COAXIAL CAVITY RESONATOR

Abstract: The present invention relates to a modified coaxial cavity resonator and a filter utilising such resonators. The cavity resonator is provided with a cavity wall (203, 204, 205), which includes on its inside at least one conductive body (201, 301, 401), which body forms an open-circuit at one end and is longitudinally, substantially shorter than a normal quarter-wave resonator. The conductive body (201, 301, 401) includes a main conductive rod (206, 405), which is in one end (209, 407) attached to the cavity wall, and at least one additional conductive protruding means (207, 302, 402, 403), such as a rod, substantially perpendicularly attached to the free end (208, 406) of the main rod (206, 405). The cross-sectional diameter of the additional rod is 0,5 to 2 times the cross-sectional diameter of the main rod (206, 405), and preferably approximately the same as the cross-sectional diameter of the main rod. By attaching the additional rod (207, 302, 402, 403) the total physical length of the conductive body is not substantially shortened, but rather redirected in another direction.

Description: Coaxial cavity resonator Technical field The present invention relates to a coaxial cavity resonator of the kind defined in the preamble of claim 1, such resonators being particularly suitable as a structural part of a filter in radio devices. The invention also relates to a filter comprising at least two resonators, as defined by the preamble of claim 13. Backgroυnd to the invention Resonators are used as the main structural part in the manufacture of oscillators and filters. The important characteristics of resonators include, for example Q-value, size, tunability, mechanical stability, temperature and humidity stability and manufacturing costs. The resonator constructions that are known so far include the following: 1) Resonators compiled of discrete components such as capacitors and inductors Resonators of this kind entail the drawback of internal dissipation of the components and therefore clearly lower Q- values compared to the other types. 2) Microstrip resonators A microstrip resonator is formed in the conductor areas on the surface of a circuit board, for example. The drawback is radiation dissipation caused by the open construction and thus relatively low Q-values. 3) Transmission line resonators In a transmission line resonator, an oscillator is constituted by a certain length of a transmission line of a suitable type. When a twin cable or coaxial cable is used, the drawback is relatively high dissipation and a relatively poor stability. When a tubular wave guide is used, stability can be improved, but the dissipation is still relatively high because of radiation when the end of the tube is open. The construction can also be unpractically large. A closed, relatively short wave guide resonator can be regarded as a cavity resonator, which is dealt with below. 4) Coaxial cavity resonators Resonators of this type have a construction which is not merely a piece of coaxial cable but a unit which was originally intended as a resonator. It includes, among other things, an inner conductor and an outer conductor, which are air-insulated from each other, and a conductive cover, which is connected with the outer conductor. A relatively good result can be achieved by this construction. The length of the resonator is at least in the order of one fourthof the wavelength, λ/4, of the variable field effective in it, which is a drawback when aiming at minimising the size. The width can be reduced by reducing the sides of the outer conductor and the diameter of the inner conductors. However, this leads to an increase of resistive dissipation. In addition, because of the reduction in the thickness of the construction, it may be necessary to support the inner conductor by a piece made of a dielectric material, which causes considerable extra dissipation in the form of dielectric loss and increases the manufacturing costs. In US 4,216,448 by Kasuga et al, see Fig. la and lb, there is described a coaxial cavity resonator, where at least one rod has a projection at the open end to capacitively couple a signal from said rod to another rod. The projection is thin compared to the diameter of the rod, less than 10% of the diameter of the rod, taken from the drawings. No dimensions are mentioned in the text for this projection, but since the purpose of the projection is to capacitively couple the signal between rods without changing the overall performance of the filter, the dimensions of the projections must be kept small. The resonator rod may be reduced in length since the electrical length of each resonator rod may be varied by attaching the projection. Longer protrusions will make the capacitive coupling to the cavity side walls and the other resonator rod stronger and hence reduce the length of the resonator rod. The drawback with the disclosed resonator is that the Q-value of the resonator will substantially decrease with increasing length of the protrusions, should one try to reduce the cavity volume for a predetermined frequency. 5) Helix-resonators This type is a modification of a coaxial cavity resonator, in which the cylindrical inner conductor is replaced by a helical conductor. Thus the size of the resonator is reduced, but the clearly increased dissipation is a drawback. Dissipation is due to the generally small wire diameter of the inner conductor. 6) Hat resonators A subclass of coaxial cavity resonators, here called hat resonators, are described in US Patent No 4,292,610 by Makimoto. This type of resonators is a coaxial cavity resonator, as described above, with an additional disc on the open end of the wave guide, having a larger diameter than the wave guide. An advantage is that the resonator can be made compact. Relatively low dissipation can be achieved. The surface of the disc and the distances to the walls of the resonator are dimensioned so that, due to an extra capacitance created between the disc and the cavity, the resonator can be made substantially smaller compared to one without the additional disc. 7) Dielectric resonators Coaxial cables or a closed conducting surface is formed on the surface of a dielectric body. The advantage is that the construction can be made in a small size. Relatively low dissipation can also be achieved. On the other hand, dielectric resonators have the drawback of relatively high manufacturing costs . 8) Cavity resonators Resonators of this type comprises a hollow structure made of a conductive material, in which electromagnetic oscillation can be excited in an interior cavity. The resonator can be boxlike, cylindrical or spherical in shape. Very low dissipation can be achieved with cavity resonators. However, their size is a drawback when the aim is to minimise the size of the construction. In addition, the tunability of most cavity resonators is poor. Summary of the invention A problem with the prior art coaxial cavity resonator is that, although projections are provided, the volume of the cavity containing the conductive body may not be changed without a significant loss of Q-value for the resonator for a predetermined resonance frequency. The object of the invention is to provide a modified coaxial cavity resonator that will have the same or increased Q-value when having a substantially reduced cavity volume for a predetermined frequency. A cavity resonator according to the invention has the features set forth in the independent claims. Some preferred embodiments of the invention are set forth in the dependent claims . The basic idea of the invention is the following: The cavity resonator is provided with a cavity wall, which include on its inside at least one conductive body, which body forms an open- circuit at one end and is longitudinally, substantially shorter than a normal quarter-wave resonator. The conductive body includes a main conductive rod, which is in one end attached to the cavity wall, and at least one additional conductive protruding means, such as a rod, substantially perpendicularly attached to the free end of the main rod. The cross-sectional diameter of the additional rod is between 0.5 and 2 times of the cross-sectional diameter of said main rod, and preferably approximately the same as the cross-sectional diameter of the main rod. By attaching the additional rod the total physical length of the conductive body is not substantially shortened, but rather redirected in another direction. The longitudinally shortening is carried out by creating air- insulated extra capacitance by means of a mechanical structure at the open end of the conductive body between the resonator cavity walls and the additional rod. The invention has the advantage that, thanks to increasing the capacitance and redirecting the length of the conductive body, the resonator can be made substantially smaller than a prior art quarter-wave resonator, which has the same Q-value. The improvement achieved can also be used partly for saving space and partly for maintaining a high Q-value compared to the Q- value for a resonator with a single top capacitance, such as a tuning screw. The present invention also causes the electric and magnetic fields to be evenly distributed in the open end of the conductive body. Furthermore, a smaller resonator according to the present invention has the advantage to allow the volume of the cavity to be substantially smaller for a specific frequency, compared to prior art solutions. In addition, the invention has the advantage that when the resonator is shortened, it becomes mechanically stronger and therefore also more stable with regard to its electrical properties. Support pieces that increase the dissipation are not needed in it, either. Furthermore, the invention has the advantage that the manufacturing costs of the resonator are relatively small. In the following, the invention will be described in more detail with reference to the accompanying drawings. Description of drawings Fig. la shows a side view of a coaxial cavity resonator according to prior art. Fig. lb shows a top view of the prior art resonator in Fig. la. Fig. 2a shows a perspective view of an embodiment of the present invention comprising one additional rod. Fig. 2b shows a top view of the embodiment in Fig. 2a. Fig. 2c shows a cavity of the embodiment in Fig. 2b. Fig. 3 shows an alternative embodiment according to the present invention comprising an asymmetric attachment of an additional rod. Fig. 4a shows an perspective view of an embodiment of the present invention comprising two additional rods. Fig. 4b shows a top view of the embodiment in Fig. 4a. Fig. 5a shows a side view of a filter comprising three resonators according to the invention. Fig. 5b shows a top view of the filter in Fig. 5a. Fig. 6 shows a top view of an alternative filter comprising three resonators according to the invention. Preferred embodiments Fig. 1 shows a coaxial cavity resonator filter 100, according to prior art, having two cavities 101a, 101b, each having a conductive body 102a, 102b. Each cavity 101a, 101b having conductive walls comprising, in turn, side walls 103, a top wall 104 and a bottom wall 105. Each conductive body 102a, 102b comprising a conductive rod 106. An end 107 of the rod 106 is in short-circuit connection with the bottom wall 105 of each cavity 101a, 101b. An opposite end 108 of each rod 106 is in open-circuit relation with the top wall 104 of each cavity 101a, 101b. A protrusion 109a, 109b is perpendicularly attached to each rod 106 adjacent to the open end 108, mainly to capacitively couple the signal from the first conductive body 102a to the second conductive body 102b, through an opening 110 in the cavity said wall between the cavities 101a, 101b. The first protrusion 109a is directed from the rod 106 of the first conductive body 102a, towards said opening 110, and the second protrusion 109b is directed from the rod 106 of the second conductive body 102b, towards said opening 110, thereby creating a capacitive coupling between the first and the second conductive body. This is clearly shown from the top view of the filter in Fig. lb. The length of a conductive rod is normally approximately a fourth of the wavelength, λ/4, of the variable field effective in it, which is a drawback when aiming at minimising the size, as previously discussed. The length L of the conductive bodies 102a, 102b is slightly reduced, but still approximately λ/4, i.e. L«λ/4, since there is also, apart from the capacitive coupling between the conductive bodies 102a and 102b, a capactive coupling between the conductive bodies 102a, 102b and the cavity side wall 103. If the resonator filter 100 in question is made for the 900 Mhz frequency, for example, the length L of the conductive rod is approximately 75 mm. The diameter D of each cavity 101a, 101b and the diameter d of each rod 106 can be selected according to the amount of dissipation permitted. However, there is an optimum value for the ratio D/d, about 3, which maximises the Q-value, if the wave form is TEM. The drawback of this prior art filter is that although the length of the conductive bodies may be slightly reduced due to the capacitive coupling between the protrusion and the cavity wall, longer protrusions will cause a decrease in Q-value which is not desirable. This is caused by a high resistance value in the protrusions, due to the small dimensions, which in turn is not capable of handling a high current and thus the Q-value is reduced. Fig. 2a and 2b shows a preferred embodiment of a cavity resonator 200 according to the present invention, comprising a conductive body 201 located in a cavity 202. The cavity 202 having conductive walls comprising, in turn, side walls 203, a top wall 204 and a bottom wall 205. The conductive body 201 comprises a conductive main rod 206 and an additional conductive rod 207 perpendicularly attached to said main rod 206. An end 208 of said main rod 206 is attached substantially in the middle of said additional rod 207. An opposing end 209 of said main rod 206 is in short- circuit relation with the bottom wall 205 of said cavity 202. Thus giving the conductive body a T-shape. The additional rod 207, including the open ends 210 of said additional rod, is in open-circuit relation to the top wall 204 of the cavity 202. The open ends 210 are arranged to be positioned at an essentially equal distance "a" from the side walls 203 and thus create capacitive couplings between the additional rod 207 and the side walls 203. The short-circuit end 209 of the main rod is attached substantially in the centre of the bottom wall. Fig. 2c illustrates the alignment of the additional conductive rod 207. The additional rod 207 is aligned on the diagonal of the square formed cavity, comprising the maximum diameter C ma χ of the cavity, to increase the capacitive coupling between the conductive body 201 and the cavity side walls 203. This is clearly in contrast to the prior art resonators as described in Fig. la and lb, where the protrusions are aligned between the sides of the cavity. A such alignment is illustrated in Fig. 2c as a minimum diameter C m i n of the cavity. The dimensions of the additional rod is described in more detail below. The length 1 of the inventive conductive body 201, is up to half the length L of one of the prior art conductive bodies 102a in Fig. la, i.e. l«λ/4, due to the extra capacitances created by the inventive conductive body, for the same resonance frequency and with maintained Q-value of the resonator. The inventive resonator show an increased Q-value of approximately 40 to 60%, or even higher, compared to the prior art resonator in Fig. la and lb, when the cavity volume and the resonance frequency is constant. This is due to the increased diameter of the additional rod 207 compared to the projections 109a, 109b, since an increased diameter results in a lower resistance value in the additional rod. A lower resistance value will in turn mean that the additional rod can handle a higher current and thus increase the Q-value of the resonator. The volume of the resonator, for a specific resonance frequency, may be reduced if the prior art conductive body 102a, 102b is replaced with an inventive conductive body 201, with a maintained, or higher, Q-value, which may be desired in applications demanding a small size of the resonator. The additional rod may be attached asymmetrically to the main rod, as is shown in Fig. 3. Fig. 3 shows an embodiment of a cavity resonator 300 with a first alternative conductive body 301 comprising an additional rod 302 located in the cavity 202. The additional rod is perpendicularly attached to the main rod 206. A first end 304 of said additional rod 302 is positioned a first distance "a", which is closer to the side walls 203 than a second distance "b" between a second end 303 of said additional rod 302 and said side walls 203. Thus giving the conductive body 301 an asymmetric T-shape, with different strong capacitive coupling at each end to said side walls 203. Furthermore, the second end 304 may also be used for coupling a signal capacitively out of the resonator through an opening 305 in the side walls 203. A separate element for coupling is thus not needed. Fig. 4 shows another embodiment of a cavity resonator 400, according to the present invention, with a conductive body 401 comprising a rectangular main rod 405, a first 402 and a second 403 rectangular additional rod located in the cavity 202. Said main rod and said additional rods have essentially an equal cross-sectional diagonal length. Said first 402 and second 403 additional rod being perpendicularly attached, in the middle, to each other, thus creating a X-shape. The additional rods 402 and 403 being perpendicularly attached to the first end 406 of the main rod 405. The second end 407 of the main rod 405 being in short- circuit connection with the bottom wall 205, and the additional rods, including the open ends 404 of said additional rods, being in open-circuit relation with the side walls 203 and the top wall 204. The open ends 404 are arranged to be positioned at equal distance c from the side walls 203 and thus create capacitive couplings between the additional rods 402 and 403 and the side walls 203. The short-circuit end 406 of the main rod is attached substantially in the centre of the bottom wall 205. As previously discussed in connection to Fig. 3a and 3b asymmetric combinations can be made using two or more additional rods. Combinations of rods with different cross- sectional shape, such as rectangular, circular or elliptic, are of course possible. This embodiment of the present invention is a more space saving embodiment than the embodiment in Fig. 2, due to a shorter main rod is needed to keep the resonant frequency for the resonator. The Q-value of the resonator can be maintained and will allow strong couplings to four directions . Fig. 5a and Fig. 5b shows a filter 500 comprising an input means 501, an output means 502, an outer conductor 503 and three resonators according to the embodiment in Fig. 2. Each resonator comprises walls, creating a cavity 504a, 504b, 504c and a conductive body 505, which, in turn, comprises a main rod 506 and an additional rod 507a, 597b, 507c. A signal is connected to a first resonator via said input means 501. The signal is coupled to a second resonator via capacitive couplings between an open end 508 of said additional rod 507a of the first resonator and the additional rod 507b of said second resonator. The open end 508 is arranged adjacent to a first opening 509 between the first 504a and second 504b adjacent cavity, thus creating a path for said capacitive coupling through said first opening 509. The signal is coupled to a third resonator via capacitive couplings between an open end 510 of said additional rod 507b of the second resonator and the additional rod 507c of said third resonator. The open end 510 is arranged adjacent to a second opening 511 between the second 504b and third 504c adjacent cavity, thus creating a path for said capacitive coupling through said second opening 511. The signal is then connected to the output means 502. Fig. 6 shows a top view of an alternative embodiment of a filter 600, having a strong capacitive coupling between the resonators in the filter. An additional rod 601 of a second resonator is rotated 90 degrees compared to the embodiment in Fig. 5b. A signal is connected to a first resonator via an input means. The signal is coupled from the first resonator to a second resonator via a capacitive coupling between an open end 602 of an additional rod 603 of the first resonator and a first open end 604 of said additional rod 601 of said second resonator. The first resonators open end 602 is arranged adjacent to a first opening 605 between a first 606 and second 607 adjacent cavity, thus creating a path for said capacitive coupling through said first opening 605. The signal is coupled to a third resonator via capacitive couplings between a second open end 608, opposite said first open end 604, of said additional rod 601 of the second resonator and an open end 609 of an additional rod 610 of said third resonator. The second open end 609 of said second resonator is arranged adjacent to a second opening 611 between the second 607 and a third 612 adjacent cavity, thus creating a path for said capacitive coupling through said second opening 611. The signal is then connected to an output means. The capacitive couplings described in connection to Fig. 5a, 5b and 6 are indicated in the drawings. In the description, a rod indicates any kind of protruding means . The drawings only show embodiments with circular and rectangular cross-sections, but the scope of the claims should not be limited to this. Other shapes of the rods are possible, such as tubular cross-section or any protruding means with varying cross-sectional area. The cross-sectional diameter of a rod, independently of the shape, should always be the longest diagonal distance of the cross-section. The diameter of the additional rod(s) has/have to be larger than 0.5 times, but less than 2 times, the diameter of the main rod for maintaining a proper function of the resonator. The upper limit is set due to undesired effects that arise when the diameter of the additional rod(s) is too big. These effects, including higher capacitive coupling to the top wall and the corners of the side walls, will lower the resonating frequency, which, in turn, reduces the Q-value of the resonator . The lower limit is set due to the undesired effect of increased resistance value of the protrusions, when the diameter is too small. This will cause a reduction in Q-value, since the protrusions cannot handle the desired amount of current . The additional rod(s) is/are not used for tuning the resonator, but instead to redirect the physical length of the conductive body, if the diameter of the additional rod(s) are too small the height of the cavity will not be reduced as much as wanted. A preferred diameter of the additional rod(s) is in the range ±20 percent of the diameter of the main rod. Sharp edges on the conductive body may degrade the performance of the resonator, i.e. the Q-value. As a general fact, sharp edges are always worse than circular shape in high frequencies . The degradation of the performance would of course depend upon the relative length of the additional bars and the frequency of the resonator - the higher the frequency is the worse it will be. Other cross-sectional shapes of the cavity are of course possible, such as elliptic, circular and rectangular. In each of these cavities the best performance of the inventive resonator is achieved when the additional rod(s) is/are aligned within the cavity along the maximum diameter, e.g. corner to corner of a square cavity. Regardless of the cross-sectional shape of the cavity there is always at least one internal distance having a maximum diameter and at least one internal distance having a minimum diameter. For a circular cavity the maximum and the minimum diameter are the same. Some ways of implementing a resonator according to the invention were described above. The invention is not restricted to the solutions described only, but the same inventive idea can also be applied in other ways without departing from the scope defined by the attached claims.

Claims: Claims 1. A coaxial cavity resonator comprising: a cavity (202) walls delimiting said cavity and forming side walls (203) , a top wall (204) and a bottom wall (205) opposing said top wall, at least one conductive body (201, 301, 401), comprising a main conductive rod (206, 405) , provided in said cavity (202), - said rod having first (209, 407) and second (208, 406) ends and a substantially constant cross-sectional diameter, a first end of said conductive body, formed by said first end (209, 407) of said main conductive rod (206, 405), being in short-circuit connection with said bottom wall (205) of said cavity, and a second end of said conductive body being in open-circuit relation with said top wall (204) of said cavity, c h a r a c t e r i s e d b y said conductive body (201, 301, 401) further comprising at least one additional conductive rod (207, 302, 402, 403), attached in a perpendicular relation to said second end (208, 406) of said main conductive rod, said additional conductive rod (207, 302, 402, 403) forming said second end of said conductive body, and - said at least one additional rod having a cross-sectional diameter being between 0.5 and 2 times said diameter of said main conductive rod (206, 405) . 2. Coaxial cavity resonator according to claim 1, wherein said cavity has, at least in a proximity of said additional conductive rod, a non-circular cross section with a first maximum diameter (Cm ax ) and a first minimum diameter (C m ι n ) , wherein said first maximum diameter (C max ) is larger than said first minimum diameter (C m i n ) , and said at least one additional conductive rod (207, 302, 402, 403) is essentially aligned with said first maximum diameter (C max ) • 3. Coaxial cavity resonator according to claim 2, wherein a length of said additional rod (207, 302, 402, 403) is greater than said first minimum diameter (C m in) • 4. Coaxial cavity resonator according to any of claims 2 or 3, wherein said non-circular cross section is a square cross section. 5. Coaxial cavity resonator according to any one of claims 1-4, wherein each additional conductive rod (207, 302, 402, 403) has two parts extending in opposite directions from said second end of said main rod. 6. Coaxial cavity resonator according to claim 5, wherein said extending parts have substantially equal length. 7. Coaxial cavity resonator according to any one of claims 5-6, wherein said open end of each of said extending parts have a substantially equal distance (a) to said side walls. 8. Coaxial cavity resonator according to claim 1, wherein said conductive body (401) further comprises a first (402) and a second (403) additional conductive rod being mutually perpendicularly attached to each other. 9. Coaxial cavity resonator according to claim 8, wherein said extending parts have substantially equal length, and said open end of each of said extending parts have a substantially equal distance (c) to said side walls (203) . 10. Coaxial cavity resonator according to any of the preceding claims, said top wall (204) is parallel with said bottom wall (205) and said side walls (203) are perpendicular to said bottom wall (205), wherein said first end (209, 407) of conductive body (201, 301, 401) is substantially centrally attached to said bottom wall (205) of said cavity. 11. Coaxial cavity resonator according to any of the preceding claims, wherein at least one of said additional conductive rods (207, 302, 402, 403) is also a capacitive coupling element for external coupling of the resonator. 12. Coaxial cavity resonator according to any of the preceding claims, wherein said at least one additional rod (207, 302, 402, 403) having a cross-sectional diameter being approximately the same as of said diameter of said main conductive rod (206, 405) . 13. A filter comprising input means (501) and output means (502) , c h a r a c t e r i s e d b y said filter further comprising at least two coaxial cavity resonators (200, 300, 400) according to any of the preceding claims, provided with at least one opening (305, 509, 511, 605, 611) , in the neighbourhood of said capacitive coupling element, in at least one side wall between two adjacent cavities, creating a path for said coupling from one resonator to another resonator.

Inventors: KANERVO ANTTI, RAETY TUOMO, KANERVO, ANTTI, RAETY, TUOMO
Applicants: ALLGON AB, KANERVO ANTTI, RAETY TUOMO, KANERVO, ANTTI, RAETY, TUOMO
Application year: 1999
Main classification: H01P7
IPC codes: H01P1, H01P7