Abstract: Arrangement for transmission and reception of electromagnetic radiation. A ransmitting and receiving part (1) transmits and receives the electromagnetic radiation, and a reflecting part (2) reflects the transmitted electromagnetic radiation back to the transmitting and receiving part while passing through a medium which transmits radiation. The transmitting and receiving part has an electromagnetic radiation source (5) and a concave main reflector (3) which is common to transmission and reception. The transmitting and receiving part has a second reflector (6), which is positioned between the radiation source (5) and the main reflector (3), so that this second reflector shades a part of the main reflector reflecting area (4) for the radiation transmitted by the radiation source towards the main reflector. This forms a shadow area (17) on the main reflector reflecting area for transmitted radiation.