Maximum gain at 90 degrees elevation for a 40 meter dipole is at 27 ft
above average ground. Anything between 23 and 33 ft will be within 0.1
dB of that. It doesn't vary a lot with ground quality, although EZNEC
thinks that 15 ft is the max gain point when over salt water. That may
be a simulation error (I don't know). Some people may think max NVIS
gain is achieved at a lower height because of a NEC simulation using a
Mininec ground. With that ground, EZNEC will give you a gain of 19.8
dBi for a 40 meter dipole 1 foot above ground (due to massive simulation
error).
I don't think there is any justification for intentionally putting a 40
meter dipole lower than about 23 ft. Originally there was a reason, or
so the story goes. It doesn't apply to amateur radio however. As you
put the antenna at a lower height, the low angle lobes get attenuated
more than the high angle lobe gets attenuated. So when you don't want
to be heard by the enemy at great distances, you put your antenna very
low, then crank up the power until you can just barely maintain
communications with the close friendly troops. Actually cranking down
the power is almost as much help as lowering the antenna. If you go
from 25 ft to 3 ft, the gain at 10 degrees elevation decreases by 12.4
dB while the gain at 90 degrees elevation decreases by 10.4 dB (over
average ground).
Maybe if you don't want to be heard by those unfriendly DX stations,
this would be a good idea.
Jerry, K4SAV
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