A basic antenna designed to form a radiating element of an array antenna comprising: - a probe for converting electricity into electromagnetic energy and vice versa; - an electromagnetic wave planar reflector bearing the probe; and - an assembly of elements made from at least two materials differing by their permittivity and/or their permeability and/or their conductivity, the assembly including: - a structure configured based on the principle of Electromagnetic Band Gap materials and having a periodicity in the direction orthogonal to the planar reflector; and - a cavity in contact with the planar reflector and the structure; the probe being contained in the cavity in contact with the planar reflector, the cavity constituting a defect in the periodicity of the structure that imparts the assembly with a behavior of an Electromagnetic Band Gap material with a defect in which the positioning of the elements in said assembly ensures the radiation and a spatial and frequency filtering of the electromagnetic waves produced or received by the probe, said filtering allowing in particular one or more operating frequencies of the basic antenna inside a frequency band gap; said basic antenna being characterized in that it comprises a wall enclosure capable of reflecting the electromagnetic waves at the operating frequency or frequencies, said wall enclosure being an extension in the direction orthogonal to the planar reflector and surrounding simultaneously and only the probe, the cavity and the structure, making it possible to generate a basic radiating surface with a shape predetermined and imposed by the wall enclosure, in that the probe is a patch antenna including a metal plate of square shape, and a dielectric substrate of square shape on which the metal plate is printed and which separates the metal plate from the planar reflector, and in that the structure comprises two strips (18, 20) of Electromagnetic Band Gap materials with same dimensions, forming a planar cross positioned across from the probe through the cavity, which is an air cavity, at a height (h) of the planar reflector, each strip having a length equal to the length of the side of the dielectric substrate and a width smaller than the length of one side of the metal plate. - a probe for converting electricity into electromagnetic energy and vice versa; - an electromagnetic wave planar reflector bearing the probe; and - an assembly of elements made from at least two materials differing by their permittivity and/or their permeability and/or their conductivity, the assembly including: - a structure configured based on the principle of Electromagnetic Band Gap materials and having a periodicity in the direction orthogonal to the planar reflector; and - a cavity in contact with the planar reflector and the structure; - a structure configured based on the principle of Electromagnetic Band Gap materials and having a periodicity in the direction orthogonal to the planar reflector; and - a cavity in contact with the planar reflector and the structure; said basic antenna being characterized in that it comprises a wall enclosure capable of reflecting the electromagnetic waves at the operating frequency or frequencies, said wall enclosure being an extension in the direction orthogonal to the planar reflector and surrounding simultaneously and only the probe, the cavity and the structure, making it possible to generate a basic radiating surface with a shape predetermined and imposed by the wall enclosure, in that the probe is a patch antenna including a metal plate of square shape, and a dielectric substrate of square shape on which the metal plate is printed and which separates the metal plate from the planar reflector, and in that the structure comprises two strips (18, 20) of Electromagnetic Band Gap materials with same dimensions, forming a planar cross positioned across from the probe through the cavity, which is an air cavity, at a height (h) of the planar reflector, each strip having a length equal to the length of the side of the dielectric substrate and a width smaller than the length of one side of the metal plate.