HIGH FREQUENCY COMB-LINE FILTER

Abstract: The invention relates to a comb-line high-frequency filter or the like, comprising an elongated casing (1) of an electrically conductive material, conductor rods of an electrically conductive material being arranged in a sequence at predetermined intervals in a substantially integral air cavity within the casing, one end of each conductor rod being short-circuited to the casing and its other end being detached therefrom so that each conductor rod forms with the casing (1) an air-insulated coaxial resonator. In each coaxial resonator, the ratio between the diameters (X, Y) of the casing (1) and the resonator (3) is within the range from about 2.8 to 3.3.

Description: High frequency comb-line filter. The invention relates to a comb-line high-fre¬ quency filter or the like, comprising an elongated casing made of an electrically conductive material, conductor rods of an electrically conductive material being arranged in a sequence at predetermined inter¬ vals in a substantially integral air cavity within the casing, one end of each conductor rod being short-circuited to the casing and its other end being detached therefrom so that each conductor rod forms with the casing an air-insulated coaxial resonator. In comb-line filters the resonators are typically positioned in a sequence one after another so that the inductive and capacitive couplings be¬ tween the resonators form the couplings between the resonators directly without any separate coupling elements. Therefore such comb-line filter structures are usually simpler and smaller in structure than conventional filter structures accomplished by separate resonators coupled together e.g. by separate coils. Comb-line filters can be realized by coaxial resonators, in which case air is used as a medium between the resonators and the electrically conductive casing surrounding the resonators. FI Patent Application 906215 discloses a comb- line filter in which the medium is air, that is, the filter is air-insulated. In this filter, the resonators consist of electrically conductive conductor rods enclosed in an integral space defined by a single electrically conductive casing. The casing is common to the resonators, forming a coaxial resonator with each resonator. Today very stringent requirements are set on the electrical properties of high-frequency filters while increasingly smaller sizes and lower costs of manufacture are also required. In filters accom¬ plished by coaxial resonators, for instance, one aims at an optimal Q factor and the smallest possible los- ses by using a theoretical optimum value of 3.6, cal¬ culated as the ratio between the diameters of the casing and the resonator of the coaxial resonator, and by manufacturing the casing and the resonator of materials having as low losses as possible (materials of high electrical conductivity), such as a copper casing and resonators coated with silver (cf. "Micro¬ wave Filters, Impedance Matching Networks, and Coupling Structures", G. Matthaei, L. Young, E.M.T. Jones, Artech House Books, Dedham, MA USA, p. 165- 168). However, such materials are relatively expensive, which increases the cost of manufacture for these filters. So one constantly has to com¬ promise between the size, properties and cost of manufacture of the filter. The object of the invention is to decrease the size and/or cost of an air-insulated comb-line filter realized with coaxial resonators without impairing the electrical properties. This is achieved by means of a comb-line high- frequency filter of the type described in the intro¬ duction, which according to the invention is charac¬ terized in that in each coaxial resonator, the ratio between the diameters of the casing and the conductor rod is within the range about 2.8 to about 3.3. The invention is based on the inventor's surprising discovery that the theoretical calculatory ratio of 3.6 between the diameters of the casing and the resonator of the coaxial resonator, which has generally been regarded as an optimum value in comb- line filters, does not, in fact, provide the best possible Q factor for the filter, but considerably better Q factors are achieved with values smaller than this theoretical ratio, ranging approximately from 2.8 to 3.3. This is obviously due to the mutual interference between the resonators in a comb-line type filter. By using a ratio in the range according to the invention, at least the following advantages are achieved: - The electrical properties of air-insulated filters previously realized with the ratio of 3.6 can be improved without increasing the size of the filter or without using better materials. - The electrical properties of air-insulated filters previously realized with the ratio of 3.6 can be achieved by ' using inferior and less expensive materials. - The electrical properties of air-insulated filters previously realized with the ratio of 3.6 can be achieved with a smaller filter without using better materials. In other words, the properties, price and size of an air-insulated filter can be influenced in a very simple way by means of the invention. The invention will now be described in more detail by means of an illustrating embodiment with reference to the drawing, in which Figure 1 is a diagrammatic sectional view of the mechanical structure of a bandpass filter accord¬ ing to the invention; and Figure 2 is a sectional top view of the band¬ pass filter shown in Figure 1, taken along the line A-A shown in Figure 1. Referring now to Figures 1 and 2, the high-fre¬ quency bandpass filter comprises a rectangular, elongated casing closed on all sides and comprising end plates 2A and 2B, a top plate 2C, a bottom plate 2D and side plates.2E and 2F. The casing may be made of an electrically conductive material and/or coated with a conductive material, such as copper or yellow chrome. The plates 2A-2F forming the casing define therebetween an integral cavity 9 extending substan¬ tially over the entire length of the casing. Six cyl¬ indrical conductor rods 3 are positioned in this cavity 9 in succession at predetermined intervals, each conductor rod being secured and short-circuited at its lower end to the bottom plate 2D of the casing and spaced at its upper end apart from the top plate 2C of the casing, so that the conductor rod 3 forms a coaxial resonator with the casing, in which the rod 3 is the inner conductor or resonator, and the casing is the outer conductor or shell, air being used as a medium. The resonator rods 3 are made of an electrically conductive material or coated with an electrically conductive material, such as copper or silver. In this way, an open comb-line type air- insulated filter structure is achieved, in which the couplings between the resonators are formed directly by the inductive and/or capacitive couplings between the resonator rods 3, as illustrated by capacitor C^- j _ and coil M ^_. In the preferred embodiment shown in Figures 1 and 2, each resonator rod 3 is secured at its lower end to the bottom plate 2D of the casing and further comprises a cylindrical knob portion 4 of a larger diameter attached to its upper end, the knob portion being preferably made of copper. The structure of a resonator rod of this kind, comprising a knob portion 4, is described in FI Patent Application 906251. In such a structure, the capacitive coupling or the inductive coupling can be made the predominant cou¬ pling type in the coupling between two adjacent co¬ axial resonators by adjusting the ratio of the dis¬ tance dl between the resonator rods of the respective coaxial resonators and the distance d2 between the knobs 4. By varying the type of coupling, the filter responses can be varied. The resonators 3 may, however, also be realized in a conventional way without the knob 4. A metal tuning screw 5 is provided in the top plate 2C of the casing 1, extending into the inner cavity 9 of the casing above the resonator rod 3. The distance of the lower end of the tuning screw 5 from the upper end of the resonator rod 3 determines the value of the earth capacitance Cl between the casing and the resonator rod 3, the earth capacitance being illustrated by capacitor Cl drawn in broken lines. The earth capacitance and thereby the resonance fre¬ quency of an individual resonator can be adjusted by means of the tuning screw 5. In the top plate 2C of the casing 1, in the area between two adjacent con¬ ductor rods, there is further provided a metal tuning screw 6 extending into the casing. This tuning screw enables the fine adjustment of the capacitance bet- ween the knobs 4 of two adjacent conductor rods and thereby the fine adjustment of the coupling between adjacent resonators. In the embodiment of Figure 1, the filter input is formed by a conductor loop 7 introduced into the casing 1 through an inlet 10 pro- vided in the bottom plate 2D, the end of the conductor loop within the casing being connected to the bottom plate 2D. The conductor loop 7 is posi¬ tioned in a space between one end plate 2A of the casing and the resonator rod 3 closest to it. The filter output is correspondingly formed by a conductor loop 8 introduced into a space between the opposite end plate 2B and the resonator rod closest to it through an inlet 11, one end of the conductor loop 8 being connected to the bottom plate 2D. The conductor loops 7 and 8 form coils which are induc¬ tively connected to the closest resonator rod 3. In accordance with the invention, in each co¬ axial resonator of the filter, the ratio between the diameter X of the outer conductor or casing and the diameter Y of the resonator is approximately within the range 2.8 to 3.3. In the rectangular casing of a comb-line filter or the like, the diameter X repres¬ ents the dimension of the casing in the direction of its width at the resonator 3, as shown in Figure 2 (the figure is not drawn to scale). The advantages obtained with the invention are illustrated by the following example. A filter of the type shown in Figures 1 and 2 was first realized with a conventional X/Y ratio of 3.6 (X=36 mm and Y=10 mm), in which case it was necessary to use an aluminium casing coated with copper and copper rod resonators coated with silver to obtain the required electrical properties. The same filter was then realized with the X/Y ratio according to the inven- tion, i.e. 3.0 (X=36 mm, Y=12 mm), and the same electrical properties were now obtained by using a casing coated with yellow chrome and resonators coat¬ ed with copper. In this way the use of silver and the coating of the casing with copper could be avoided completely. Instead, the casing could be coated with the less expensive yellow chrome, such coating also being easier to perform. The figures and the description relating there¬ to are only intended to illustrate the present inven- tion. In its details, the filter according to the invention may vary within the scope of the attached claims.

Claims: Claims : 1. Comb-line high-frequency filter or the like, comprising an elongated casing (1) made of an electrically conductive material, conductor rods of an electrically conductive material being arranged in a sequence at predetermined intervals in a sub¬ stantially integral air cavity within the casing, one end of each conductor rod being short-circuited to the casing and its other end being detached therefrom so that each conductor rod forms with the casing (1) an air-insulated coaxial resonator, c h a r a c ¬ t e r i z e d in that, in each coaxial resonator, the ratio between the diameters (X, Y) of the casing (1) and the resonator (3) is within the range from about 2.8 to 3.3. 2. Comb-line high-frequency filter according to claim 1, c h a r a c t e r i z e d in that said casing (1) is coated with yellow chrome and the con- ductor rods (3) are made of copper or coated with copper, and that the ratio between the diameters of the casing and the resonator of each coaxial resonator is about 3.0 at the resonator.

Inventors: PIIRAINEN RISTO, PIIRAINEN, RISTO
Applicants: TELENOKIA OY
Application year: 1992
Main classification: H01P1
IPC codes: H01P1