Thanks for reading this. I've just been through a very confusing antenna
trial, and I'd really appreciate some info from hams who know more about
them then I.

My brother, W3TDH, called me up yesterday and asked me to help test some
80 and 40 meter antennas that his club is preparing for field day. He
told me that the club is going to serve their target coverage area by
using two dipoles, spaced 1/2 wave apart and fed in phase, so as to
maximize NVIS (Near-Vertical Incidence Skywave)radiation.

I'm not arguing with the design: antennas spaced 1/2 wave apart and fed
in phase always have maximum radiation at right angles to a line
bisecting both antennas. Ergo, two dipoles, horizontally mounted, etc.,
will indeed produce their maximum radiation straight up and down.

My question is: why would that be the best design? I thought that NVIS
radiation was a byproduct of having a radiator less than 1/2 wave above
the ground, and that it wasn't to be sought after, but rather avoided.
Leaving aside the losses due to 1/2 the power pointing straight down,
why would an antenna on 80 or 40 meters be most effective by radiating
almost straight up? Wouldn't the coverage be improved by phasing the
antennas so as to maximize radiation toward the horizon?

73,

W1AC

--
Bill Horne, W1AC

(Remove QRM from my address for direct replies.)

In article ,
Bill Horne wrote:
Wouldn't the coverage be improved by phasing the
antennas so as to maximize radiation toward the horizon?

Depends on the coverage you want. You want far away? Aim for the horizon.
You want 100-200 miles? Aim up.

On Mon, 16 Jun 2008 23:10:31 EDT, (Mark Kramer) wrote:

In article ,
Bill Horne wrote:
Wouldn't the coverage be improved by phasing the
antennas so as to maximize radiation toward the horizon?

Depends on the coverage you want. You want far away? Aim for the horizon.
You want 100-200 miles? Aim up.

NVIS is great for in-state communications.

However, the object of Field Day is to contact as many stations as possible.
Using a NVIS antenna will severely limit the number of stations which can be
contacted.
73 de Dick, AC7EL

Using a NVIS antenna will severely limit the number of stations which can be
contacted.
73 de Dick, AC7EL


Well, maybe and then again, maybe not.

During SS weekends I temporarily rig a very low 75M dipole (25 feet up) to snag
the "close in" mults like WI, IA, MB, etc. It's not NVIS per se, but the idea
is to get a high take-off angle.

Out here in flyover country it's simply a mult-catcher, but in a dense
population area (like the east coast or Ohio valley), having an NVIS choice on
the antenna switch can be a great advantage.

73, de Hans, K0HB

KØHB wrote:
Using a NVIS antenna will severely limit the number of stations which
can be
contacted.
73 de Dick, AC7EL


Well, maybe and then again, maybe not.

During SS weekends I temporarily rig a very low 75M dipole (25 feet up)
to snag
the "close in" mults like WI, IA, MB, etc. It's not NVIS per se, but t
he idea
is to get a high take-off angle.

Out here in flyover country it's simply a mult-catcher, but in a dense
population area (like the east coast or Ohio valley), having an NVIS ch
oice on
the antenna switch can be a great advantage.

73, de Hans, K0HB




Congratulations, You spotted the strategy involved on the first pass.
We are in Maryland about twenty miles north of the District of Columbia.
This is a very contact rich environment. The antenna in question is
one of many we plan to deploy.
--
Tom Horne, W3TDH

"This alternating current stuff is just a fad. It is much too dangerous
for general use." Thomas Alva Edison

Dick Grady AC7EL wrote:
On Mon, 16 Jun 2008 23:10:31 EDT, (Mark Kramer) wrote:

In article ,
Bill Horne wrote:
Wouldn't the coverage be improved by phasing the
antennas so as to maximize radiation toward the horizon?
Depends on the coverage you want. You want far away? Aim for the horizon.
You want 100-200 miles? Aim up.

NVIS is great for in-state communications.

However, the object of Field Day is to contact as many stations as possible.
Using a NVIS antenna will severely limit the number of stations which can be
contacted.


An NVIS antenna is usually referring to a low dipole. There are a lot of
misconceptions about the antenna. They do work on DX (just not as well)
and they make a pretty fair Field day antenna.

The reasons are that they radiate fairly equally in all angles, as
compared to a similar antenna at a height that would allow it to have a
lower radiation pattern. This might sound odd, but if you model an NVIS
antenna, then compare it to a higher one you'll see that is the case.

I think that we some times get tricked by terms such as "Take off Angle"
or similar terms. That dipole is radiating in all directions. Some just
not as much as others. That can get us thinking that the signal comes
off the antenna as a "blob" that is heading out at some ideal or
non-ideal angle.

People have earned DXCC using NVIS antennas.


To complicate matters, Propagation effects are not static. I performed
experiments a few years ago, using a dipole which was NVIS on 80/75
meters, and a Vertical antenna (Butternut HF6V) to answer the perennial
question " Which is better, a horizontal or a vertical antenna?" In
receive mode, I used a decade attenuator box. To transmit, that has to
be removed, lest I get a smokey crunchy attenuator.


Which is better? The answer is a resounding yes! And not always in the
way we would think. The propagation effects made the dipole perform
better sometimes when I expected the vertical to, and vice versa.

What's more the conditions can change in the middle of a QSO. I had many
cases of fading, when the Op would mention how the band was changing.
Switch from vertical to dipole, or vice versa, and it brought the signal
back up. Note I'm not saying that this will compensate for the band
closing up.

So that NVIS antenna might not be so bad for Field day as we may think.
It won't have as much oomph at lower takeoff angles as the low takeoff
angle antenna will, but it will have output at those angles none the less.

- 73 de Mike N3LI -

Dick Grady AC7EL wrote:

However, the object of Field Day is to contact as many
stations as possible.

I thought the purpose of Field Day was to serve as a training
ground for emergency preparedness. And a time to experiment
with new techniques and technology in an "emergency" setting
without it being a "real" emergency.

Considering the relative ease of setting up an NVIS antenna
system, and the number of other stations on the air, Field
Day makes a perfect time to see how it performs and to map
it's effectiveness for short range HF communications.

Maybe I'm going blind in my old age, but I just don't see
contesting under 47CFR97.1

Jeff-1.0
wa6fwi

Jeffrey D Angus wrote:
Dick Grady AC7EL wrote:

However, the object of Field Day is to contact as many
stations as possible.

I thought the purpose of Field Day was to serve as a training
ground for emergency preparedness.

It certainly does that. It can be a test of how quickly a group can
gets stations up and running. It can be a test of which groups are
better organized and it can certainly be (and is) a test of how
efficient operators can be in copying a message quickly and accurately.
They keep and publish scores, do they not? That's a contest as well as
an exercise.

And a time to experiment
with new techniques and technology in an "emergency" setting
without it being a "real" emergency.

I'm going to be bold in stating that I believe Field Day is a terrible
time to be fiddling with new techniques/technology. It is a simple
matter to do that well in advance of Field Day. You certainly wouldn't
want to experiment with new techniques/technology in an actual emergency.

Considering the relative ease of setting up an NVIS antenna
system, and the number of other stations on the air, Field
Day makes a perfect time to see how it performs and to map
it's effectiveness for short range HF communications.

I could have done that last Tuesday. The bands are loaded with stations
almost all of the time. The results of installing an NVIS antenna are
well known. They work better for working stations within a few hundred
miles of your location. Install a low dipole or install a dipole at
medium height above ground with a reflector under it and voila!

Maybe I'm going blind in my old age, but I just don't see
contesting under 47CFR97.1

You're right. Neither will you see anything about discussing
non-functional body parts on 75m, participating in a large roundtable
discussion or DXing. There are numerous things not addressed. That
does not mean that they are prohibited.

Dave K8MN

Put your dipole up between and two (or three, if it is an inverted vee)
connect the coax and start screaming. This idea of two phased antennas
isn't going to make any difference. If it does, please come back and let us
know but I will bet you wont see one iota of difference in your FD
performance.
CQ Field Day, CQ Field Day. One alpha Norhtern New Jersey (or whatever).

Jeffrey D Angus wrote:
Dick Grady AC7EL wrote:

However, the object of Field Day is to contact as many
stations as possible.

I thought the purpose of Field Day was to serve as a training
ground for emergency preparedness. And a time to experiment
with new techniques and technology in an "emergency" setting
without it being a "real" emergency.

I think you are both correct. While the purpose is emergency
preparedness, the format seems rather contest like. Keeping score,
different classes, posting the scores in QST. It's just a way to inject
some fun in the whole process. Get folks on the air, and nothing like a
little competition to get a person to improve their skills.


Considering the relative ease of setting up an NVIS antenna
system, and the number of other stations on the air, Field
Day makes a perfect time to see how it performs and to map
it's effectiveness for short range HF communications.

Let's look at Field day. We're pretty much not DX'ing to begin with.
Excepting Alaska and Hawaii, the distances aren't that vast.

So a Low takeoff angle antenna isn't even the best choice, IMO. My
experiments show the low angle antenna to have a very strong local
signal, then for a few hundred miles out it is not very good, then it
starts improving, and for very long distances it tends to be better than
the NVIS type.

That was done on 75 meters, and keep in mind that it was not over a long
period of time - around 2 weeks, but I'm fairly confident that I'm at
least in the ball park.

I worked Field Day once overnight on 75 meters using 100 watts, and an
off center fed dipole at 40 feet. I used that setup to work the west
coast, and Hawaii and Alaska. I ran and held frequencies the whole night
long.

I would recommend if a group was going to put up a "DX angle" antenna,
they would be well served to put up an NVIS antenna to complement it.


Maybe I'm going blind in my old age, but I just don't see
contesting under 47CFR97.1

Ever since cavemen started rolling boulders around in the woods, they
figured out how to turn it into a competition. It seems to be what we
humans do best. ;^)


- 73 de Mike N3LI -