Rick wrote in newsan.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

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?

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

"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

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

Rick wrote in
news
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.

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:

"J. Mc Laughlin" writes:

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.

Or, in the case of 80 m, between 16 and 24 m up (for the metric
challenged: between 50 and 75 ft). Excellent for those who can, but
most can't.

73 de LA4RT Jon

LA4RT Jon ?Q?K=C3=A5re?= Hellan wrote in
:

"J. Mc Laughlin" writes:

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.

Or, in the case of 80 m, between 16 and 24 m up (for the metric
challenged: between 50 and 75 ft). Excellent for those who can, but
most can't.

A horizontal quad loop at 50 feet is almost ideal for NVIS.


--
Dave Oldridge+
ICQ 1800667

On 10 abr, 06:34, 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?

Hello Rick,

At 100 miles distance, the TOA is almost vertical (maybe 75 degr), so
with respect to signal strength, it is practically impossible to
detect whether the antennas are broad side. Because of the distance,
ground wave propagation loss is far higher with respect to NVIS
propagation loss under these circumstances.

One would mention polarization of the waves going up and down. I
would not matter about this. These low frequencies are strongly
affected by faraday rotation. On the way up and down, the polarization
rotates several times and several wave fronts do exist.

While you transmit (nearly vertical) with linear polarization, the
down coming wave may have a strong circular component.

With respect to the radiation pattern, you are right, the differences
in pattern are minimal below 30 feet. However the overall efficiency
is strongly depended on antenna height and soil properties. I did some
simulation and practice. I made a short document of it (for JOTA
porpuse), however the document is in Dutch Language (http://
www.tetech.nl/divers/NVISantenneNL1.pdf). Maybe you can get some
useful info out of it. At low heights, much power is dissipated into
the ground (resulting in a useful bandwidth).

Best Regards,

Wim
PA3DJS