This invention concerns a method of manufacturing a cavity resonator according to the preamble of claim 1 and a cavity resonator according to the preamble of claim 2.

Wave guide resonators of the cavity resonator type are used as filters, wherein electromagnetic energy is transported to the resonator. The cavity supports a desired resonant frequency while suppressing undesired frequency modes. The use of radio frequency resonators as filters is well known and the shape of the cavity defining structure may be configured in various ways.

A prior art resonant cavity consists of a box-like structure, forming a so called rectangular resonator, or more appropriate¬ ly a parallelepipedic resonator. According to the prior art a rectangular resonator is typically assembled by the top wall being soldered to the remaining box shaped structure in order to obtain a closed cavity. Great care has to be taken with respect to choice of material (typically silver-plated invar metal) in order to guarantee dimensional stability. Presence of solder material causes resistance losses in solder joints.

The Q-value of a cavity resonator is determined by the geometry as well as the ohmic resistance of the inside surface region. Wall part solder joints are however difficult to obtain having a conductivity which is comparable to that of the surface region of the walls themselves. Also with perfect joints problems with ohmic resistance occur, because the current lines in the conventional cavity resonator have to cross the soldered joints. Since the ohmic resistance affects the efficiency and the Q-value of the cavity resonator, ohmic resistance is undesired and should be minimized. When used in combiner filters for connecting several radio transmitters to one common transmitting antenna, ohmic resistance in the cavity resonator is equal to reduced transmitted effect.

It is an object of this invention to eliminate or at least reduce the above problems and to provide a cavity resonator which is inexpensive in manufacture and have an excellent Q- value. This aim is achieved by the method and the resonator as above being characterized by the characterizing features of the claims 1 and 2 respectively.

By the wall sections being shaped such that joints between neighbouring wall sections are oriented essentially in parallel with the electric currents occurring in use, significant gain its obtained due to reduced ohmic resistance losses in wall joints as compared to resonators according to the prior art.

This makes the assembling of the cavity resonator far less critical, because essentially no current lines cross the wall joints. The cavity resonator may even be assembled without any soldering operation, as long as the wall sections are mutually secured in an appropriate manner.

By the wall sections being formed from a plate shaped material and in particular comprising flanges which meet along at least a portion of each joint, easy manufacture is obtained, resul¬ ting in a resonator having excellent Q-value compared to a similar resonator which is manufactured conventionally. The invention thus provides a resonator allowing more energy to be stored within the resonator.

Claims 5 - 8 define resonators which are easily manufactured and have relatively high Q-value.

The invention will now be described in greater detail with reference to embodiments which appear on the drawings, wherein:

Fig. 1 illustrates a resonator according to the invention in a plan view,

Fig. 2 illustrates the resonator of Fig. 1 in a side view,

Fig. 3 illustrates another resonator according to the invention in a plane view,

Fig. 4 illustrates the resonator of Fig. 3 in a side view, and

Fig. 5 show diagramatically a blank for making a part of the resonator of Figs 3 and 4.

Fig. 1 and 2 show diagrammatically a cavity resonator 1 having a rectangular or parallelepipedic shape. The resonator 1 is constructed such that four wall sections 2 complete the closed box-shape of the resonator. For that reason each wall section 2 consists of two triangular end wall parts 2 ' which together with three similar end wall parts form the respective end walls of the resonator. The end wall parts of each wall section 2 are perpendicular to a side wall 2" and located at opposite sides thereof. At the meeting edges of the respective end wall parts 2', the wall sections 2 are provided with meeting flanges 3 abutting corresponding flanges 3 of the neighbouring wall section end parts 2 ' . At the side edges of each side wall 2" each wall section 2 is provided with flanges 4, in a similar manner contacting the flanges 4 of the neighbouring wall sections 2.

In use the resonator functions such that a magnetic field is established inside the cavity whereas the associated electric wall currents flow at the innermost inside surface layer of the walls. The construction of the resonator is such that current lines divert essentially radially from the centre point 5 of the end wall. Further the electric current lines go essentially in parallel with the flanges 4 along the side walls 2" of the resonator. This way essentially no electric currents will cross the wall joints and essentially no ohmic resistance losses will occur in said wall joints.

Assembling of the cavity resonator according to the invention may be achieved by a soldering operation in such a way that the flanges 3 and 4 are soldered together. Since there are reduced

quality demands with respect to the joints, the wall sections 2 may also be assembled using mechanical fastening means, such as clips, clamps or screws. As a whole, the invention allows radically simplified manufacture.

A magnetic field coupling device 6 ' in use transmits energy into the cavity via conductor 8 which is intended to be coupled to an outside source. The device is arranged such that an insulator element is secured within a through-hole in the wall of the resonator. Inside the cavity the conductive element is shaped such that it forms a loop portion 9 (see fig 2) so as to produce the magnetic field when the resonator is in use. The resonator further comprises a second magnetic field coupling device 6'' which functions as an output for the resonator.

Figs 3 and 4 show a second embodiment of the invention having a hexagonal cross section. Like elements bear the same reference numbers as in figs 1 and 2. This resonator consists of three wall sections 2 each of which are bent along edge 11 such that they comprise two side wall portions 2 ''a and 2 ' 'b forming an angle relative to each other. Further each wall section 2 include end wall portions 2 'a and 2 'b which are provided with flanges 3 on rim portions which are intended to be placed against adjacent portions of neighbouring wall sections. If the wall sections are manufactured in a metal bending process from a metal plate material, the blank may be formed generally as is shown in fig 5. The completed joint 10 between portions 2 'a and 2 'b extends radially and may be a solder or a weld joint.

The resonator according to this invention is not restricted to the described embodiments. Resonators having circular, hexagonal cross or octagonal cross-section are examples within the scope of this invention. A resonator according to this invention may also be spherical or have any other curved configuration.

In a resonator having circular cross-section, the current lines divert radially from the centre of one of the end walls, extend

mainly axially along the envelope surface so as to converge radially to the centre of the second end wall. Such a resonator may be divided into wall sections in such a way that each section consists of two end wall parts similar to 2 ' of fig 1 and a side wall comprising a portion of the envelope surface. When divided into four sections, each wall section comprises one fourth of the envelope surface, each wall section being cake piece shaped, although having a shell structure. A possible process of manufacturing wall sections having curved configuration is using plate metal deep-draw techniques. It should however be noted that invar-materials often are difficult to deep-draw.

The invention further may allow the use of more inexpensive materials. Considerable ohmic losses do remain in the wall surfaces but because reduced heating due to ohmic resistance losses in fact are reduced in a resonator according to the invention, the need for extremely dimensional stable material may be correspondingly reduced.