A circuit for a loop antenna having a first antenna terminal and a second antenna terminal with an antenna impedance, and method for tuning an overall impedance that has an antenna impedance of a loop antenna and a tuning impedance, with an output amplifier for amplifying a transmit signal that has an output for connection to the first antenna terminal of the loop antenna, with a tuning device designed for automatic tuning that has a terminal, which is separated from the output of the output amplifier for connection to the second antenna terminal, in which the tuning device has an adjustable tuning impedance that is connected to the terminal, in which the tuning device has a measurement device that is connected to the tuning impedance in order to measure a voltage amplitude across the tuning impedance, in which the tuning device has a computing unit that is connected to the measurement device and the adjustable tuning impedance, and in which the computing unit is designed for automatic adjustment of the tuning impedance based on evaluation of the voltage amplitude and the tuning impedance.