I have the L-400B LW antenna and what they mean about the metal mast is
that as long as the mast is parallel to the first few inches of the
antenna at the bottom where you would clamp it you will be OK. But
don't let the middle to upper part of the antenna be close to metal
objects. I found that even 20' proximity to lush trees had a big
impact on the L-400B reception on LW. As long as I kept it in the
clear of metal or wet objects it performed well. If no near objects
were present, mounting any higher than about 10' made no difference.

Frank
Tucson