All,

I am creating a 'scaled' model of a loaded vertical. The model is complete and
in place. However I immediately realized I had no idea of the 'theoretical'
response. How can I calculate the expected impedance response of a loaded
vertical? It a mid load construction,

the base in #10 solid copper 4 inchs,
the coil is .6 pitch, 1 inch diameter, 5 turns #12, measuring .72 uH,
and the top is 4 inch #12.

The antenna appears to resonate at about 112 MHz.

The instrumentation, a directional coupler measuring reflection is located at
the base, directly under the 'ground plane'. This allows measuring input and
reflected signal strength.

The question is: How can I plot the expected impedance from say 100 MHz to 130 MHz?

Thanks - Dan

dansawyeror wrote:
All,

I am creating a 'scaled' model of a loaded vertical. The model is
complete and in place. However I immediately realized I had no idea of
the 'theoretical' response. How can I calculate the expected impedance
response of a loaded vertical? It a mid load construction,

the base in #10 solid copper 4 inchs,
the coil is .6 pitch, 1 inch diameter, 5 turns #12, measuring .72 uH,
and the top is 4 inch #12.

The antenna appears to resonate at about 112 MHz.

The instrumentation, a directional coupler measuring reflection is
located at the base, directly under the 'ground plane'. This allows
measuring input and reflected signal strength.

The question is: How can I plot the expected impedance from say 100 MHz
to 130 MHz?

Thanks - Dan

The free EZNEC demo from http://eznec.com will give you the information
you need.

Roy Lewallen, W7EL

Roy,

Thanks, I was able to enter the data into eznec and run the simulation. The
output was 'confusing'. Eznec predicted 100 Ohm over various grounds. Other
models predict 5 Ohms for the antenna plus ground resistance with a total of 10
Ohms or so. Which is correct? How can the differences be reconciled?

Thanks again - Dan

Roy Lewallen wrote:
dansawyeror wrote:

All,

I am creating a 'scaled' model of a loaded vertical. The model is
complete and in place. However I immediately realized I had no idea of
the 'theoretical' response. How can I calculate the expected impedance
response of a loaded vertical? It a mid load construction,

the base in #10 solid copper 4 inchs,
the coil is .6 pitch, 1 inch diameter, 5 turns #12, measuring .72 uH,
and the top is 4 inch #12.

The antenna appears to resonate at about 112 MHz.

The instrumentation, a directional coupler measuring reflection is
located at the base, directly under the 'ground plane'. This allows
measuring input and reflected signal strength.

The question is: How can I plot the expected impedance from say 100
MHz to 130 MHz?

Thanks - Dan


The free EZNEC demo from http://eznec.com will give you the information
you need.

Roy Lewallen, W7EL

I apologize -- I read your earlier posting too hastily. Your antenna
requires the full version of EZNEC because of the inductor. While a
short inductor can be modeled as a lumped load, that doesn't work well
in cases like this where the current changes significantly from one end
of the coil to the other due to radiation. The coil has to be modeled as
a helix.

A full model of your antenna with the coil properly modeled shows a
feedpoint impedance of 5.03 - j212 ohms at 112 MHz over perfect ground.
Loss in a real ground system will of course increase the resistance. It
resonates at 171 MHz, where the feedpoint resistance is about 17 ohms.
My result is far from your finding of resonance around 112 MHz, so maybe
I didn't interpret the design correctly -- the antenna I modeled is a
total of 11 inches high, the center 3 inches of that being the 1 inch
diameter 5 turn coil.

With EZNEC and similar programs, you can only connect directly only to
perfect and MININEC-type ground; connection to Real, High-Accuracy
ground results in an unpredictable resistance that has no physical
meaning. More information can be found in the Modeling Ground chapter of
the EZNEC manual under Building The Model.

Roy Lewallen, W7EL

dansawyeror wrote:
Roy,

Thanks, I was able to enter the data into eznec and run the simulation.
The output was 'confusing'. Eznec predicted 100 Ohm over various
grounds. Other models predict 5 Ohms for the antenna plus ground
resistance with a total of 10 Ohms or so. Which is correct? How can the
differences be reconciled?

Thanks again - Dan

Roy Lewallen wrote:

dansawyeror wrote:

All,

I am creating a 'scaled' model of a loaded vertical. The model is
complete and in place. However I immediately realized I had no idea
of the 'theoretical' response. How can I calculate the expected
impedance response of a loaded vertical? It a mid load construction,

the base in #10 solid copper 4 inchs,
the coil is .6 pitch, 1 inch diameter, 5 turns #12, measuring .72 uH,
and the top is 4 inch #12.

The antenna appears to resonate at about 112 MHz.

The instrumentation, a directional coupler measuring reflection is
located at the base, directly under the 'ground plane'. This allows
measuring input and reflected signal strength.

The question is: How can I plot the expected impedance from say 100
MHz to 130 MHz?

Thanks - Dan



The free EZNEC demo from http://eznec.com will give you the
information you need.

Roy Lewallen, W7EL

Roy said -
While a
short inductor can be modeled as a lumped load, that doesn't work
well
in cases like this where the current changes significantly from one
end
of the coil to the other due to radiation. The coil has to be
modeled as
a helix.

=======================================
The non-uniform current does not arise from radiation but from the
distributed capacitance of the wire turns on the helix to its
surroundings.

Distributed inductance and capacitance of the helix behave as a short
transmission line and there is a corresponding phase shift between one
end and the other in addition to the non-uniform current distribution.

The effects can be estimated by approximate calculations with an
adequate degree of accuracy but it requires the right type of computer
program (which probably doesn't exist) to provide exact answers.
----
Reg.

Reg Edwards wrote:
Roy said -

While a
short inductor can be modeled as a lumped load, that doesn't work

well

in cases like this where the current changes significantly from one

end

of the coil to the other due to radiation. The coil has to be

modeled as

a helix.


=======================================
The non-uniform current does not arise from radiation but from the
distributed capacitance of the wire turns on the helix to its
surroundings.

You're correct. I apologize for the error.

Distributed inductance and capacitance of the helix behave as a short
transmission line and there is a corresponding phase shift between one
end and the other in addition to the non-uniform current distribution.

The effects can be estimated by approximate calculations with an
adequate degree of accuracy but it requires the right type of computer
program (which probably doesn't exist) to provide exact answers.

Moment method programs such as NEC-2, EZNEC, or even MININEC, do a very
good job.

Roy Lewallen, W7EL

Reg Edwards wrote:
The non-uniform current does not arise from radiation but from the
distributed capacitance of the wire turns on the helix to its
surroundings.

How about the fact that it is a *standing wave antenna* with a
forward traveling wave in one direction and a reverse traveling
wave in the other direction? The net current is the sum of
those two current components and their phasors are rotating
in opposite directions.
--
73, Cecil http://www.qsl.net/w5dxp

It is a free antenna simulation software NEC2 at http://www.nec2.org/
You must be able to model your antenna and much more with it.

Here is an exemple of what this software can do:
http://www.extremetech.com/article2/...1641191,00.asp

Sorry windose user, I believe in free software. If you want this
software, it is time to try linux. You must have at least 1 free
partition, and the installation process of most of the linux
distributions will install a double boot windows-linux for you.

Best,
Dominique

On Sat, 03 Dec 2005 22:43:16 -0800
Roy Lewallen wrote:

I apologize -- I read your earlier posting too hastily. Your antenna
requires the full version of EZNEC because of the inductor. While a
short inductor can be modeled as a lumped load, that doesn't work
well in cases like this where the current changes significantly from
one end of the coil to the other due to radiation. The coil has to be
modeled as a helix.

A full model of your antenna with the coil properly modeled shows a
feedpoint impedance of 5.03 - j212 ohms at 112 MHz over perfect
ground. Loss in a real ground system will of course increase the
resistance. It resonates at 171 MHz, where the feedpoint resistance
is about 17 ohms. My result is far from your finding of resonance
around 112 MHz, so maybe I didn't interpret the design correctly --
the antenna I modeled is a total of 11 inches high, the center 3
inches of that being the 1 inch diameter 5 turn coil.

With EZNEC and similar programs, you can only connect directly only
to perfect and MININEC-type ground; connection to Real, High-Accuracy
ground results in an unpredictable resistance that has no physical
meaning. More information can be found in the Modeling Ground chapter
of the EZNEC manual under Building The Model.

Roy Lewallen, W7EL

dansawyeror wrote:
Roy,

Thanks, I was able to enter the data into eznec and run the
simulation. The output was 'confusing'. Eznec predicted 100 Ohm
over various grounds. Other models predict 5 Ohms for the antenna
plus ground resistance with a total of 10 Ohms or so. Which is
correct? How can the differences be reconciled?

Thanks again - Dan

Roy Lewallen wrote:

dansawyeror wrote:

All,

I am creating a 'scaled' model of a loaded vertical. The model is
complete and in place. However I immediately realized I had no
idea of the 'theoretical' response. How can I calculate the
expected impedance response of a loaded vertical? It a mid load
construction,

the base in #10 solid copper 4 inchs,
the coil is .6 pitch, 1 inch diameter, 5 turns #12, measuring .72
uH, and the top is 4 inch #12.

The antenna appears to resonate at about 112 MHz.

The instrumentation, a directional coupler measuring reflection
is located at the base, directly under the 'ground plane'. This
allows measuring input and reflected signal strength.

The question is: How can I plot the expected impedance from say
100 MHz to 130 MHz?

Thanks - Dan



The free EZNEC demo from http://eznec.com will give you the
information you need.

Roy Lewallen, W7EL