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.

Bill,

NVIS antennas are in fact desirable in some applications, where you wish to
communicate only with stations nearby, say out to about 250 miles. I can see
them useful in emergency communcations scenarious to get good HF coverage of a
local disaster area, or for use on a section traffic net, or other similar
"local" communications needs. In a "contest" situation like Field Day they'd be
tactically useful to have in your bag if your station was located in a high
population density area with a lot of nearby stations like the east coast
corridor, so that you'd somewhat avoid having a skip-zone, especially if the low
bands (40 and 80) "go long" at night.

73, de Hans, K0HB

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

Bill Horne writes:

The goals with NVIS are twofold:
- Avoiding nulls in local area coverage.
- Eliminating distant noise and QRM. If you're, after local coverage,
you also want that.

Local is a relative term. If the reflective layer is 200 km up, ranges
up to 480 km have a takeoff angle of 40 degrees or higher. A dipole a
quarter wave up above average ground is less than 2 dB down at that
elevation. At very low heights, the dipole is 3 dB down at 45
degrees. (It also radiates just a small fraction of the power you feed
it.)

As to why this would be the best design, it won't be for winning the
contest! Your brother may have another goal, like simply wanting to
test the design.

By the way, only part of the power going down is lost. The rest is
reflected. How much depends on height and ground characteristics.

73
LA4RT Jon

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.)

"Bill Horne" wrote in message
t...
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

------------

It's just a W.A.G., but most of the old publications reflected the culture
of the time and the culture in those days was oriented to working DX, not
locals. Today, I enjoy talking out to several hundred miles much more than
speaking to someone a continent or two away who is restricted (many times)
as to what they can or cannot say by their government. Then throw in foreign
accents, my aging hearing, etc., and one understands why speaking with my
fellow Americans is a much more pleasant experience - most of the time. I
mean no offense to foreign amateur radio ops. I enjoy speaking with them
too, when in a relaxed, non competitive atmosphere.

Ed, NM2K

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

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