On Dec 30, 8:50*am, Eric wrote:


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

Thanks...

You need to use verticals if you want to use the ground wave on
the lower bands.
There basically is no ground wave when horizontal. Only a space
wave, which is not likely to go as far.
So in order to use the ground wave on the low bands, you need
to be using a vertical antenna to transmit.
How well that will do depends on the antenna, amount of noise, etc.
50 miles should be a good average, with some times better, maybe
others worse. IE: you will almost always do a lot better in the
winter,
than summer just due to lower noise.
Try it in the daytime. The noise is usually lower, and the ground
wave doesn't care what time it is.. :/ It should always be about
the same no matter what time of day. You lose a lot of ionospheric
clutter in the day, and can get a better idea what the ground
wave is doing. But there is still some skip in the day..
So if you end up working someone 200 miles away, it's
probably *not* via the ground wave. More likely the D layer
or whatever. But on the other hand, if you can work someone
50 miles away, and the signal is the same appx level every
day, day in, day out, it's probably the ground wave.
There could be minor season and moisture changes, but
overall it should stay fairly stable strength.
I know when I ran a 40m dipole at 36 ft, and also had a full size
ground plane with the base at the same height, the difference
in local coverage was drastic. Some times the band conditions
"NVIS" would stretch out, and I would lose the locals that were
across town if I were on the dipole.
But I could switch to the ground plane, and no problem.
In the daytime on 40m, I could often work people way out
west of town in mobiles, that the locals on dipoles wouldn't hear
due to the often long skip and lower signals due to being mobile
and vertical. Some of these were 70-90+ miles away.
I don't know how much of the total was ground wave vs space
wave, vs the ionosphere , but when I would switch to the dipole,
they would drop way down.
The ground plane did put out a pretty decent space wave
being it was elevated. Way better than the same antenna
ground mounted. And it was fairly efficient, which helps
no matter which path it takes.
The lower the frequency, the better the ground wave.
IE: tune AM radio in the daytime. Any station you pick
up that is out of town is coming to you via the ground wave.
With a good radio and antenna, you can receive quite a
distance. IE: from Houston, I can hear Dallas, San Antonio,
even farther, no problem at all. Almost pure ground wave,
and being such, it's easy to null out if needed vs sky wave
which usually comes from multiple paths.
But say take a 10m vertical at 50 ft in the air. The space
wave is the primary path in case of working local.
Not the ground wave like the low frequencies.

Since your operation seems to be NVIS rather than ground wave, I highly
recommend _Near Vertical Incidence Skywave Communication: Theory,
Techniques, and Validation_, by Fiedler and Farmer, published by
Worldradio Books. Don't know if it's still in print, but it shouldn't be
hard to find a copy on the web.

Roy Lewallen, W7EL

On Dec 30 2008, 8:50*am, Eric wrote:
Can anyone tell me the best way to estimate the groundwave coverage
I'm likely to get on 75 meters?
___________

NEC shows this for a theoretically perfect 1/4-wave vertical monopole
system:

Frequency = 3.9 MHz
Conductivity = 5 mS/m
Applied Power = 100 watts

Groundwave Field at 10 miles = 31.7 µV/m
Groundwave Field at 20 miles = 14 µV/m

RF

Richard Fry wrote:
On Dec 30 2008, 8:50 am, Eric wrote:
Can anyone tell me the best way to estimate the groundwave coverage
I'm likely to get on 75 meters?
___________

NEC shows this for a theoretically perfect 1/4-wave vertical monopole
system:

Frequency = 3.9 MHz
Conductivity = 5 mS/m
Applied Power = 100 watts

Groundwave Field at 10 miles = 31.7 µV/m
Groundwave Field at 20 miles = 14 µV/m

RF

Wouldn't a 200 degree vertical work better?

On Jan 10, 9:06*am, Dave wrote:
Richard Fry wrote:
On Dec 30 2008, 8:50 am, Eric wrote:
NEC shows this for a theoretically perfect 1/4-wave vertical
monopole system: etc

I need to correct the values I posted earlier. I mis-read the larger
table I had constructed, which was apparent when calculating the
values for a ~200 degree vertical.

Frequency = 3.9 MHz
Conductivity = 5 mS/m
Applied Power = 100 watts

Groundwave Field at 10 miles = 131 µV/m (Correction)
Groundwave Field at 20 miles = 31.7 µV/m (Correction)

Wouldn't a 200 degree vertical work better?

Yes, by a little over 2 dB (other things equal).

Groundwave Field at 10 miles = 167 µV/m
Groundwave Field at 20 miles = 41 µV/m

RF

"Richard Fry" wrote in message
...
On Jan 10, 9:06 am, Dave wrote:
Richard Fry wrote:
On Dec 30 2008, 8:50 am, Eric wrote:
NEC shows this for a theoretically perfect 1/4-wave vertical
monopole system: etc

I need to correct the values I posted earlier. I mis-read the larger
table I had constructed, which was apparent when calculating the
values for a ~200 degree vertical.

I don't work the low bands enough , but I would have thought someone would
have just gave some prctical experiance instead of all the NEC stuff. Not
many hams are going to put up a 60 some foot vertical and the required
ground system for 80 meters.

Could not someone say that with horizontal dipoles about 30 feet up (or
whatever is being used) you may get so many miles ground wave and during
the day so many miles skywave and so many more at night ?

NEC and other programs are fine for predicting the coverage, but it does not
take into account all the variatables that can be answered by the
experiance of actual operations.

On Sat, 10 Jan 2009 11:49:11 -0500, "Ralph Mowery"
wrote:

I don't work the low bands enough , but I would have thought someone would
have just gave some prctical experiance instead of all the NEC stuff. Not
many hams are going to put up a 60 some foot vertical and the required
ground system for 80 meters.

That's OK, not many hams read these threads either. They fully expect
they are doing their best, and certainly their experience proves it.

Could not someone say that with horizontal dipoles about 30 feet up (or
whatever is being used) you may get so many miles ground wave and during
the day so many miles skywave and so many more at night ?

Those same Hams that don't read these threads, and don't build 60+
foot radiators with elaborate ground systems don't realize that
horizontal antennas don't have ground waves anyway. So what coverage
they do get is perfect and they probably got an ARRL award for
horizontal antenna ground wave WAC already when they sent in the
box-top of their favorite breakfast.

NEC and other programs are fine for predicting the coverage, but it does not
take into account all the variatables that can be answered by the
experiance of actual operations.

NEC doesn't predict coverage, it is an antenna modeler, not a
propagation modeler (which would fit into your "other programs"). That
aside, these programs account for more variables than imagined by
those Hams that don't read these threads and don't build 60+ foot
radiators with elaborate ground systems. They have already had the
experience of actual operations and nothing is better than that.

So, why are we writing about those experienced, award winning Hams who
don't read these threads, expect ground wave from their horizontal
antenna, couldn't list more than one variatable and are satisfied with
sub-par to mediocre performance?

73's
Richard Clark, KB7QHC

Ralph Mowery wrote:

I don't work the low bands enough , but I would have thought someone would
have just gave some prctical experiance instead of all the NEC stuff. Not
many hams are going to put up a 60 some foot vertical and the required
ground system for 80 meters.

Could not someone say that with horizontal dipoles about 30 feet up (or
whatever is being used) you may get so many miles ground wave and during
the day so many miles skywave and so many more at night ?

The basic problem is that there isn't a simple answer to your question.
Any simple answer you'd get would be wrong much or most of the time.

It's safe to say that you won't get any ground wave communication at all
with a horizontal antenna, unless a vertical feedline is radiating. The
range with a vertical antenna depends on the noise level, which changes
day to day, season to season, and day to night, as well as ground
conductivity and power level. The ARRL Antenna Book gives a "typical"
ground wave range of around 60 miles at 3.5 MHz, but of course this
depends on the factors I've mentioned, among others -- it's one of those
"simple" answers.

Sky wave communication range depends on the condition of the ionosphere.
Sometimes you'll be able to communicate hundreds of miles, sometimes
zero. There is no range you can depend on. If you're interested in the
performance of a low dipole on 80 meters, I highly recommend _Near
Vertical Incidence Skywave Communication_ by Fiedler and Farmer.

NEC and other programs are fine for predicting the coverage, but it does not
take into account all the variatables that can be answered by the
experiance of actual operations.

Unfortunately, experience doesn't take into account all the variables
either. No one or group of people have experience with all possible
antennas, ground conditions, ionospheric conditions, and noise levels,
so one person's experience is likely to be different from another's.
The value of modeling is that it allows you to see which factors are
important and in what way, so you can get a better idea of what
performance you might get under your particular set of circumstances --
rather than someone else's.

Roy Lewallen, W7EL

Save wrote:
"Wouldn`t a 200 degree vertical wotk better?"

Field strength versus tower height rises with height up to about 225
degrees. See Fig. 2.1 on page 80 of E.A. laport`s "Radio Antenna
Engineering".

Best regards, Richard Harrison, KB5WZI