On Friday, November 14, 2014 11:46:03 AM UTC-6,
Generally for DX a takeoff angle of 30 degrees or less is the rule of
thumb for best general performance.

Of course the antenna still "works" at other heights, but if DX is what
you want to achieve, then best results, on the average over average
ground, the antenna will work best for that at a height of .5 lambda
or better.

Yep, for 80m, it's usually easier to put up a good vertical for dx
than a high dipole. And even then sometimes the vertical will do the
best. W8JI talks a lot about this comparing his 160m verticals and his
high 160m dipoles. Most times, his verticals still win to long paths.
I forgot how high his dipole was, but it's pretty high vs what most
people have.
People talk about short antennas being poor radiators, but on 40m with
my appx 40 ft tall full size dipole fed with coax, my mobile antenna
would beat it most every night from Houston to Jacksonville FL.
I thought maybe it was a fluke, but I tested it a few more times, and
it almost always won. So the most efficient antenna does not always win
the race if the less efficient antenna puts more rf at the lower angles
where you want for longer paths, vs the highly efficient antenna like my
coax fed dipoles. At 40 ft on 40m, it's still shooting a lot of rf at
fairly high angles, and not so much at the low angles.
Less than my mobile antenna did.

I remember one night I was at the coast fishing, and I actually ran a wide
braid ground wire from the truck body into the ocean just to add that extra
gusto.
On longer paths, I was smoking some people using dipoles and running amps
vs my extended 14 ft tall mobile antenna sitting on the beach with 100w.
So much for small antennas always being poor radiators.. :/
Efficiency isn't always everything. But it usually is for NVIS paths,
which is why I've always preferred coax fed dipoles for my usual 75m NVIS
chatter.