Here are a few reasons I've seen for using a lower NVIS antenna:
Yes, the antenna gain goes down so there is less signal. But there is
also less noise and interference, and in many cases the noise drops
faster when the antenna is lowered, such that the S/N improves.
Depends on the source of the noise, of course. So as a receiving
antenna for high elevation angle signals it becomes better.
Depending on ground and the water table in your area, the effective
conducting layer may be some distance down, and then the antenna
should be lower to be the same height above it.
With these rotten sunspot conditions, you can't play NVIS much higher
than 80 meters, if even that at night. Up a quarter wavelength on 80
or 160 is not practical for most of us.
Low works well for field expedient antennas. Even though they might
not be optimal, they are good enough for (non-)government work.
Some NVIS antennas add a low reflector on or a little above the ground
surface, like a yagi. As I understand it, the optimum spacing from a
reflector wire IS less than a quarter wavelength when that reflector
is inductive (cut ~5% longer) ... as opposed to a large or infinite
ground plane where 1/4 wavelength gives highest gain.
I've rarely (if ever) seen hiding your signal from the enemy as a
reason, nor simulation results. Most folks who recommend lower
antennas do so based on experience ... either that they found the
lower antenna worked better, or that it is good enough and the extra
height isn't necessary.
Andy
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