NVIS Dipoles Directional?
 

My experience seems to be that the performance of NVIS dipoles (at or
under around 30 feet high for 80 meters) doesn't noticeably change with
orientation. In other words ... and despite what modeling programs say
.... BVIS dipoles are pretty much omnidirectional.

I haven't scientifically tested that but it does seem to me that since
most NVIS radiation goes approximately straight up (definition of NVIS),
it really shouldn't matter much how the antenna is oriented.

Have you all found that to be true, or false?

If I do want near-omnidirectional performance, in the real world (versus
in a software program's modeling), am I better off with an inverted vee
that's up 30 feet in the center, vs a dipole that's 30 feet up along its
full length?

How about an inverted vee that's up 50 feet in the center and 10 feet at
the ends, versus a dipole that's 30 feet up along its full length?

NVIS Dipoles Directional?
 

Rick wrote:
I haven't scientifically tested that but it does seem to me that since
most NVIS radiation goes approximately straight up (definition of NVIS),
it really shouldn't matter much how the antenna is oriented.

Have you all found that to be true, or false?

With a 130 ft dipole at 30 feet, the azimuthal radiation
pattern at 60 degrees is 3 dB down off the ends of the
dipole. I have never seen the minimum NVIS angle
defined anywhere.
--
73, Cecil http://www.w5dxp.com

NVIS Dipoles Directional?
 

Rick wrote in news:pan.2007.04.09.16.49.43.321645
@reply.in.gp:


My experience seems to be that the performance of NVIS dipoles (at or
under around 30 feet high for 80 meters) doesn't noticeably change with
orientation. In other words ... and despite what modeling programs say
... BVIS dipoles are pretty much omnidirectional.

I don't know that modelling programs are in conflict with the the
observation that dipoles low over real ground are nearly omni directional
at high elevations.


I haven't scientifically tested that but it does seem to me that since
most NVIS radiation goes approximately straight up (definition of
NVIS),
it really shouldn't matter much how the antenna is oriented.


I wrote an article on the design of an antenna for local contacts on 40m,
it is at http://www.vk1od.net/7MDipole/7MDipole.htm . It may be of
interest.


Have you all found that to be true, or false?

If I do want near-omnidirectional performance, in the real world
(versus
in a software program's modeling), am I better off with an inverted vee
that's up 30 feet in the center, vs a dipole that's 30 feet up along
its
full length?

I think you are on the wrong tram about the modelled performance.


How about an inverted vee that's up 50 feet in the center and 10 feet
at
the ends, versus a dipole that's 30 feet up along its full length?


Mounting a half wave dipole low over real ground modifies the pattern
from the free space pattern, the the whole pattern is deformed by the
presence of the ground and nulls previously off the end are not nearly as
pronounced.

When you rig the dipole as an inverted V, the pattern is further deformed
and the nulls are even less pronounced.

Ask yourself the question "is a dipole horizontal over ground (at any
height) omni-directional at the zenith?". If you found a modelling
program that, used properly, says otherwise, tell us about it.

NVIS isn't strictly about the zenith, but an antenna that is omni at the
zenit, and doesn't have deep nulls (dipole low over real ground), is
close to omni at high elevations. The exact variation will depend on
frequency, mounting height, ground parameters, and leg angles if an
inverted V. If you could rotate such a dipole, I would defy you to
reliably measure the out-of-omni above 50 deg elevation with an S meter.

Owen

NVIS Dipoles Directional?
 

Rick wrote in news:pan.2007.04.09.16.49.43.321645
@reply.in.gp:


My experience seems to be that the performance of NVIS dipoles (at or
under around 30 feet high for 80 meters) doesn't noticeably change with
orientation. In other words ... and despite what modeling programs say
... BVIS dipoles are pretty much omnidirectional.

NVIS propagation is pretty high angle stuff. If you look at the three
dimensional patterns for NVIS antennas you will see that they have a large
lobe at high angles and an almost circular omnidirectional pattern at those
angles. We're looking at 80 degrees and up mostly here, maybe 70 at the
low end....so that antennas are mainly designed to illuminate the patch of
ionosphere directly above the antenna.

--
Dave Oldridge+
ICQ 1800667

NVIS Dipoles Directional?
 

On Mon, 09 Apr 2007 23:36:56 +0000, Dave Oldridge wrote:

NVIS propagation is pretty high angle stuff. If you look at the three
dimensional patterns for NVIS antennas you will see that they have a large
lobe at high angles and an almost circular omnidirectional pattern at those
angles. We're looking at 80 degrees and up mostly here, maybe 70 at the
low end....so that antennas are mainly designed to illuminate the patch of
ionosphere directly above the antenna.

Right. That's my point. So, what I'm claiming ... and trying to get
someone who knows more about this stuff than I do (which is just about all
of you) to confirm or deny ... is that with an NVIS dipole, someone 100
miles away from me would not be able to perceive the difference if my
antenna was broadside to him or oriented in line with him. True, or false?

NVIS Dipoles Directional?
 

Rick wrote:
... is that with an NVIS dipole, someone 100
miles away from me would not be able to perceive the difference if my
antenna was broadside to him or oriented in line with him. True, or false?

The broadside radiation is mostly horizontally polarized
while the radiation off the ends is mostly vertically
polarized. I wonder if that would make a measurable
difference?
--
73, Cecil http://www.w5dxp.com

NVIS Dipoles Directional?
 

Rick wrote:
On Mon, 09 Apr 2007 23:36:56 +0000, Dave Oldridge wrote:

NVIS propagation is pretty high angle stuff. If you look at the three
dimensional patterns for NVIS antennas you will see that they have a large
lobe at high angles and an almost circular omnidirectional pattern at those
angles. We're looking at 80 degrees and up mostly here, maybe 70 at the
low end....so that antennas are mainly designed to illuminate the patch of
ionosphere directly above the antenna.

Right. That's my point. So, what I'm claiming ... and trying to get
someone who knows more about this stuff than I do (which is just about all
of you) to confirm or deny ... is that with an NVIS dipole, someone 100
miles away from me would not be able to perceive the difference if my
antenna was broadside to him or oriented in line with him. True, or false?

Most likely TRUE. Other conditions may affect the signal but for the
most part the signal goes almost straight up in all directions and
almost straight down in all directions. Your coverage can be up to 300
to 500 miles depending on conditions. The MUF for NVIS is dependent on
the position of the sun and drops drastically at and after sun set.

Dave WD9BDZ

NVIS Dipoles Directional?
 

Rick wrote in
:

Right. That's my point. So, what I'm claiming ... and trying to get
someone who knows more about this stuff than I do (which is just about
all of you) to confirm or deny ... is that with an NVIS dipole,
someone 100 miles away from me would not be able to perceive the
difference if my antenna was broadside to him or oriented in line with
him. True, or false?


I thought that was the meaning of this para that I wrote for you befo

NVIS isn't strictly about the zenith, but an antenna that is omni at the
zenith, and doesn't have deep nulls (dipole low over real ground), is
close to omni at high elevations. The exact variation will depend on
frequency, mounting height, ground parameters, and leg angles if an
inverted V. If you could rotate such a dipole, I would defy you to
reliably measure the out-of-omni above 50 deg elevation with an S meter.

NVIS Dipoles Directional?
 

Dear Rick (no call sign):

A 0.5 WL dipole almost on the ground will be inefficient. What the
dipole does radiate will, at high take-off-angles, be close to circular
(independent of azimuth) well past 100 miles.

If both efficiency and circularity are important, an excellent solution
is what the CCIR calls a TR2/2/.2. That is two, parallel one-wavelength
dipoles both in a horizontal plane 0.2 WL above the ground that are 0.5 WL
apart. With the dipoles fed in the middle with equal lengths of open line
that meet at a point midway between the dipoles, one can achieve a
reasonable input impedance.

In any case, I suggest you avail yourself of the benefit of raising
whatever you use to a height of something like 0.2 WL and less than 0.3 WL.

73, Mac N8TT
--
J. Mc Laughlin; Michigan U.S.A.
Home:

NVIS Dipoles Directional?
 

"Cecil Moore" wrote in message
t...
Rick wrote:
... is that with an NVIS dipole, someone 100
miles away from me would not be able to perceive the difference if my
antenna was broadside to him or oriented in line with him. True, or
false?

The broadside radiation is mostly horizontally polarized
while the radiation off the ends is mostly vertically
polarized. I wonder if that would make a measurable
difference?
--
73, Cecil http://www.w5dxp.com

I think polarization will play in the picture. Dealing with NVIS, we do not
get much of the polarization being rolled around as with signals coming from
refractions/reflections via ionosphere.
The signals at the receiving end would be coming from "above", but I would
suspect that with distinct polarization component that should be detectable
with receiving antenna if rotated.
So I would vote that there would be difference in NVIS signals most likely
characterized by sharp minimum at the opposite polarization receiving
antenna orientation.
Any RF signals have a distinct polarization at any point and time.
Add some constructive or destructive interference and one would get the
picture.

73 Yuri, K3BU.us