A directional coupler comprising a first connection for the inlet or outlet of a shaft, a first decoupling connection which is used to decouple a coupled shaft, a second connection for the inlet or outlet of the inlet or outlet shaft from the first connection and a second decoupling connection which is used to decouple the coupled shaft. The first connection and the first decoupling connection are connected to the internal conductor and to the external conductor of a coaxial conductor on the first connection surface thereof, by means of a first network. The second connection and the second decoupling connection are connected to the internal conductor and to the external conductor of the coaxial conductor on the second connection surface thereof, by means of a second network. The coaxial conductor is curved in such a manner that it is arranged in a parallel manner in relation to the first and second connection surfaces thereof, with a planar circuit board containing the first connection, the second connection, the first decoupling connection and/or second decoupling connection.