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From U-Boat archive:
http://www.uboatarchive.net/KTBNotesCommunications.htm
U-boat Radio Equipment
U-boats carried a suite of radio equipment giving them the ability to transmit, receive and direction find on several frequency bands.
At the beginning of the war all HF and MF transmitters used forward jumping wire/antenna and the U-boat received HF transmissions on the trailing jumping wires/antennas.
Because of the polarization of the wire, the boat sometimes had to point toward or away from the station it was transmitting or receiving to/from. Later a telescopic rod antenna extending from the port side of the bridge was added.
U-boats received and transmitted messages in encrypted Morse code.
U-boats seldom, if ever, transmitted or received by voice.
The boat would have to be surfaced to transmit or receive messages on HF but could receive messages on VLF when submerged at shallow depth using the DF-loop antenna.
Snorkel-equipped U-boats carried a drum-shaped aerial atop the snorkel which enabled them to transmit and receive HF while snorkeling.
U-boats communicated almost exclusively with Control ashore.
They did not send messages or beacon signals to other U-boats unless specifically instructed to do so by Control.
A typical radio suite would consist of the following equipment:
The antenna was retracted while at periscope depth. less objects to create a tell tale wake.
