An elementary antenna of the microstrip type, comprising a stack of layers, the elementary antenna being able to assume a flat configuration, in which the layers are substantially flat and perpendicular to a stacking axis (z) along which the layers are stacked, the stack comprising a first conductive radiating element and an excitation device coupled to the first radiating element so as to allow the radiating element to be excited according to two orthogonal linear polarisations, the excitation device comprising: - a first elementary excitation device configured and coupled to the first radiating element so as to be able to excite a first pair of excitation points, formed by a first excitation point and a second excitation point arranged on a first straight line of the first radiating element, the first elementary excitation device comprising a first conducting line, a second conducting line and a first power distributor capable of distributing a power of an input signal received at an input/output (I/O) point of the first power distributor on the first conducting line and the second conducting line, the first conducting line and the second conducting line extending linearly perpendicular to the first straight line; - a second elementary excitation device configured and coupled to the first radiating element so as to be able to excite a second pair of excitation points, formed by a third excitation point and a fourth excitation point arranged on a second straight line of the first radiating element, the second elementary excitation device comprising a third conducting line, a fourth conducting line and a second power distributor capable of distributing a power of an input signal received at an input/output (E/S') of the second power distributor on the third conducting line and the fourth conducting line, the third conducting line and the fourth conducting line extending linearly perpendicular to the second straight line, the first, second, third and fourth conducting lines being interposed between the first radiating element and the first and second power distributors (11, 21), along the stacking axis (z), the first power distributor and the second power distributor being coplanar. - a first elementary excitation device configured and coupled to the first radiating element so as to be able to excite a first pair of excitation points, formed by a first excitation point and a second excitation point arranged on a first straight line of the first radiating element, the first elementary excitation device comprising a first conducting line, a second conducting line and a first power distributor capable of distributing a power of an input signal received at an input/output (I/O) point of the first power distributor on the first conducting line and the second conducting line, the first conducting line and the second conducting line extending linearly perpendicular to the first straight line; - a second elementary excitation device configured and coupled to the first radiating element so as to be able to excite a second pair of excitation points, formed by a third excitation point and a fourth excitation point arranged on a second straight line of the first radiating element, the second elementary excitation device comprising a third conducting line, a fourth conducting line and a second power distributor capable of distributing a power of an input signal received at an input/output (E/S') of the second power distributor on the third conducting line and the fourth conducting line, the third conducting line and the fourth conducting line extending linearly perpendicular to the second straight line,