Cecil Moore wrote in message ...
Mark Keith wrote:
Bravo. Pretty much sums it all up in a nutshell. No one has shown an
example of gross modeling error to date.

What is the matter with the one I posted last week with phase-
reversing coils as described by Kraus on page 824 of _Antennas_
for-all_Applications_, third edition? If you missed it, look at

http://www.qsl.net/w5dxp/current.htm

What example? I look at the page, and all I see is a current
distribution model for an antenna using stubs. ?? Where is the model
of the one using lumped inductance? I tried modeling a couple of
antennas myself using multiple 1/2 wave elements and inductance as
phasing coils just to test this out. But I used three elements instead
of four. I do not see any major change in current distribution when
compared to feeding all three with three separate sources. Minimum
current is at the ends of each section in both cases.
But I do see a bit less gain with the lumped inductance version, and
not quite as tight a pattern. This *might* be a point of error, but
I'd have to look more carefully into that. It's quite possible that
feeding with separate sources is superior in the real world, and it's
reflected in the model. But if this is an error in the model, I would
think that it only applies to arrays with lumped inductance used for
phasing coils. I very seriously doubt this error would apply to
modeling short coil loaded verticals. Or even large arrays that used
lumped inductance for loading, and not phasing purposes. Like I've
said, when I phase elements, I usually use separate sources to feed.
That way I can control the phase angle to whatever I want it to be.
MK

Mark Keith wrote:
Cecil Moore wrote in message ...

Mark Keith wrote:
Bravo. Pretty much sums it all up in a nutshell. No one has shown an
example of gross modeling error to date.

What is the matter with the one I posted last week with phase-
reversing coils as described by Kraus on page 824 of _Antennas_
for-all_Applications_, third edition? If you missed it, look at

http://www.qsl.net/w5dxp/current.htm

What example? I look at the page, and all I see is a current
distribution model for an antenna using stubs.

The example is four 1/2WL phased sections with "PHASE-REVERSING"
coils. The current is flowing into both ends of the coils at the
same time. Please show me an EZNEC model that allows current to
flow into both ends of the coil at the same time. You can model
that antenna using stubs but you cannot model that antenna using
lumped inductance. This is an example of an antenna that EZNEC
models with a "gross modeling error". With EZNEC, the current
never reverses phase as it must for that antenna to work properly.
--
73, Cecil http://www.qsl.net/w5dxp



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Mark Keith wrote:

Cecil Moore wrote:

Mark Keith wrote:
Bravo. Pretty much sums it all up in a nutshell. No one has shown an
example of gross modeling error to date.

What is the matter with the one I posted last week with phase-
reversing coils as described by Kraus on page 824 of _Antennas_
for-all_Applications_, third edition? If you missed it, look at

http://www.qsl.net/w5dxp/current.htm

What example? I look at the page, and all I see is a current
distribution model for an antenna using stubs. ?? Where is the model
of the one using lumped inductance?

Scroll on down. It's four 1/2WL array sections separated by a "phase
reversing coil".

I tried modeling a couple of
antennas myself using multiple 1/2 wave elements and inductance as
phasing coils just to test this out. But I used three elements instead
of four. I do not see any major change in current distribution when
compared to feeding all three with three separate sources.

Turn on the CI feature, display the current phase, and you will
see the difference. With EZNEC's lumped coils, the currents in the
outside sections are 180 degrees out of phase with what they need
to be.

But I do see a bit less gain with the lumped inductance version, and
not quite as tight a pattern. This *might* be a point of error, but
I'd have to look more carefully into that.

When you turn on the 'display current phase' feature, you will
see the difference. The currents in each 1/2WL sections are supposed
to be in phase. With EZNEC's lumped coils, they are out of phase no
matter what coil reactance is chosen. That's why there's more gain
using multiple sources. The errors are minimized using multiple sources.
--
73, Cecil, W5DXP

Cecil Moore wrote in message

Scroll on down. It's four 1/2WL array sections separated by a "phase
reversing coil".

I see a diagram, but no pattern plots or current distribution plots.
I'm curious to see how your's compared to mine. So far, after juggling
many different coil inputs, about the best gain I can get broadside is
3.54 dbi. "three 1/2 wave elements set end to end as a single wire
with the coils in the proper places on the wire ends" With separate
sources, I can get nearly 5 dbi. One note...End to end 1/2 waves is
not an optiumum spacing for max gain...

I tried modeling a couple of
antennas myself using multiple 1/2 wave elements and inductance as
phasing coils just to test this out. But I used three elements instead
of four. I do not see any major change in current distribution when
compared to feeding all three with three separate sources.

Turn on the CI feature, display the current phase, and you will
see the difference. With EZNEC's lumped coils, the currents in the
outside sections are 180 degrees out of phase with what they need
to be.

I wasn't using eznec...

But I do see a bit less gain with the lumped inductance version, and
not quite as tight a pattern. This *might* be a point of error, but
I'd have to look more carefully into that.

When you turn on the 'display current phase' feature, you will
see the difference.

Again, wasn't using eznec, but I looked in eznec for that feature and
couldn't find it. Is it in the demo version?
The currents in each 1/2WL sections are supposed
to be in phase. With EZNEC's lumped coils, they are out of phase no
matter what coil reactance is chosen.

Dunno, mine seem to be fairly well in phase. But not as good a
pattern or as much gain as using separate sources and elements. But
being as the overall current distribution seems little changed either
way, I'm not going to speculate on the reason at this point. With the
separate source antenna, I placed the elements as close together as
possible in order to be close to the loaded version in total length.

That's why there's more gain
using multiple sources. The errors are minimized using multiple sources.

This is quite possible. But I still don't think this would cause
noticable error when modeling short coil loaded antennas, or arrays
where the coil didn't act as a phasing device. Surely not a short
mobile whip. I doubt even with complex arrays if all the coils are for
loading purposes only. Sure, there may be some error, but not much
anyone can do about it, unless they design a new modeling engine. I
would think any error would be a db or less in most cases. Not really
worth worrying about. Or to me anyway...:/ When phasing elements, I
prefer separate sources anyway. And when designing mobile antennas, I
don't model them.
I calculate in terms of efficiency. The appx pattern and current
distribution is a given...
Seriously speaking...I think you can take Reg's vertload program, and
get as close as you would ever need to get as far as designing a
mobile whip, or short loaded vertical. It will tell you the best appx
place for the coil, the effects of wire dia, coil dia, coil length,
adding lower masts, adding upper stingers, ground loss, etc, etc,
about as well as you will ever need to use for designing such an
antenna. And if you feed it an accurate ground loss number, you know
your appx efficiency. After farting around with it a couple of years,
it seems to be pretty accurate comparing it to the real world. And
it's not even zipped up... MK

Mark Keith wrote:
I see a diagram, but no pattern plots or current distribution plots.
I'm curious to see how your's compared to mine. So far, after juggling
many different coil inputs, about the best gain I can get broadside is
3.54 dbi. "three 1/2 wave elements set end to end as a single wire
with the coils in the proper places on the wire ends" With separate
sources, I can get nearly 5 dbi. One note...End to end 1/2 waves is
not an optiumum spacing for max gain...

That difference in gain is because one cannot model a phase reversing
coil with a lumped inductive reactance. A workaround is needed and
yours is to use multiple sources. Mine is to use inductive stubs.

Again, wasn't using eznec, but I looked in eznec for that feature and
couldn't find it. Is it in the demo version?

I'm using DOS EZNEC 2.0 but I assume 3.0 has the same features. On
2.0 a Ctrl-I changes the current display from magnitude only to
magnitude and phase.

Dunno, mine seem to be fairly well in phase. But not as good a
pattern or as much gain as using separate sources and elements. But
being as the overall current distribution seems little changed either
way, I'm not going to speculate on the reason at this point.

That's the point. The current distribution is NOT the same either way.
I'm pretty sure you are displaying the current magnitude, not the
current phase. Given that the EZNEC lumped inductive reactance cannot
reverse the phase, the phases will always be 180 degrees apart in
adjacent 1/2WL sections and will not result in maximum gain or
minimum beamwidth.

This is quite possible. But I still don't think this would cause
noticable error when modeling short coil loaded antennas, or arrays
where the coil didn't act as a phasing device.

If I remember correctly, all you asked for was an example where EZNEC
didn't provide the correct results. Antennas using phasing coils is
that example. The lumped inductive reactance will not model a phasing
coil. It's not a big deal since there exist workarounds. I'm going to
try a parallel inductance and capacitance and see what happens to the
phase of the currents in EZNEC.
--
73, Cecil, W5DXP