Eznec modeling loading coils?
 

It's not practical, and often not possible (due to inability to model
proximity effect), to model a loading coil on a turn-by-turn basis.
Where a loading coil is physically long enough to have a current
gradient across it, I've always believed it's adequate to model it as a
wire whose outer diameter is the same as the outer diameter of the coil.
Usually, a single lumped inductive/resistive "load" placed in the center
of that wire is adequate, although if the coil is particularly long, you
might want to split the load into several equal pieces distributed along
the fat wire.

This approach assumes that the current gradient across a coil is the
same as for a wire of the same outside diameter as the coil. I don't,
however, have any measured data to confirm this. It would be a fairly
simple matter to put toroidal current transformers just above and below
a coil and compare their values to what EZNEC or other programs report.
If anyone has done this or is interested in doing it, I'd love to see
the result along with a detailed description of the methodology.

Roy Lewallen, W7EL

Yuri Blanarovich wrote:
Question to Roy,
can EZNEC model loading coils in loaded antennas, like mobile whips including
the physical properties of the coil?
I just had some exchange with W8JI on eHam.net
http://www.eham.net/articles/5998
where he is ignoring the fact that loading coil is part of the radiator and its
physical properties, where length to diameter ratio have impact on the current
distribution in the coil.
He is claiming that modeling programs confirm his statement that the current is
the same at the both ends of the loading coil (close in short coils) . I am
having hard time to see how the modeling program is capturing physical
properties of the coil besides inductance, Q etc.
W9UCW has done considerable amount of work on the subject and has measured that
the current diminishes in the loading coil away from the feedpoint.

Thanks!

Yuri, K3BU

On 12 Aug 2003 11:23:27 GMT, oSaddam (Yuri Blanarovich)
wrote:

Question to Roy,
can EZNEC model loading coils in loaded antennas, like mobile whips including
the physical properties of the coil?
I just had some exchange with W8JI on eHam.net
http://www.eham.net/articles/5998
where he is ignoring the fact that loading coil is part of the radiator and its
physical properties, where length to diameter ratio have impact on the current
distribution in the coil.
He is claiming that modeling programs confirm his statement that the current is
the same at the both ends of the loading coil (close in short coils) . I am
having hard time to see how the modeling program is capturing physical
properties of the coil besides inductance, Q etc.
W9UCW has done considerable amount of work on the subject and has measured that
the current diminishes in the loading coil away from the feedpoint.

Thanks!

Yuri, K3BU

Hi Yuri,

When I open the load dialog window, it presents R in some form or
another (the answer to your Q provision). Are you concerned the
program will treat the load as an infinitesimally small object? If it
does, you could decimate the coil inductance and distribute it across
10 segments whose span is equivalent to your anticipated physical
length. Perhaps a lot of trouble (and perhaps there's another, better
way), but it seems to be the price of your query.

73's
Richard Clark, KB7QHC

Why not seperate the inductance into
four separate loads over a length that equals one load?




oSaddam (Yuri Blanarovich) wrote in message ...
Question to Roy,
can EZNEC model loading coils in loaded antennas, like mobile whips including
the physical properties of the coil?
I just had some exchange with W8JI on eHam.net
http://www.eham.net/articles/5998
where he is ignoring the fact that loading coil is part of the radiator and its
physical properties, where length to diameter ratio have impact on the current
distribution in the coil.
He is claiming that modeling programs confirm his statement that the current is
the same at the both ends of the loading coil (close in short coils) . I am
having hard time to see how the modeling program is capturing physical
properties of the coil besides inductance, Q etc.
W9UCW has done considerable amount of work on the subject and has measured that
the current diminishes in the loading coil away from the feedpoint.

Thanks!

Yuri, K3BU

This approach assumes that the current gradient across a coil is the
same as for a wire of the same outside diameter as the coil. I don't,
however, have any measured data to confirm this. It would be a fairly
simple matter to put toroidal current transformers just above and below
a coil and compare their values to what EZNEC or other programs report.
If anyone has done this or is interested in doing it, I'd love to see
the result along with a detailed description of the methodology.

Roy Lewallen, W7EL


Barry, W9UCW has done extensive measurements using termocuples and other
methods minimizing disturbance to measured system. He found that current in
(esp. longer) coils is diminishing towards the other end.
If you read the thread and some more velleyball on Topband reflector last
month, there is more info.

Yuri, K3BU

That's what EZNEC would predict, modeled as I suggested.

Roy Lewallen, W7EL

Yuri Blanarovich wrote:
This approach assumes that the current gradient across a coil is the
same as for a wire of the same outside diameter as the coil. I don't,
however, have any measured data to confirm this. It would be a fairly
simple matter to put toroidal current transformers just above and below
a coil and compare their values to what EZNEC or other programs report.
If anyone has done this or is interested in doing it, I'd love to see
the result along with a detailed description of the methodology.

Roy Lewallen, W7EL



Barry, W9UCW has done extensive measurements using termocuples and other
methods minimizing disturbance to measured system. He found that current in
(esp. longer) coils is diminishing towards the other end.
If you read the thread and some more velleyball on Topband reflector last
month, there is more info.

Yuri, K3BU

Yuri Blanarovich wrote:
Barry, W9UCW has done extensive measurements using termocuples and other
methods minimizing disturbance to measured system. He found that current in
(esp. longer) coils is diminishing towards the other end.

If you add 1/4WL to the top of the antenna, the current in the coil
will *increase* towards the other end. In a standing wave antenna, the
total current is the sum of the forward current and the reflected
current. Whether the total current increases or decreases depends
on the phasing of the two currents at the point where the coil is
placed.
--
73, Cecil http://www.qsl.net/w5dxp



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When I open the load dialog window, it presents R in some form or
another (the answer to your Q provision). Are you concerned the
program will treat the load as an infinitesimally small object? If it
does, you could decimate the coil inductance and distribute it across
10 segments whose span is equivalent to your anticipated physical
length. Perhaps a lot of trouble (and perhaps there's another, better
way), but it seems to be the price of your query.

73's
Richard Clark, KB7QHC

That sounds reasonable.
Including physical properties of the coil (Length/diameter ratio, physical
size) in the calculations of the model could help to optimize the system, allow
more precise calculations and participation in the optimization routines.
If the load is treated as point, the current is considered the same at the
entrance and exit of the coil, which is not the case especially in longer
loading coils. Question is to what extent is program capable of capturing and
then massaging it and what is the correlation to real life situations.
I got some RF ammeters, it will be interesting to see wasaaap.

Yuri, K3BUm

If you add 1/4WL to the top of the antenna, the current in the coil
will *increase* towards the other end. In a standing wave antenna, the
total current is the sum of the forward current and the reflected
current. Whether the total current increases or decreases depends
on the phasing of the two currents at the point where the coil is
placed.
--
73, Cecil http://www.qsl.net/w5dxp

No standing or laying waves allowed. We are talking about the (mostly)
electrical quarter wave loaded radiators (aka mobile antennas) as used in
shootouts.

Yuri

Yuri Blanarovich wrote:
No standing or laying waves allowed. We are talking about the (mostly)
electrical quarter wave loaded radiators (aka mobile antennas) as used in
shootouts.

Those mobile antennas are standing-wave antennas. The forward current is
relatively constant and is reflected at the end of the whip. The reflected
current adds to the forward current to give the total current. A standing-
wave antenna is similar to a transmission line with standing waves except
that it is designed to radiate and the transmission line is not. The major
reason that the current drops through the loading coil is the same as for
a 1/4WL monopole without a loading coil. The total current is maximum at
the feedpoint and drops to zero at the end of the whip because the forward
and reflected currents add to those values. The forward current through the
coil is virtually the same value at each end of the coil. The reflected
current back through the coil is virtually the same value at each end of
the coil. The phasing between those two currents is what determines the
total current which is indeed different at each end of the coil.
--
73, Cecil http://www.qsl.net/w5dxp



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Yuri,
I think you would better understand what is happening
if you view the inductance in question as half of
a transformer, where the other half is the aether.
This allows you to bring in the formular NxI one side =
NxI the otherside. This way you can see that I is constant.
If it wasn't a constant then the lines of flux would have a
very distorted shape at one end of the inductance.
The fact that transformers have impedances that are not
totally resistive is because of other factors than Inductance.
If I am incorrect with this analogy I would apreciate any
corrections as I am sure Yuri would to
Art



oSaddam (Yuri Blanarovich) wrote in message ...
Question to Roy,
can EZNEC model loading coils in loaded antennas, like mobile whips including
the physical properties of the coil?
I just had some exchange with W8JI on eHam.net
http://www.eham.net/articles/5998
where he is ignoring the fact that loading coil is part of the radiator and its
physical properties, where length to diameter ratio have impact on the current
distribution in the coil.
He is claiming that modeling programs confirm his statement that the current is
the same at the both ends of the loading coil (close in short coils) . I am
having hard time to see how the modeling program is capturing physical
properties of the coil besides inductance, Q etc.
W9UCW has done considerable amount of work on the subject and has measured that
the current diminishes in the loading coil away from the feedpoint.

Thanks!

Yuri, K3BU