Eric wrote:
Can anyone tell me the best way to estimate the groundwave coverage
I'm likely to get on 75 meters?

I tried R. J. Edwards (G4FGQ] GRNDWAV3.EXE, answered all the questions
about type of terrain, frequency, power output, and antenna
efficiency, and got an answer of around 50 miles for an S-5 signal at
the other end.

(With the noise level around here, S-5 is about the minimum to shoot
for around here...)

And yet, in the real world, I am lucky to be able to establish
reliable contact with another station that's a bit less than 10 miles
away.

We are both using horizontal NVIS antennas, which I guess aren't as
good for groundwave as vertical antennas. In any case I have no idea
what to put in to G4FGQ's program for "antenna efficiency". I tried
25 percent and got the 50 miles for S-5, then reduced antenna
efficiency to 10 percent and got 40 miles for S-5... still well below
what I am seeing in the real world.

I switched to a terrain type of "City blocks, roads, streets,
industrial areas, rivers" and got 32 miles for S-5, then switched to
"Mountainous regions, bare rock, vegetation in valleys" (we do have a
few molehills around here that people call mountains) and got 28
miles... still beyond what I'm really seeing.

So, how can I estimate the longest reliable groundwave distance on 75?

Thanks...

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.

73

John Passaneau W3JXP