Before, during and after WW2, submarines had "jumping wires" which were rigged from forward to aft external of the boat, over the top of the casing and the conning tower.  The conning tower was fitted with stanchions which supported the wire, set in such a way that personnel and equipment [periscopes etc] were not affected by the presence of the hefty wire.

The function of the wire was to support a loop aerial in the forward section [i.e., bow to bridge {conning tower}] which would act as the receiving aerial when dived. In earlier fits, the Captain had to point the boat towards the direction of the transmitting source to maximise the strength of the received signal.

Here are two fits showing the jumping wire.  The first picture comes from a book called the "U-Boat Commander's Handbook" and shows a U-Boat drawing dating from 1943. The wire can clearly be seen in the top picture.

This second picture is of a British 'A' Boat not yet modified to have a sail-fin.  It is of HM Submarine Alaric in Malta. The stanchions and the wire are very clear.

This picture is of one of Alaric's sister-boats the Alderney.  She has a sail-fin and no jumping wire.  Her loop aerials [plural] Forward-Aft and Port-Starboard are sited in the top of the fin, F-A forward and the P-S aft. They fed the outfit Type ALF.

The actual loop aerial, was a continuous cable, part secured to the bow-to-stern support wire, part on or under the free flood casing/conning tower area and in a very simple drawing, looked like this