Frequency-tunable microwave bandpass filter comprising in combination at least the following elements: a waveguide of rectangular section comprising a first fixed conductive portion and a second moving conductive portion, said first fixed portion comprising three longitudinal conductive partitions,, forming three sides of the waveguide G, the section of the waveguide having a large side (a) defined by the position of the moving portion when it is inserted into the portion and a small side (b), said first portion comprising a number of first conductive partitions (105 1, 105 2 and 105 4, 105 5) constituted of one or more conductive obstacles which associated with complementary openings (O i) in the section of the waveguide form the irises of capacitive type, said first partitions being mounted transversely to the propagation of the wave in the guide and defining a number of cavities (K i) (106 1, 106 2) in the longitudinal direction of the guide and integral to the first portion and a number of second conductive partitions (105 3) with one or more openings (i) defining the irises of capacitive type and which form, in association with two guide lengths (L3, L4) adjacent a second conductive partition (105 3) immitance inverters (Ji), said first partitions (105 1, 105 2 and 105 4, 105 5) forming a succession of resonant cavities (Ki) coupled together by the immitance inverters (Ji), said second moving conductive portion comprising a wall, parallel to the small side (b) of the guide, forming the fourth face of the waveguide, said wall defining the value of dimension (a) of the large side of the guide, and thus the centre frequency of the filter, the moving portion comprising a number of slots receiving the partitions of the portion which form the irises of capacitive type, the cavities Ki thus being formed when the portion and the portion are fitted together, means (30, 32, 33) for ensuring electrical contact between the first fixed conductive portion and the second moving conductive portion. a waveguide of rectangular section comprising a first fixed conductive portion and a second moving conductive portion, said first fixed portion comprising three longitudinal conductive partitions,, forming three sides of the waveguide G, the section of the waveguide having a large side (a) defined by the position of the moving portion when it is inserted into the portion and a small side (b), said first portion comprising a number of first conductive partitions (105 1, 105 2 and 105 4, 105 5) constituted of one or more conductive obstacles which associated with complementary openings (O i) in the section of the waveguide form the irises of capacitive type, said first partitions being mounted transversely to the propagation of the wave in the guide and defining a number of cavities (K i) (106 1, 106 2) in the longitudinal direction of the guide and integral to the first portion and a number of second conductive partitions (105 3) with one or more openings (i) defining the irises of capacitive type and which form, in association with two guide lengths (L3, L4) adjacent a second conductive partition (105 3) immitance inverters (Ji), said first partitions (105 1, 105 2 and 105 4, 105 5) forming a succession of resonant cavities (Ki) coupled together by the immitance inverters (Ji), said second moving conductive portion comprising a wall, parallel to the small side (b) of the guide, forming the fourth face of the waveguide, said wall defining the value of dimension (a) of the large side of the guide, and thus the centre frequency of the filter, the moving portion comprising a number of slots receiving the partitions of the portion which form the irises of capacitive type, the cavities Ki thus being formed when the portion and the portion are fitted together, means (30, 32, 33) for ensuring electrical contact between the first fixed conductive portion and the second moving conductive portion.