About BdU-Uboat, Uboat-BdU and Uboat-Uboat radio communications:
http://www.uboat.net/articles/35.html
http://www.uboatarchive.net/KTBNotesCommunications.htm
http://www.ibiblio.org/hyperwar/ETO/.../SRH025-8.html
http://en.wikipedia.org/wiki/Goliath_transmitter
Seems that U-boat radio equipment was able to intercept VLF signals even underwater, at variable depths depending on the distance from the transmitter.
I am still looking for a complete list of HF/VLF radio transmitters used by Kriegsmarine with their respective power/range. Does anyone know where I can collect this information?