Christiansen is a Senior Principal Research Officer in the Radiophysics Laboratory working on radio astronomy. His contributions to high resolution studies of the Sun are outstanding. In 1952 he devised an instrument giving high resolution in one dimension, in 1957 another giving resolution in two dimensions and producing actual pictures of the Sun in the "light" of 21 cm radio waves. Christiansen's observations and interpretation have supplied a major part of our knowledge of this aspect of solar radio astronomy. His equipment has since been copied in many laboratories. Prior to joining the Radiophysics Laboratory, he worked on the development of rhombic aerials for long distance communication and the forms he developed are now in use in many countries.