Russ,

I don't think I've seen a photo, but there's a description, including wavelengths used, in "German Warships of World War I: The Royal Navy's Official Guide to the Capital Ships, Cruisers, Destroyers, Submarines, and Small Craft, 1914-1918."

Note also that use (sending) of radio messages seems to have varied considerably by command -- the High Seas Fleet boats seem to have radioed a lot, the Flanders-based boats very little. This goes a long way to explaining why the Royal Navy had a lot better understand of HSF submarine losses as compared to Flanders submarine losses.

Best wishes,
Michael