A measurement apparatus (10, 30) for estimating a value of a physiological characteristic of a subject, the measurement apparatus comprising: - a radio-frequency, RF, power generating apparatus comprising: - a transmit antenna for emitting RF radiation; - an RF signal generator; and - a transmission line connecting the RF signal generator to the transmit antenna; - at least one sensor configured to record data in respect of the subject; - a detector arranged to measure a variation of at least one parameter correlated with attenuation of the emitted RF radiation by monitoring a level of DC current supplied to the RF signal generator, during a measurement period, the at least one parameter comprising one of: a level of DC power input to the RF signal generator; and the level of DC current input to the RF signal generator; and - a controller (15, 35) in communication with the RF power generating apparatus and the detector, and arranged to: - cause the RF power generating apparatus to emit RF radiation at a predetermined level of output power during the measurement period; - control the RF power generating apparatus to maintain the predetermined level of output power during the measurement period; - receive measurements of the variation of the at least one parameter from the detector; and - calculate a value of a physiological characteristic of a subject based on the received measurements and on the data recorded by the at least one sensor. - a radio-frequency, RF, power generating apparatus comprising: - a transmit antenna for emitting RF radiation; - an RF signal generator; and - a transmission line connecting the RF signal generator to the transmit antenna; - a transmit antenna for emitting RF radiation; - an RF signal generator; and - a transmission line connecting the RF signal generator to the transmit antenna; - at least one sensor configured to record data in respect of the subject; - a detector arranged to measure a variation of at least one parameter correlated with attenuation of the emitted RF radiation by monitoring a level of DC current supplied to the RF signal generator, during a measurement period, the at least one parameter comprising one of: a level of DC power input to the RF signal generator; and the level of DC current input to the RF signal generator; and - a controller (15, 35) in communication with the RF power generating apparatus and the detector, and arranged to: - cause the RF power generating apparatus to emit RF radiation at a predetermined level of output power during the measurement period; - control the RF power generating apparatus to maintain the predetermined level of output power during the measurement period; - receive measurements of the variation of the at least one parameter from the detector; and - calculate a value of a physiological characteristic of a subject based on the received measurements and on the data recorded by the at least one sensor. - cause the RF power generating apparatus to emit RF radiation at a predetermined level of output power during the measurement period; - control the RF power generating apparatus to maintain the predetermined level of output power during the measurement period; - receive measurements of the variation of the at least one parameter from the detector; and - calculate a value of a physiological characteristic of a subject based on the received measurements and on the data recorded by the at least one sensor.