Radio-frequency identification tag (6, 100, 110, 120, 140, 160, 180) adapted to indicate to a reader, via a wireless connection, that a physical quantity has reached a predetermined threshold, this physical quantity being selected from the group comprised of a temperature of the radio-frequency identification tag and a magnetic field in which the radio-frequency identification tag is immersed, this radio-frequency identification tag comprising: - an electrically insulating substrate, - an antenna for establishing the wireless connection with the reader, this antenna being placed entirely on and secured to the insulating substrate without any degree of freedom, - a floating ground plane made of an electrically conductive material and electrically insulated from the antenna, this ground plane being mobile relative to the antenna between a first position wherein the impedance of the antenna is equal to a first value and a second position wherein the impedance of the antenna is equal to a second value different from the first value, and - an actuator made of a transducer material adapted to convert a variation in the physical quantity into a mechanical displacement of the ground plane between its first and second positions when this variation leads the physical quantity to cross the predetermined threshold to indicate this event to the reader, the transducer material being selected from a group consisting of a thermal shape-memory material, a magnetostrictive material and a magnetic shape-memory material, characterized in that: - the transducer material is also an electrically conductive material, and - the ground plane and the actuator are both formed by a single strip (66; 112; 126; 142, 144; 162; 182) made of this transducer material and a mobile portion of which moves relative to the antenna between the first and second positions when the physical quantity crosses the predetermined threshold. - an electrically insulating substrate, - an antenna for establishing the wireless connection with the reader, this antenna being placed entirely on and secured to the insulating substrate without any degree of freedom, - a floating ground plane made of an electrically conductive material and electrically insulated from the antenna, this ground plane being mobile relative to the antenna between a first position wherein the impedance of the antenna is equal to a first value and a second position wherein the impedance of the antenna is equal to a second value different from the first value, and - an actuator made of a transducer material adapted to convert a variation in the physical quantity into a mechanical displacement of the ground plane between its first and second positions when this variation leads the physical quantity to cross the predetermined threshold to indicate this event to the reader, the transducer material being selected from a group consisting of a thermal shape-memory material, a magnetostrictive material and a magnetic shape-memory material, - the transducer material is also an electrically conductive material, and - the ground plane and the actuator are both formed by a single strip (66; 112; 126; 142, 144; 162; 182) made of this transducer material and a mobile portion of which moves relative to the antenna between the first and second positions when the physical quantity crosses the predetermined threshold.