how to model a loaded vertical ??
 

All,

I am trying to model a loaded vertical using EZNEC. It is fairly easy to start
with the vertical model and extend or shorten it. I also think I created a load
correctly. However I seem unable to connect the load to the vertical segment.
How can that connection be verified?

Also, what type of also load supports a second connection to a second wire to
simulate a mid load?

Thanks,
Dan

Hi Dan,

?? In the "Loads (R+JX)" window, you enter a desired position on a
particular wire number, and the value of the load, which presumably
would be a coil or a capacitor. It will tell you where the load
actually got put on the wire, as it must be in the middle of a segment.
You can adjust the number of segments to put the load closer to where
you want, if it didn't come out close enough the first time.

A good way to verify that the load is doing something is to change the
value of the load and look at what happens to the feedpoint impedance,
which you can see in the "Src Dat" (Source Data) window. For example,
if you start with a 10.3 meter vertical, 40mm diameter, 11 segments,
over perfect ground, with the feedpoint (source) at the middle of the
lowest segment, you should find that a coil of 0+j582 ohms gives you
very close to resonance: close to zero imaginary component in the
feedpoint impedance. Raising the coil reactance causes the feedpoint
to become inductive: positive imaginary component. And lowering it
causes the feedpoint to become capacitive.

Cheers,
Tom

To add to Tom's excellent explanation, let me add that you only need one
wire to model the vertical. I suggest beginning with the Vert1.ez
example antenna to get an idea of how it works. Look at the model with
the View Antenna display. Then specify a load in wire 1, 50% of the way
from end 1. The load will appear in the View Antenna display as a little
square. The Loads Window (where you specified the load) will show that
the actual position is 55% of the way from end 1, which is the closest
EZNEC could place it with the specified number of wire segments. If you
change the number of segments to an odd number, EZNEC will be able to
place the load exactly at the middle. You can specify other positions in
the same fashion, but it might require more segments to get it really
close to where you want.

Source and load positions are specified in exactly the same way.

Roy Lewallen, W7EL

dansawyeror wrote:
All,

I am trying to model a loaded vertical using EZNEC. It is fairly easy to
start with the vertical model and extend or shorten it. I also think I
created a load correctly. However I seem unable to connect the load to
the vertical segment. How can that connection be verified?

Also, what type of also load supports a second connection to a second
wire to simulate a mid load?

Thanks,
Dan

Let me make sure I understand. First I used the LCR configuration, that should
not make a difference? Are you saying a load is placed in the middle of a
segment? If I wanted to create a center loaded vertical one way would be to
create a 1 wire antenna and place the load in the middle? Is that correct?

Thanks,
Dan

K7ITM wrote:
Hi Dan,

?? In the "Loads (R+JX)" window, you enter a desired position on a
particular wire number, and the value of the load, which presumably
would be a coil or a capacitor. It will tell you where the load
actually got put on the wire, as it must be in the middle of a segment.
You can adjust the number of segments to put the load closer to where
you want, if it didn't come out close enough the first time.

A good way to verify that the load is doing something is to change the
value of the load and look at what happens to the feedpoint impedance,
which you can see in the "Src Dat" (Source Data) window. For example,
if you start with a 10.3 meter vertical, 40mm diameter, 11 segments,
over perfect ground, with the feedpoint (source) at the middle of the
lowest segment, you should find that a coil of 0+j582 ohms gives you
very close to resonance: close to zero imaginary component in the
feedpoint impedance. Raising the coil reactance causes the feedpoint
to become inductive: positive imaginary component. And lowering it
causes the feedpoint to become capacitive.

Cheers,
Tom

Roy and Tom,

Thank you. I actually tried that as one experiment. The loads window showed the
connection as you described. When the antenna is configured close to what I
expect there is a resonance near the intended frequency. So that is progress.
Now I am just trying to tweak the parameters.

On the SWR scan is there a way to print or plot the step outputs? I am trying to
determine the resonance point.

Thanks,
Dan

Roy Lewallen wrote:
To add to Tom's excellent explanation, let me add that you only need one
wire to model the vertical. I suggest beginning with the Vert1.ez
example antenna to get an idea of how it works. Look at the model with
the View Antenna display. Then specify a load in wire 1, 50% of the way
from end 1. The load will appear in the View Antenna display as a little
square. The Loads Window (where you specified the load) will show that
the actual position is 55% of the way from end 1, which is the closest
EZNEC could place it with the specified number of wire segments. If you
change the number of segments to an odd number, EZNEC will be able to
place the load exactly at the middle. You can specify other positions in
the same fashion, but it might require more segments to get it really
close to where you want.

Source and load positions are specified in exactly the same way.

Roy Lewallen, W7EL

dansawyeror wrote:

All,

I am trying to model a loaded vertical using EZNEC. It is fairly easy
to start with the vertical model and extend or shorten it. I also
think I created a load correctly. However I seem unable to connect the
load to the vertical segment. How can that connection be verified?

Also, what type of also load supports a second connection to a second
wire to simulate a mid load?

Thanks,
Dan

dansawyeror wrote:
Thank you. I actually tried that as one experiment. The loads window
showed the connection as you described.

There's one point of caution. The lumped load function will
not accurately predict the phase shift through a real-world
coil. Many antenna designs use phasing coils. The lumped load
feature of EZNEC will not accurately model those antennas.
If phasing coils are used, the EZNEC helix option should
be used which does seem to model the phase shift through the
coil.
--
73, Cecil http://www.qsl.net/w5dxp


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All,

Thank you for your help to get the basic model in place. It seems to be working
except that ground resistance does not appear to be modeled. This may be an
oversimplification my part in how the source is defined or connected. It is a
simple source connected to the first, lower, segment. The end of that segment is
also connected to a radial system of 8 quite short radials. I would expect to
see the feedpoint impedance reflect the ground system. However, it seems to
reflect the vertical element only. That value is very close to a theoretical
loaded element over a perfect ground. This ground is a medium ground so it
should have some visible effect.

Is it obvious what am I doing wrong?

Thanks,
Dan

Cecil Moore wrote:
dansawyeror wrote:

Thank you. I actually tried that as one experiment. The loads window
showed the connection as you described.


There's one point of caution. The lumped load function will
not accurately predict the phase shift through a real-world
coil. Many antenna designs use phasing coils. The lumped load
feature of EZNEC will not accurately model those antennas.
If phasing coils are used, the EZNEC helix option should
be used which does seem to model the phase shift through the
coil.

dansawyeror wrote:
All,

Thank you for your help to get the basic model in place. It seems to be
working except that ground resistance does not appear to be modeled.
This may be an oversimplification my part in how the source is defined
or connected. It is a simple source connected to the first, lower,
segment. The end of that segment is also connected to a radial system of
8 quite short radials. I would expect to see the feedpoint impedance
reflect the ground system. However, it seems to reflect the vertical
element only. That value is very close to a theoretical loaded element
over a perfect ground. This ground is a medium ground so it should have
some visible effect.

Is it obvious what am I doing wrong?

It sounds like you're using a MININEC-type ground, which isn't
appropriate for this application. Be sure to read the manual sections
involving ground modeling.

Roy Lewallen, W7EL

Roy,

Thanks, I did and I have tried all the combinations.

I would suspect that 'free space' would show a mis match for 'short radials'. It
does not. Simulations of free space dipoles seem accurate. Am I missing something?

I also tried to simulate several values of real earth. None of them produced
high input impedance values. What ground values represent suburban clay?

Thanks again,
Dan

Roy Lewallen wrote:
dansawyeror wrote:

All,

Thank you for your help to get the basic model in place. It seems to
be working except that ground resistance does not appear to be
modeled. This may be an oversimplification my part in how the source
is defined or connected. It is a simple source connected to the first,
lower, segment. The end of that segment is also connected to a radial
system of 8 quite short radials. I would expect to see the feedpoint
impedance reflect the ground system. However, it seems to reflect the
vertical element only. That value is very close to a theoretical
loaded element over a perfect ground. This ground is a medium ground
so it should have some visible effect.

Is it obvious what am I doing wrong?


It sounds like you're using a MININEC-type ground, which isn't
appropriate for this application. Be sure to read the manual sections
involving ground modeling.

Roy Lewallen, W7EL

dansawyeror wrote:
Roy,

Thanks, I did and I have tried all the combinations.

I would suspect that 'free space' would show a mis match for 'short
radials'. It does not. Simulations of free space dipoles seem accurate.
Am I missing something?

You'll need to send me the model description (.EZ file) so I can see
what you've got. Attach it to an email to me.

I also tried to simulate several values of real earth. None of them
produced high input impedance values. What ground values represent
suburban clay?

We have clay here, and by comparing a dipole and vertical at various
times of year, I've estimated that the soil is about equivalent to
EZNEC's "poor" quality ground in the summer when it's bone dry and "very
good" ground in the winter when soaking wet. It's an approximation at
best, though.

Roy Lewallen, W7EL

Roy, if he reads the impedance value he can determine resonance by the
absence of the reactive term.

Do you agree that the SWR scan is a poor method of determining resonance?

Roy Lewallen wrote:
dansawyeror wrote:


On the SWR scan is there a way to print or plot the step outputs? I am
trying to determine the resonance point.


The SWR scan automatically produces a plot. To print, click File, then
Print in the SWR display window.

Roy Lewallen, W7EL

Ham op wrote:
Roy, if he reads the impedance value he can determine resonance by the
absence of the reactive term.

Do you agree that the SWR scan is a poor method of determining resonance?

No, I usually use the SWR sweep to determine resonance. Stepping the
cursor and looking at the impedance at each step quickly identifies the
resonant frequency to within the closest step. Additional narrower scans
are done as necessary to get more precision. What method do you use
that's better?

Roy Lewallen, W7EL