the cavity tuning. Supporting disks RS1, RS2 are made of quartz. This material can offer consistent advantages with respect to the previously examined materials: extremely-low dielectric losses (tgδ=10⁻⁴ at 10 GHz); better thermal conductivity than that of foamy materials, namely silica foam and plastics; very high operating temperature. These characteristics make the cavity resonator, provided by the invention, present low losses and be particularly suited to handle high-power signals. That is due both to the fact that the amount of heat produced, proportional to losses, is low, and to the fact that the thermal conductivity of quartz, and hence the dissipation of heat produced, is among the best that can be obtained with dielectric materials. Machining of quartz disks does not present any particular problems, since it can be carried out by using normal diamond tools or by abrasive lapping. Fig. 2 shows a view from top of the same resonator as in Fig. 1. In this coupling irises IS and tuning screws T can be more clearly seen. Fig. 3 shows a partial section, wherein also part CS presents an increase of the inner diameter like that of part CE, so as to obtain a supporting step for the group of dielectric elements. A few drops of adhesive C, placed at regular intervals along the circumference between the two supporting bases and disks RS1 and RS2, ensure a good mechanical stability and a certain protection against vibrations. Quality factor reduction, due to the adhesive introduction, is limited since the electromagnetic field is mostly concentrated in the dielectric resonator and is minimum along the cavity walls. It is clear that what described has been given by way of non limiting example. Variations and modifications are possible without going out of the scope of the invention claims. E.g., the cavity could present a square instead of a circular section. In this case also RS1 and RS2 would have a square shape. Besides the axial hole of RS1 and RS2 could be left out to favour the dissipation of the heat produced in dielectric cylinder RC.

Claims: A dielectric-loaded cavity resonator, comprising a cavity defined by a housing (CE, CS) and extending along a cavity axis so as to have a bottom face, an upper end face and one or more side wall faces, said housing comprising a closed metallic body consisting of two parts (CE, CS) and containing a dielectric cylinder (RC) defined by a cylinder circumference face and two cylinder bases, said cylinder being held in place coaxial with the cavity by two dielectric supports (RS1, RS2), each provided with a centering indentation apt to house one of the bases of dielectric cylinder (RC), the group formed by the dielectric cylinder (RC) and the supports (RS1, RS2) being held in a fixed position inside the cavity, characterized in that said metallic body is subdivided transversally to the axis into said two parts (CE, CS), the first part (CE) presenting inside in its side wall face(s) a circumferential step by a slight increase of the cavity inner transverse size; that said dielectrical supports are formed as plates (RS1, RS2) held inside the cavity and maintained at a convenient distance from the bottom thereof by the step in the side wall face(s) of the first part (CE) in a plane transversal to said axis, which step has a depth equal to the height of the group of the dielectric cylinder (RC) and the support plates (RS1, RS2); that the second part (CS) of the metallic body presents an inner transversal size slightly smaller than the one of said plates (RS1, RS2); and that the first and the second part (CE, CS) of the metallic body are mutually fastened by a number of screws (V). A cavity resonator as in claim 1, characterized in that in the plates (RS1, RS2) an axial hole is each provided having a diameter smaller than that of the said indentation. A cavity resonator as in claim 1 or 2, characterized in that said second part (CS) also presents an inner circumferential step at a depth of a certain axial length by a step down of the transverse size to a dimensioning smaller than the transverse size of the plates (RS1, RS2), the transverse size of the cavity in the second part (CS) for the rest of the axial length being equal to the one of said first part (CE) near the joining plane between the two parts (CE, CS) of the metallic body, the plates (RS1, RS2) being held by adhesive means (C).

Inventors: ACCATINO LUCIANO, BERTIN GIORGIO, ACCATINO, LUCIANO, BERTIN, GIORGIO
Applicants: CSELT CENTRO STUDI LAB TELECOM, CSELT CENTRO STUDI E LABORATORI TELECOMUNICAZIONI S.P.A.
Application year: 1989
Main classification: H01P7
IPC codes: H01P7, H01P1, H01P11