A communication antenna device (D), on board a motor vehicle, for communicating with an antenna of a portable device, includes: a communication antenna (A) including a first part (A1) having a maximum number of primary windings (E1MAX) and a second part (A2) having a maximum number of secondary windings (E2MAX), a microcontroller (10) electrically connected to the antenna (A), a first switching element (M1), for connecting the first part and/or the second part to the microcontroller (10), a second switching element (M2), for connecting a determined number of primary windings (E1i) of the first part to the microcontroller (10), a third switching element (M3), for connecting a determined number of secondary windings (E2j) of the second part to the microcontroller (10), and elements for controlling (20) the first, second and third switching elements.