If it helps, here's the full extract from "U-boats, History, Development and Equipment" by David Miller:
FuMO-391 Lessing
The air warning set,
FuM0-391 Lessing, was developed for use in the Type XXI, where it used the "
runddipol" antenna atop the
schnorchel.
Lessing indicated the presence of an aircraft but not its height or bearing. Range on a surfaced U-boat was given as approximately 10,900yd (10,000m) against low-flying aircraft and approximately 18 miles (30km) against aircraft flying at 6,560ft (2,000m). It could also be used when the U-boat was
schnorchelling, but the range was much less: approximately 13,200yd (12,070m) for an aircraft at 3,300ft (1,000m).
Also from the same book,
XXI Sensor placement diagramme.
And
schnorchel/runddipol closeup
***Let me stress again that this all means nothing to me - hopefully it means something to all you guys and helps you all out.