COAXIAL RESONATOR STRUCTURE

Abstract: The object of the invention is a coaxial resonator structure. According to the invention, a smaller coaxial resonator structure is achieved when the structure comprises two or more coaxial resonators positioned coaxially one within another (1, 2). The resonator structure according to the invention can be used e.g. for providing a wave trap in a high frequency filter.

Description: Coaxial resonator structure The invention relates to a coaxial resonator structure. High frequency filters are generally realized as coaxial resonators, the frequency response of the filter being determined by the number, dimensions, and mutual couplings of the coaxial resonators. A general solution is to realize each coaxial resonator as a conductor rod positioned in an entirely separate metal housing. The coupling between the resonators is pro¬ vided, for example, by separate coil structures, such as a conductor wire extending from one resonator housing to another through coupling openings. Filter structures realized as resonators of this kind are usually complic¬ ated and large in size. A slightly smaller size can be achieved in a modification of this filter structure wherein the resonators are positioned in a common metal housing and separated from one another to separate com- partments by means of metallic intermediate walls. An even smaller size is made possible by a so-called Comb- Line filter wherein instead of separate metal housings or compartments separated by intermediate walls the resonators are positioned in the same space defined by the housing such that an open filter structure is formed wherein the resonators are coupled directly to each other without any separate coupling structures. In ceramic filters a coaxial resonator is usually a resonator hole provided in a ceramic body and coated with electrically conducting material. and The smallest size is achieved with a ceramic filter wherein all the resonator holes are provided successively in the same ceramic body. Yet another general coaxial resonator type is a helix resonator, which instead of a direct conductor or a conductor rod is formed by a conductor wound to provide a cylindrical coil, whereby a smaller size is achieved. Since the constant aim with various radio apparatuses is to decrease their size, increasingly greater requirements are set to the size of the filters used therein. The object of the present invention is to pro¬ vide a coaxial resonator which makes it possible to provide filter structures with a smaller size than before. This is achieved with a coaxial resonator, which according to the invention is characterised in that the coaxial resonator structure is formed by two or more coaxial resonators positioned coaxially one within another. The basic idea of the invention is to utilize the empty space conventionally found inside a coaxial resonator by positioning another resonator therein. By the invention either the size of the filter can be decreased or the number of the resonators in the filter can be increased by replacing at least one of the conventional resonators with a resonator structure according to the invention. A further object of the invention is to use a resonator structure according to the invention for pro¬ viding a wave trap. By means of the inner resonator of the resonator structure according to the invention a relatively narrowband band-stop circuit or wave trap for the desired frequency can be easily provided to at- tenuate e.g. the carrier frequency or the harmonics thereof in radio apparatuses. In the following the invention is described in greater detail by means of illustrating embodiments, with reference to the attached drawing, wherein Figure 1 shows a cross-sectional side view of a coaxial resonator structure positioned in a metal housing, comprising, two coaxial resonators one within the other, Figure 2 shows a cross-sectional side view of another coaxial resonator structure comprising three coaxial resonators one within another, Figure 3 shows a cross-sectional side view of a coaxial resonator structure provided in a ceramic body, comprising two coaxial resonators one within the other, Figure 4 depicts a frequency response of a band-pass filter provided with a wave trap. Reference is now made to Figure 1, wherein a coaxial resonator formed by a cylindrical conductor rod 1 preferably made of copper is positioned in a space 4 defined by a housing 3 made of electrically conducting material and closed on all sides thereof. The conductor rod 1 is secured and short-circuited at the lower end thereof to the bottom of the housing 3. The upper end of the conductor rod 1 is spaced from the cover of the housing 3. Further, a hollow tubular inner space 5 of the conductor rod opens, at the lower end of the con¬ ductor rod, to the outside of the housing through an opening on the bottom of the housing 3. Through coupling openings 9 and 10 on the bottom of the housing 3, coupling conductors 7 and 8 are introduced into the housing, the sections of the conductors that are inside the housing being bent to form loops connected to the bottom of the housing. The conductor loops 7 and 8 form coils that are coupled inductively to the resonator 1 on both sides thereof, thus forming the input and the output, IN and OUT, of the resonator. Inside the tubular conductor rod 1, and coaxially therewith, is positioned another coaxial resonator formed by the conductor 2, the lower end thereof extending to the outside of the housing 3 through the open lower end of the conductor rod 1. A coaxial resonator structure according to the invention with two coaxial resonators 1 and 2 one within the other has now been realized. By connecting the lower end of the inner resonator 2 to the coupling conductor 9 of the resonator 1, i.e. to the input or output, a narrowband band-stop circuit or wave trap for the frequency desired is' provided by the resonator 2 to attenuate e.g. the carrier frequency or the harmonics thereof in radio apparatuses. The resonance frequency is determined by the length of each resonator 1 and 2, and the Q value or the width of the resonance peak is determined by the diameter, whereby each resonator can be tuned independ- ently to different frequencies. Thus in spite of mechanic integration, the resonators 1 and 2 are elec¬ trically two separate mutually independent coaxial resonators. It is possible to have more than two coaxial resonators within one another. Figure 2 shows a cross- sectional side view of a coaxial resonator structure wherein a coaxial cable is positioned coaxially within the tubular conductor rod 1 forming the outermost co¬ axial resonator, the cylindrical outer conductor 2 of the cable forming a first inner resonator and a central conductor 12 surrounded by a dielectric 11 forming a second inner resonator._ Also, several resonators posi¬ tioned one within another can be tuned independently to different frequencies as described above. Figure 3 shows a cross-sectional side view of a coaxial resonator structure that can be used in connection with ceramic filters. In Figure 3 the outer¬ most coaxial resonator is formed by an electrically conducting coating 31 of a resonator hole 35 provided in a body 34 made of dielectric material, the coating joining, at the lower end of the hole 35, to an electrically conducting coating 33 covering essentially the entire dielectric body 34. The coating forming the resonator is separated from the coating of the body 34 at the upper end of the hole 35. A conductor 32 forming a second coaxial resonator is positioned coaxially in the hole 35. Alternatively, the inner resonator may be e.g. a helix resonator. One or more of the resonators of the filter can be replaced with the coaxial resonator according to the invention. The resonator structure according to the invention is particularly advantageous when a narrowband band-stop filter or wave trap is to be introduced into the filter in order to attenuate e.g. the carrier frequency or the harmonics thereof in radio apparatuses. Figure 4 depicts a frequency response of a band-pass filter in which additional attenuation for the frequency f τ is provided by a narrowband wave trap. Such a wave trap can be provided by an inner resonator / resonators of a coaxial resonator structure according to the inven¬ tion, e.g. by means of the circuit illustrated in Figure 1, without increasing the size of the filter. The figures and the specification relating thereto are intended only to illustrate the invention. The details of the coaxial resonator structure according to the invention may vary within the limits defined by the attached claims.

Claims: Claims 1. A coaxial resonator structure, c h a r a c ¬ t e r i s e d in that the coaxial resonator structure comprises two or more coaxial resonators coaxially one within another (1,2,12). 2. A coaxial resonator structure according to claim 1, c h a r a c t e r i s e d in that a conductor (2) forming a second coaxial resonator is coaxially within a cylindrical conductor (1) forming the outermost coaxial resonator. 3. A coaxial resonator structure according to claim 1, c h a r a c t e r i s e d in that the outer¬ most coaxial resonator is formed by a tubular metal rod (1) positioned within a housing (3) made of electrically conducting material and grounded to the housing (3) at one end thereof. 4. A coaxial resonator structure according to claim 1, c h a r a c t e r i s e d in that the outer- most coaxial resonator is formed by an electrically conducting coating (31) of a hole (35) in a dielectric block (34), the coating joining at the other end of the hole to an electrically conducting coating (33) cover¬ ing essentially the entire dielectric block (34). 5. A coaxial resonator structure according to claim 1, 2, 3 or 4, c h a r a c t e r i s e d in that the conductor forming a second coaxial resonator is a tubular conductor (2), a conductor (12) forming a third coaxial resonator being positioned coaxially there- within. 6. A coaxial resonator structure according to claim 5, c h a r a c t e r i s e d in that the second coaxial resonator and the third coaxial resonator are formed by an outer conductor (2) and a central conductor (12), respectively, of the coaxial cable positioned within the outermost coaxial resonator (1). 7. A coaxial resonator structure according to claim 1, c h a r a c t e r i s e d in that the inner coaxial resonator is a helix resonator. 8. A coaxial resonator structure according to any one of the preceding claims, c h a r a c t e r ¬ i s e d in that one end of the conductor (2) forming the second coaxial resonator is coupled to the output or input of the outermost coaxial resonator ( 1 ) . 9. A coaxial resonator structure according to any one of the preceding claims, c h a r a c t e r ¬ i s e d in that the inner coaxial resonator (2) forms a wave trap. 10. Use of a coaxial resonator structure according to claim 1 for forming a wave trap in a high frequency filter.

Inventors: PIIRAINEN RISTO, PIIRAINEN, RISTO
Applicants: TELENOKIA OY
Application year: 1992
Main classification: H01P7
IPC codes: H01P7