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

Dear Wim PA3DJS:

How I wish that I could read Dutch. However, in looking over all of
your work, it continually occurred to me that much of the base of English
comes from Northern Nederland.

I looked at the first table on page 20 of your work. The information so
intrigued me that I ran almost the same information through NEC4, which does
well with antennas close to the ground. I assumed: a center frequency of
3.6 MHz, a horizontal half wave dipole slightly adjusted to be resonant at
3.6 MHz at each height, the use of Cu wire 2 mm in diameter, and earth with
a sigma of 0.01 and a relative dielectric constant of 15. This is what I
found without too obsessive an amount of tweaking:

The data is in this order:
height of the dipole in meters
the real part of the dipole's impedance in ohms
(the phase angle of the impedance was kept under 2 degrees)
the approximate SWR=2 BW in a 50 ohm system in kHz
the total apparent loss of the system in dB
(this is found by integrating gain in all directions)
the peak antenna gain in dBi

4.2 38.1 125 6.25 2.0
5.8 37.7 120 4.31 4.1
8.3 42.1 130 2.59 5.8
10.4 49.0 140 1.81 6.5
15 66.5 145 1.04 6.9
20.8 85.7 125 0.7 6.5

Most of my results are close to your results. It is interesting to know if
I used the same assumptions. Please let me know.

I encourage you to consider translating your work, or at least parts of it,
into English.

73, Mac N8TT
--
J. Mc Laughlin; Michigan U.S.A.
Home:
"Wimpie" wrote in message


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

On 11 abr, 06:14, "J. Mc Laughlin" wrote:
Dear Wim PA3DJS:

How I wish that I could read Dutch. However, in looking over all of
your work, it continually occurred to me that much of the base of English
comes from Northern Nederland.

I looked at the first table on page 20 of your work. The information so
intrigued me that I ran almost the same information through NEC4, which does
well with antennas close to the ground. I assumed: a center frequency of
3.6 MHz, a horizontal half wave dipole slightly adjusted to be resonant at
3.6 MHz at each height, the use of Cu wire 2 mm in diameter, and earth with
a sigma of 0.01 and a relative dielectric constant of 15. This is what I
found without too obsessive an amount of tweaking:

The data is in this order:
height of the dipole in meters
the real part of the dipole's impedance in ohms
(the phase angle of the impedance was kept under 2 degrees)
the approximate SWR=2 BW in a 50 ohm system in kHz
the total apparent loss of the system in dB
(this is found by integrating gain in all directions)
the peak antenna gain in dBi

4.2 38.1 125 6.25 2.0
5.8 37.7 120 4.31 4.1
8.3 42.1 130 2.59 5.8
10.4 49.0 140 1.81 6.5
15 66.5 145 1.04 6.9
20.8 85.7 125 0.7 6.5

Most of my results are close to your results. It is interesting to know if
I used the same assumptions. Please let me know.

I encourage you to consider translating your work, or at least parts of it,
into English.

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

Dear Mac,

Thank you for sharing your results with me. You are fully right, the
results are almost the same. The difference with my assumptions is
wire diameter only. I used 10mm. I expected more difference in useful
bandwidth, because of the factor 5 in wire diameter.

The simulations were carried out on IE3D with a lossy dielectric as
ground. The first table on page 17, shows the results for a perfect
ground.

I wasn't satisfied with the bandwidth, so the actual antenna consists
of 2 wires (1m separation). This gave me sufficient bandwidth to use
the antenna without a tuner.

The results between the brackets are with 3 floating reflector wires
under the antenna (about 1 m above ground). This gives an evident
increase in gain. The gain increases somewhat by raising the 3 wires
to about 1.5..2m above ground.

At this moment there are no plans to translate the document into
English language. As you have seen, the text in the images is in
English language, so you never know what happens during a vacation.....
The birdth of the Dutch version was in Spain.

Best Regards,

Wim
PA3DJS.

Dear Wim:

Thank you for the additional information. The information fills in the
rest of the story.

Before I shut down the dedicated computer used for the calculations, I
looked at the last entry again. The 20.8 m high dipole in a 50 ohm system
seemed to have significantly smaller BW than what you reported. I changed
the impedance to 85.7 ohms and then found (using the same data) that the
SWR=2 BW is about 250 kHz. With a 1.3:1 turns ratio transformer, this is
also the expected BW that would be seen in a 50 ohm system.

In short, my reported BW numbers may need qualification. It is
interesting to see how two different simulation techniques reach similar
conclusions. I too expect that increasing the wire to 10 mm has little
effect.

Vacations can be very creative times. Thank you for your work. 73,
Mac N8TT

--
J. Mc Laughlin; Michigan U.S.A.
Home:
"Wimpie" wrote in message
oups.com...
On 11 abr, 06:14, "J. Mc Laughlin" wrote:
Dear Wim PA3DJS:

How I wish that I could read Dutch. However, in looking over all of
your work, it continually occurred to me that much of the base of
English
comes from Northern Nederland.

I looked at the first table on page 20 of your work. The
information so
intrigued me that I ran almost the same information through NEC4, which
does
well with antennas close to the ground. I assumed: a center frequency
of
3.6 MHz, a horizontal half wave dipole slightly adjusted to be resonant
at
3.6 MHz at each height, the use of Cu wire 2 mm in diameter, and earth
with
a sigma of 0.01 and a relative dielectric constant of 15. This is what
I
found without too obsessive an amount of tweaking:

The data is in this order:
height of the dipole in meters
the real part of the dipole's impedance in ohms
(the phase angle of the impedance was kept under 2 degrees)
the approximate SWR=2 BW in a 50 ohm system in kHz
the total apparent loss of the system in dB
(this is found by integrating gain in all directions)
the peak antenna gain in dBi

4.2 38.1 125 6.25 2.0
5.8 37.7 120 4.31 4.1
8.3 42.1 130 2.59 5.8
10.4 49.0 140 1.81 6.5
15 66.5 145 1.04 6.9
20.8 85.7 125 0.7 6.5

Most of my results are close to your results. It is interesting to know
if
I used the same assumptions. Please let me know.

I encourage you to consider translating your work, or at least parts of
it,
into English.

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

Dear Mac,

Thank you for sharing your results with me. You are fully right, the
results are almost the same. The difference with my assumptions is
wire diameter only. I used 10mm. I expected more difference in useful
bandwidth, because of the factor 5 in wire diameter.

The simulations were carried out on IE3D with a lossy dielectric as
ground. The first table on page 17, shows the results for a perfect
ground.

I wasn't satisfied with the bandwidth, so the actual antenna consists
of 2 wires (1m separation). This gave me sufficient bandwidth to use
the antenna without a tuner.

The results between the brackets are with 3 floating reflector wires
under the antenna (about 1 m above ground). This gives an evident
increase in gain. The gain increases somewhat by raising the 3 wires
to about 1.5..2m above ground.

At this moment there are no plans to translate the document into
English language. As you have seen, the text in the images is in
English language, so you never know what happens during a vacation.....
The birdth of the Dutch version was in Spain.

Best Regards,

Wim
PA3DJS.