Multi surface coupled coaxial resonator

A coaxial resonator, the electrical length of which is a quarter-wave, comprising an inner conductor (301), an outer conductor (302) and a conductive cover (303), which inner conductor (301) is essentially air-insulated from, and in open-circuit relation with, said outer conductor (302) and said conductive cover (303), said outer conductor, in turn, comprises at least one external wall (302a, 302b, 302c, 302d), a first end of said outer conductor (302) being short-circuit connected to a first end of said inner conductor (301) and an opposite second end of said outer conductor (302) being covered with said conductive cover (303), which is conductively connected to the outer conductor (302), characterised in that the inner conductor (301) has an extension (304) at an open end, opposite said first end, of the inner conductor, essentially parallel with the conductive cover (303), which extension forms extra capacitance substantially evenly with the conductive cover (303), said extension having at least two projections (305, 306a, 306b) that are essentially parallel with the external walls (302a, 302b, 302c, 302d) of the outer conductor (302), and at least one of said projections forms extra capacitance with an upper part (302y) of the outer conductor (302), whereby:

• at least one (305) of said projections is a capacitive coupling element for external coupling of the resonator, and
• at least one (306a) of said projections is a tuning element of the resonator.