Electromagnetic wave spatial filter with circular polarization and a Cassegrain antenna comprising such a filter

A spatial filter comprising first and second parallel grids (15,17) disposed on opposite sides of said filter;

• first and second dielectric layers (18,19) disposed between said grids;
• and a central network (16) between said dielectric layers participating with the said first grids and said first dielectric layer for conversion of incident circularly polarized waves into lineary polarized waves and with said second grids and said second dielectric layer for conversion of lineary polarized waves to circulatory polarized waves, said central network being totally reflecting for lineary polarized waves of a first polarization and transparent for waves of a second polarization perpendicular to said first polarization, wherein said two grids are each formed from conducting wires parallel to a same direction and having a pitch close to a quarter of the wave length .lambda. at the central frequency of the operating band of said filter;
• and said central network is formed from two networks of conducting wires, the first of said two networks comprising parallel wires having a pitch close to .lambda./4, the second of said two networks comprising parallel wires having a closely spaced pitch less than .lambda./4, and with a direction perpendicular to that of the wires forming said first network, the direction of the wires of said two grids forming an angle of 45.degree. with those of the wires of said two respective networks.