On Wed, 17 Jan 2007 11:26:18 -0500, "Yuri Blanarovich"
wrote:


The question is why is he modeling antenna in free space??
Planning to go there anytime soon???
Why would anyone model especially vertical antennas in free space (besides
trying to see how the donut looks)?
Verticals especially work with ground in forming the pattern and impedance.
I use and model my verticals on earth. JMO

73 Yuri, K3BU

On my "Things to do List" I plan to spend some time with EzNec and a
ground plane in free space.

What I hope to better understand is the effect of the the quality of
the ground plan in the are between a modeled "real ground" and a
modeled "perfect ground".

I don't know the free space model will be valid, but I hope to learn
from the exercise.

I could just ask another series of "dumb questions" here but I fear
some of those who are so generous will tire of my noise!

When I modeled the vertical with a perfect ground I was expecting half
a vertical dipole and that is not what I got!

John Ferrell W8CCW

"Owen Duffy" a écrit dans le message de news:


Where does the 36 ohms come from? Is it for a quarter wave monopole
mounted on a perfect ground plane?


yes


Is the radiation pattern of a quarter wave monopole mounted over 4
quarter wave radials the same as a quarter wave monopole mounted on a
perfect ground plane?


at first, it seems it is the same, but in fact there are some slight
differences, and it explains the difference in gain and radiation
resistance.

Though you might use the term GPA to refer to a quarter wave monopole
mounted over 4 quarter wave radials, it is not the same as a quarter wave
monopole mounted on a perfect ground plane.

right!
and it was the reason.

73 Owen and thank you

André
http://f5ad.free.fr/

"chuck" a écrit dans le message de news:
...

André, you might find this interesting:

http://www.cebik.com/gp/gp3.html
On Ground Planes


fine this page on ground planes Chuck

I thank you and everybody who tried to help me in understanding the
phenomenons with ground plane antennas.

with GPA = a vertical pole and four radials, it appears that:

-the vertical patern of a vertical dipole is not quite identical; it is
difficult to see on the graphs, but it is different, so the gains are
slightly different
-four radials are not a perfect plane (not a surprise!)
-the radiation resistance is low, 22 Ohms in free space, consequently the
losses with a real ground are greater than with 36 or 72 Ohms
-with my 40m GPA, the efficiency falls to 56% at the ground level (half the
power to warm the ground...);
to reach 90% the antenna must be ten meter above the ground, or use a
multitude of radials.

very pleased with this newsgroup, despite the difficulty, for me, with
english.

73

André F5AD
http://www.youtube.com/watch?v=Do11t0Btrpw

John Ferrell wrote:

On my "Things to do List" I plan to spend some time with EzNec and a
ground plane in free space.

What I hope to better understand is the effect of the the quality of
the ground plan in the are between a modeled "real ground" and a
modeled "perfect ground".

I don't know the free space model will be valid, but I hope to learn
from the exercise.

I could just ask another series of "dumb questions" here but I fear
some of those who are so generous will tire of my noise!

When I modeled the vertical with a perfect ground I was expecting half
a vertical dipole and that is not what I got!

John Ferrell W8CCW

If you model a vertical connected to perfect ground, you should get
almost exactly half the impedance of a dipole of twice the length in
free space, with the same radiation pattern except 3 dB greater in
amplitude and of course with the lower half missing. If you use an odd
number of segments and a single source for the dipole, there will be a
small difference because of the difference in source placement between
the dipole and vertical. This difference will become less as the number
of segments is increased in both models.

However, you can make the models virtually identical by using a "split
source". Here's an example you can even do with the demo program:

Open the EZNEC example file Vert1.ez or d_Vert1.ez. Click Src Dat to see
that the source Z is 36.65 + j2.971. (It's using MININEC type ground, so
the source Z is the same as for perfect ground.) Then change the Ground
Type to Free Space. Add a second wire with end coordinates 0, 0, 0 and
0, 0, -10.3 meters, 40 mm diameter, 10 segments, to be an exact mirror
image of the vertical. Then open the Sources Window and change the
source type to SV (split voltage) or SI (split current) and click Src
Dat. The reported Z is now 73.3 + j5.942, exactly twice the Z of the
vertical. To compare patterns, you'll need to use Perfect ground for the
vertical rather than MININEC type ground.

Roy Lewallen, W7EL

On Fri, 19 Jan 2007 14:41:46 -0800, Roy Lewallen
wrote:



If you model a vertical connected to perfect ground, you should get
almost exactly half the impedance of a dipole of twice the length in
free space, with the same radiation pattern except 3 dB greater in
amplitude and of course with the lower half missing. If you use an odd
number of segments and a single source for the dipole, there will be a
small difference because of the difference in source placement between
the dipole and vertical. This difference will become less as the number
of segments is increased in both models.

However, you can make the models virtually identical by using a "split
source". Here's an example you can even do with the demo program:

Open the EZNEC example file Vert1.ez or d_Vert1.ez. Click Src Dat to see
that the source Z is 36.65 + j2.971. (It's using MININEC type ground, so
the source Z is the same as for perfect ground.) Then change the Ground
Type to Free Space. Add a second wire with end coordinates 0, 0, 0 and
0, 0, -10.3 meters, 40 mm diameter, 10 segments, to be an exact mirror
image of the vertical. Then open the Sources Window and change the
source type to SV (split voltage) or SI (split current) and click Src
Dat. The reported Z is now 73.3 + j5.942, exactly twice the Z of the
vertical. To compare patterns, you'll need to use Perfect ground for the
vertical rather than MININEC type ground.

Roy Lewallen, W7EL

I will spend more time here. The last model I worked out with a
perfect ground appeared to produce a 0 degree take off angle.

Mornings are best for me to push my gray matter!
John Ferrell W8CCW

John Ferrell wrote:

I will spend more time here. The last model I worked out with a
perfect ground appeared to produce a 0 degree take off angle.
. . .

That is the correct result. The pattern is exactly the same as half the
pattern of a vertical dipole, which has a maximum at zero degree
elevation angle, except that the gain of the vertical is 3 dB greater
because the same power is concentrated in only one hemisphere.

Roy Lewallen, W7EL

On Fri, 19 Jan 2007 14:41:46 -0800, Roy Lewallen
wrote:



If you model a vertical connected to perfect ground, you should get
almost exactly half the impedance of a dipole of twice the length in
free space, with the same radiation pattern except 3 dB greater in
amplitude and of course with the lower half missing. If you use an odd
number of segments and a single source for the dipole, there will be a
small difference because of the difference in source placement between
the dipole and vertical. This difference will become less as the number
of segments is increased in both models.

However, you can make the models virtually identical by using a "split
source". Here's an example you can even do with the demo program:

Open the EZNEC example file Vert1.ez or d_Vert1.ez. Click Src Dat to see
that the source Z is 36.65 + j2.971. (It's using MININEC type ground, so
the source Z is the same as for perfect ground.) Then change the Ground
Type to Free Space. Add a second wire with end coordinates 0, 0, 0 and
0, 0, -10.3 meters, 40 mm diameter, 10 segments, to be an exact mirror
image of the vertical. Then open the Sources Window and change the
source type to SV (split voltage) or SI (split current) and click Src
Dat. The reported Z is now 73.3 + j5.942, exactly twice the Z of the
vertical. To compare patterns, you'll need to use Perfect ground for the
vertical rather than MININEC type ground.

Roy Lewallen, W7EL

Varying the parameters of the free space vertical, reading the EZNEC
help file and computing various scenarios with the G4FGQ program
RADIAL_3 has satisfied my curiosity for the moment.

I will next look at the physical problems of erecting a vertical for
use on 40/80/160.

Initially I will start with my 28 foot radiator over a radial field of
16 seventy foot radials fed with the SG-237 tuner. If I can work out
the matter of keeping that up with acceptable guys I can step up to a
48 foot radiator out of my existing aluminum supply. Based on that
experience, I will consider buying a little more aluminum to get to
the 66 foot height. That is the reason for the 70 foot radial length.

Most of the problems will be mechanical for a while.
John Ferrell W8CCW