Flat scanning antenna comprising, at transmission and at reception, at least one main array having waveguides with radiating slots, wherein the main array having waveguides with radiating slots comprises two dielectric substrates, namely the lower substrate and the upper substrate respectively, one superposed above the other, the two, lower and upper substrates comprising the same number of waveguides (10, 11) corresponding thereto, each waveguide of the upper substrate communicating with a corresponding single waveguide of the lower substrate via a coupling slot, characterized in that: - each waveguide of the upper substrate further includes a plurality of radiating slots, all the radiating slots being mutually parallel and oriented in the same direction parallel to a longitudinal axis of the waveguides; - each waveguide of the lower substrate includes an individual internal supply circuit comprising an individual internal phase-shift /amplification electronic circuit, - the antenna further includes, at transmission and at reception: ∘ an auxiliary array having waveguides with radiating slots, the main array and the auxiliary array each comprising a first internal phase-shift circuit (21, 22), (31, 32) set to the same phase value, the auxiliary array having radiating slots oriented at a non-zero angle (α) inclined to the slots of the main array; ∘ a second phase-shift circuit placed at the input of the auxiliary array, the second phase-shift circuit being intended to compensate for a rotation of the plane of polarization of a wave transmitted by the main array and comprising a phase shifter having a variable phase between 0° and 180°, and a variable-gain amplifier. - each waveguide of the upper substrate further includes a plurality of radiating slots, all the radiating slots being mutually parallel and oriented in the same direction parallel to a longitudinal axis of the waveguides; - each waveguide of the lower substrate includes an individual internal supply circuit comprising an individual internal phase-shift /amplification electronic circuit, - the antenna further includes, at transmission and at reception: ∘ an auxiliary array having waveguides with radiating slots, the main array and the auxiliary array each comprising a first internal phase-shift circuit (21, 22), (31, 32) set to the same phase value, the auxiliary array having radiating slots oriented at a non-zero angle (α) inclined to the slots of the main array; ∘ a second phase-shift circuit placed at the input of the auxiliary array, the second phase-shift circuit being intended to compensate for a rotation of the plane of polarization of a wave transmitted by the main array and comprising a phase shifter having a variable phase between 0° and 180°, and a variable-gain amplifier. ∘ an auxiliary array having waveguides with radiating slots, the main array and the auxiliary array each comprising a first internal phase-shift circuit (21, 22), (31, 32) set to the same phase value, the auxiliary array having radiating slots oriented at a non-zero angle (α) inclined to the slots of the main array; ∘ a second phase-shift circuit placed at the input of the auxiliary array, the second phase-shift circuit being intended to compensate for a rotation of the plane of polarization of a wave transmitted by the main array and comprising a phase shifter having a variable phase between 0° and 180°, and a variable-gain amplifier.