A polarising screen comprising an arrangement of at least one polarising cell (112; 172; 262; 312; 372; 412; 512; 932) made of an electrically conductive material, which is frequency- and polarisation-selective, for transforming the linear polarisation of the electric field E of an incident electromagnetic TEM wave, which field is received as input and is decomposable into two electric field signals E V, E H, the vertical and horizontal polarisations of which are linear and orthogonal, into a circular polarisation of an output electric field, and wherein each polarising cell (112; 172; 262; 312; 372; 412; 512; 932) contains a section of waveguide having two orthogonal, i.e. vertical and horizontal, pairs of lateral walls that are parallel to one another and that extend longitudinally in a direction of propagation of an incident electromagnetic TEM wave, the polarising screen being characterised in that the four lateral walls (124, 125, 126, 127; 194, 195, 196, 197; 274, 275, 276, 277; 324, 325, 326, 327; 374, 375, 376, 377; 414, 415, 416, 417) of each polarising cell (112; 172; 262; 312; 372; 412; 512; 932) are each open over their entire length due to a median continuous slot (134, 135, 136, 137;194, 195, 196, 197; 284, 285, 286, 287; 324, 325, 326, 327; 384, 385, 386, 387; 424, 425, 426, 427), parallel to the direction of propagation of the incident electromagnetic wave, so as to form four angled electrically conductive plates, and each polarising cell includes electrically conductive rods which interconnect the lateral walls and the four angled plates so that they are partially or completely coupled and which form one or more successive elementary electrical discontinuities, which are arranged at the end of or inside the section of waveguide forming the polarising cell and produce one or more inductive or capacitive loads, or one or more resonators (L, C) equivalent to an inductor and a capacitor connected in parallel or in series; and the longitudinally open slots (134, 135, 136, 137; 194, 195, 196, 197; 284, 285, 286, 287; 324, 325, 326, 327; 384, 385, 386, 387; 424, 425, 426, 427), of the lateral walls and the elementary electrical discontinuities of each polarising cell have geometric shapes and dimensions which perform total transmission of the incident wave, which is associated with a phase anisotropy of +90° or -90° according to the components E V and E H. the polarising screen being characterised in that the four lateral walls (124, 125, 126, 127; 194, 195, 196, 197; 274, 275, 276, 277; 324, 325, 326, 327; 374, 375, 376, 377; 414, 415, 416, 417) of each polarising cell (112; 172; 262; 312; 372; 412; 512; 932) are each open over their entire length due to a median continuous slot (134, 135, 136, 137;194, 195, 196, 197; 284, 285, 286, 287; 324, 325, 326, 327; 384, 385, 386, 387; 424, 425, 426, 427), parallel to the direction of propagation of the incident electromagnetic wave, so as to form four angled electrically conductive plates, and each polarising cell includes electrically conductive rods which interconnect the lateral walls and the four angled plates so that they are partially or completely coupled and which form one or more successive elementary electrical discontinuities, which are arranged at the end of or inside the section of waveguide forming the polarising cell and produce one or more inductive or capacitive loads, or one or more resonators (L, C) equivalent to an inductor and a capacitor connected in parallel or in series; and the longitudinally open slots (134, 135, 136, 137; 194, 195, 196, 197; 284, 285, 286, 287; 324, 325, 326, 327; 384, 385, 386, 387; 424, 425, 426, 427), of the lateral walls and the elementary electrical discontinuities of each polarising cell have geometric shapes and dimensions which perform total transmission of the incident wave, which is associated with a phase anisotropy of +90° or -90° according to the components E V and E H.