The invention relates to a microwave filter comprising a coaxial structure, consisting of a tubular outer conductor and an inner bar conductor. According to the invention, the inner bar conductor extends in an axial direction inside the outer tube and, together with said tube, forms a series of concentric slots in the axial direction thereby defining successive coaxial line segments with low characteristic impedance and coaxial line segments with high characteristic impedance. The aforementioned concentric slots are produced in a synthetic foam block.