On Tue, 30 Dec 2008 10:35:41 -0500, John Passaneau
wrote:

I don't think that a NVIS antenna is a "ground wave" antenna. It works
by sending the signal straight up and bouncing it off the ionosphere
back down covering the area around the antenna.

But you should do better than what your doing if you are using on both
ends a NVIS antenna. It strikes me that maybe the antennas are not at
the right hight to function correctly. Also if only one station is using
a NVIS antenna and the other is using a vertical or dipole that is high
off the ground that could be the problem. Those antennas have a reduced
response to signals coming in from high angles. It is a well known effect.

We're both using horizontal antennas... his is a dipole up about 25
feet, and mine is an inverted vee that's up about 40 feet in the
center and 10 feet at the ends.

I know that a vertically polarized antenna would work groundwave
better, but we need to be able to maintain reasonably reliable
communications pretty much everywhere within about a 200-mile circle.
We can use 160, 80, and 40 meters to do it, and theoretically we can
use any amount of power up to the legal limit but in reality, as a
practical matter we are limited to 100-200 watts.

There are some in our group who really, REALLY think that the vertical
is the way to go, and I need some debating points. So, if I had a
decent ground-mounted vertical, should I expect to get the ground wave
distances that G4FGQ indicates in his calculation program?

Is there any area / radius that would be covered by a ground-mounted
vertical that wouldn't be covered by an NVIS horizontal (assuming that
we're operating below the critical frequency)? I have always assumed
that by the theory behind NVIS, the answer would be no... NVIS should
cover everything out to a radius well beyond groundwave distance.
But, I suppose we could install NVIS antennas AND verticals and switch
between them to see which one works best at any given time. Does seem
to be a wasted effort but WDIK?

While I'm at it, how do I estimate antenna efficiency? What is the
average range of efficiencies normally seen with a quarter-wave
ground-mounted vertical, with an adequate ground radial system, in
average soil?