A chart showing 40M NVIS antenna characteristics -- gain vs take
off angle for different heights.
Interesting that a good DX dipole at 100' works well for NVIS, although
it also would be collecting noise and signals incoming at 0 - 45
The 40M DX dipole at 66' is very poor for NVIS.
This chart suggests a dipole (flat top or inv V) at 10-20' is good.