The invention relates to a high-frequency filter comprising
a block-like housing made of a dielectric material, the housing comprising ends and side surfaces, the section formed on the side surfaces between the ends being provided with a conductive plating acting as the filter""s outer conductor,
at least two holes plated with a conductive material and located on the section extending between the ends of the block-like housing to allow at least two inner conductors to be provided;
a connection base provided with a conductive pattern, the block-like housing being arranged onto the base;
and the conductive pattern of the filter comprising connection areas for coupling the inner conductors to the connection base. 
The invention relates particularly to making capacitive couplings in a filter having a block-like housing made of a ceramic or other kind of dielectric material. 
High-frequency filters, such as radio frequency filters, for example resonator filters, are used for providing high-frequency circuits for example at base stations, mobile stations and other radio transceivers used in mobile communications networks.  One possibility is to use radio frequency filters for example as interface circuits and filtering circuits in the amplifiers of base station transmitter units or receiver units. 
One of the filter types is a filter which has a block-like housing made of a ceramic or other kind of dielectric material, the sides of the block-like housing being provided with a plating made of a conductive material and the block being provided with holes having a conductive plating which acts as the inner conductor of the filter, the above mentioned conductive plating on the side surfaces of the block acting as the filter""s outer conductor. 
Capacitive couplings are made between the inner conductors and ground potential of a filter.  They are implemented by arranging a suitable amount of capacitance between the inner conductors and the ground plane, the capacitance having an effect on resonance frequency.  The frequency control of each resonance circuit of the filter is based on the fact that by increasing the capacitance, i.e. by strengthening the capacitive coupling, between a free end of the resonator inner conductor and the ground potential, where the outer connector is, the resonance frequency of the resonance circuit decreases, whereas by decreasing the capacitance, i.e. by weakening the capacitive coupling, the resonance frequency increases. 
Capacitive couplings are also made between the inner conductors of two resonance circuits of a filter.  This usually takes place through a conductive material arranged in the area between the inner conductors of the resonators, the material influencing the strength of the capacitive coupling between the inner conductors of the two, usually adjacent, resonance circuits. 
By making capacitive couplings, i.e. by controlling them, it is possible to make the filter work in a desired manner, which in turn allows the entity formed by several resonance circuits to be made to implement a desired frequency response, such as a pass band in a band-pass filter, the filter allowing the signals within the band to pass through.  The pass band may be for example a 25-Mhz frequency band of a GSM base station employing the TDMA technique, the bandwidth being within the frequency range from 925 to 960 MHz, which is the range where single 200-kHz wide GSM channels are located. 
The couplings made in filters having ceramic, or otherwise dielectric, block-like housings, have conventionally been controlled by making grooves on the block, or what is known as the xe2x80x9cstonexe2x80x9d; in other words, the material of the block-like housing is removed from the side surfaces of the block.  The grooved block is plated, the plating thus being narrower at the grooved portions, i.e. the outer conductor of the filter is narrower at the grooved portions in question.  This type of solution is described in WO publication 83/02853.  A problem with this solution is the extra work arising from the cutting, because it complicates the manufacturing process.  A particular problem is that for different kinds of applications, differently shaped blocks are required. 
Another way to control the couplings in filters having block-like housings made of a ceramic or other kind of dielectric material has been to form conductive patterns on a surface of the block-like housing, the patterns having an effect on the capacitive and/or inductive couplings of the resonance circuits.  A solution of this type is described in EP publication 694983.  A drawback of this solution is that the manufacturing processes involved become more complicated when the conductive patterns are made onto the side surface of the block-like housing. 
A third method is the structure known from Fl publication 87406 in which one of the four side surfaces of the block-like housing is left unplated, and the resonator couplings are influenced by means of a connection base arranged against the unplated side of the block, the connection base surface between the unplated side and the connection base being provided with conductive patterns that allow capacitive couplings to the inner conductors and connections between the inner conductors to be made.  This solution requires, however, a plane arranged on the other side of the connection base to serve as ground potential which in a way replaces the plating that has been left out of the fourth side surface.  The circuit board and the conductive pattern thus become in a way an integral, operational part of the filter structure, thereby producing a non-homogenous structure because the structure is covered by different kinds of conductive materials, such as a ceramic block and a circuit board provided with a conductive pattern.  The problem is further emphasized if the dielectric block and the circuit board have different dielectricity constants, because the electromagnetic field of materials having differing dielectricity constants will behave differently.  Another at least equally serious problem is the alignment of the block and the connection base, or the circuit board, because any inaccuracies in the positioning of the block with regard to the circuit board will cause a corresponding inaccuracy in the positioning of the conductive pattern on the circuit board with regard to the inner conductors of the filter.  A further problem is that there will be variation in the shape of the conductively covered inner space of the filter structure, defined by the three block surfaces and the conductive plating arranged on the opposite side of the circuit board, if the positioning is not as designed.  Moreover, problems may arise from how to make the clearance between the unplated surface of the block arranged against the connection base as small as possible.  The paste, glue or other attaching agent used for joining the unplated block side to the connection base must be very carefully dosed, and the drying and other processing of the glue, or the like, must be extremely well controlled to ensure that in mass production the process is always repeated in the same way. 
It is therefore an object of the present invention to provide a novel filter which allows the problems and drawbacks of the prior art solutions to be avoided. 
To achieve the above object, the filter of the invention is characterized in that the filter, which is connected to the conductive pattern of the connection base, the connection base being outside the conductive plating acting as the filter""s outer conductor and arranged on the surface of its block-like housing, comprises one or more coupling members for making capacitive couplings between the inner conductors. 
The invention is based on the idea of arranging the coupling members on the connection base outside the plated block. 
An advantage of the filter structure of the invention is that the coupling members, particularly the members for making capacitive couplings, are arranged on the connection base which is outside the filter""s outer conductor and on which the block-like housing having a side surface plated with an outer conductor plating is arranged.  This allows the invention to be implemented using an extremely simple block where all side surfaces are plated, i.e. also the surface facing the circuit board, the circuit board thus not forming a structural part that would affect the operation of electric fields and/or magnetic fields.  The same applies to the coupling members arranged onto the circuit board; they do not affect the operation of the electric and/or magnetic fields either because, similarly to the circuit board, i.e. the connection base, they are also outside the filter""s outer conductor.