'Antrum' (ii) was a British operation by the Special Operations Executive to send K. J. Aarsæther back to the Ålesund area with two wireless sets, crystals, and two signals plans ('Red' and 'Blue') (4 March 1943/May 1945).
One of the stations was to be used for the provision of intelligence on the movements of German warships and coastal shipping.
Aarsæther sailed from the UK on 4 March, and 'Antrum Red' came on air on 19 March. 'Antrum Blue' became an important contact and link for the 'Vestige' and 'Barbara' operations. Aarsæther also contacted local organisations, trained a local telegraphist, and contacted a group in the Molde area, but eventually fell ill and was returned to the UK in January 1944.
Reinforcement arrived by sea during October in the form of H. Hoel and I. Næss. In January K. J. Aarsæther and the Milorg district leader were brought back to the UK for discussions in which it was decided that Knut Aarsæther would work better with the district leader, and he was sent back to the Ålesund area by sea on 13/14 February 1944. Radio contact was re-established on 17 February. Organisation within the area continued and by mid-April there were 1,500 men prepared but without weapons, although in the final weeks of the German occupation there was an increase in weapon supplies.