Filter device (100; 200; 300; 400; 450) with an electrically conducting strip structure, comprising: - a transmission line (102; 202; 302; 402; 452) formed by an electrically conducting strip printed on a surface of an electrically insulating substrate (104; 204; 304; 404; 454), said conducting strip having two ends (102 IN, 102 OUT; 202 IN, 202 OUT; 302 IN, 302 OUT; 402 IN, 402 OUT; 452 IN, 452 OUT) respectively forming the two sole input and output connection ports of the filter device (100; 200; 300; 400; 450), - a plurality of resonators (106 1,..., 106 M; 206 1,..., 206 M; 306 1,..., 306 M; 406 1,..., 406 M; 456 1,..., 456 M), each resonator comprising an electrically conducting strip printed on said surface of the substrate (104; 204; 304; 404; 454), - the conducting strip of each resonator (106 1,..., 106 M; 206 1,..., 206 M; 306 1,..., 306 M; 406 1,..., 406 M; 456 1,..., 456 M) has a first end (108 1,..., 108 M; 208 1,..., 208 M; 308 1,..., 308 M; 408 1,..., 408 M; 458 1,..., 458 M) coupled to the transmission line between the two connection ports and at least one second end (110 1,..., 110 M; 210 1,..., 210 M; 310 1,..., 310 M; 410 1,..., 410 M; 460 1,..., 460 M) that is free or connected to a ground so as to create an effective fundamental resonant wavelength specific to each resonator on said surface of the substrate, the filtering device being characterized in that: - for each pair of neighbouring resonators of the plurality of resonators, the distance (e 1,..., e M-1) between the first ends of two neighbouring resonators of this pair is less than one tenth of the smallest effective fundamental resonant wavelength of the plurality of resonators (106 1,..., 106 M; 206 1,..., 206 M; 306 1,..., 306 M; 406 1,..., 406 M; 456 1,..., 456 M) on said surface of the substrate. - a transmission line (102; 202; 302; 402; 452) formed by an electrically conducting strip printed on a surface of an electrically insulating substrate (104; 204; 304; 404; 454), said conducting strip having two ends (102 IN, 102 OUT; 202 IN, 202 OUT; 302 IN, 302 OUT; 402 IN, 402 OUT; 452 IN, 452 OUT) respectively forming the two sole input and output connection ports of the filter device (100; 200; 300; 400; 450), - a plurality of resonators (106 1,..., 106 M; 206 1,..., 206 M; 306 1,..., 306 M; 406 1,..., 406 M; 456 1,..., 456 M), each resonator comprising an electrically conducting strip printed on said surface of the substrate (104; 204; 304; 404; 454), - the conducting strip of each resonator (106 1,..., 106 M; 206 1,..., 206 M; 306 1,..., 306 M; 406 1,..., 406 M; 456 1,..., 456 M) has a first end (108 1,..., 108 M; 208 1,..., 208 M; 308 1,..., 308 M; 408 1,..., 408 M; 458 1,..., 458 M) coupled to the transmission line between the two connection ports and at least one second end (110 1,..., 110 M; 210 1,..., 210 M; 310 1,..., 310 M; 410 1,..., 410 M; 460 1,..., 460 M) that is free or connected to a ground so as to create an effective fundamental resonant wavelength specific to each resonator on said surface of the substrate, the filtering device being characterized in that: - for each pair of neighbouring resonators of the plurality of resonators, the distance (e 1,..., e M-1) between the first ends of two neighbouring resonators of this pair is less than one tenth of the smallest effective fundamental resonant wavelength of the plurality of resonators (106 1,..., 106 M; 206 1,..., 206 M; 306 1,..., 306 M; 406 1,..., 406 M; 456 1,..., 456 M) on said surface of the substrate.