Mark Keith wrote:
As far as the reverse currents Cecil mentions, I'd have to ponder that
a while.

While you are pondering, here is a quote from _Antenna_Theory_, by Balanis.

"Standing wave antennas, such as the dipole, can be analyzed as traveling
wave antennas with waves propagating in opposite directions (forward and
backward) and represented by traveling wave currents 'If' and 'Ib' in
Figure 10.1(a)."

Standing wave antennas necessarily have standing waves caused by forward
waves and reflected waves. Analyze any coil subjected to forward current
and reflected current and you will be forced to agree that the current
at one end of the coil is not the same as the current at the other end
of the coil. W8JI is thinking lumped circuits when he should be thinking
distributed networks. The phase shift through the coil changes the
phase relationship between the forward current and reflected current, so
of course, their superposed value will be different at each end of the coil.
--
73, Cecil http://www.qsl.net/w5dxp



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Cecil Moore wrote in message ...
Mark Keith wrote:
As far as the reverse currents Cecil mentions, I'd have to ponder that
a while.

While you are pondering, here is a quote from _Antenna_Theory_, by Balanis.

"Standing wave antennas, such as the dipole, can be analyzed as traveling
wave antennas with waves propagating in opposite directions (forward and
backward) and represented by traveling wave currents 'If' and 'Ib' in
Figure 10.1(a)."

Standing wave antennas necessarily have standing waves caused by forward
waves and reflected waves. Analyze any coil subjected to forward current
and reflected current and you will be forced to agree that the current
at one end of the coil is not the same as the current at the other end
of the coil. W8JI is thinking lumped circuits when he should be thinking
distributed networks. The phase shift through the coil changes the
phase relationship between the forward current and reflected current, so
of course, their superposed value will be different at each end of the coil.

How much though? What would be an average ratio difference you would
be likely to see on a 8 ft center loaded whip? Or lets go one
better...What would be a likely "worse case" scenario? Will this vary
from antenna to antenna? I would think so. I've never said there would
not be a difference. I actually expect a small difference. But I still
don't think it would be a large amount. Will this change in value be
enough to cause large errors in modeling these antennas? It's already
obvious to me that any info I may gleen from these tests will have no
impact on the position of my loading coils, being I already use them
at the optimum heights. Or as close as physically possible anyway. So
any info gleaned from these tests would only be useful from a modeling
aspect. And I'm not in a position to really comment on that too much.
I don't design modeling engines. Is it your opinion that the modeling
we now see with these antennas and coils is quite flawed? It's obvious
Yuri seems to think so. Myself, I really don't know at this point.
I've never worried about it too much. I don't model shorter than 1/4
wave verticals. MK

Mike,
the differences in current are in order of 40 - 60%, that is significant.
The lower the band, the shorter the antenna, the bigger the effect, the more
important where the coil is. It will vary from antenna to antenna, depending on
the coil "shortening" factor. If the coil is closer to the feedpoint, the
current difference is lees, but efficiency suffers most. As you move coil up
the radiator, turns increase, current difference increases and effciency goes
up. If you replace (part of) coil with top loading, current differences
decrease (0 difference at 0 deg. long coil) and your efficiency goes up.
Efficiency or radiated power of loaded antenna is roughly proportional to the
area under the corresponding current curve of the remaining (straight)
radiator. Coil "eats" part of the radiator and its current carrying (radiating)
capabilities, this is why the current will be significantly different at the
ends of the coil. I hope this illustrates the situation?

As Cecil showed, modeling is not accounting for the effect and now that Roy is
on, we hope to sort things out and come up with ways to best implement the
phenomena in modeling programs. Right now, it appears that the best way to
approximate the effect is to use loading stubs of the same inductance as
intended coil.

Barry and Cecil agreed to cooperate on the article describing in detail (and in
civil manner :-) this subject and we hope that Roy will join us adding the
modeling aspect to it.

Yuri, K3BU/m

Mark Keith wrote:
How much though? What would be an average ratio difference you would
be likely to see on a 8 ft center loaded whip?

A lot on 75m. Not much on 12m.

Or lets go one
better...What would be a likely "worse case" scenario?

The worse case I can think of is a short center-loaded whip
on 160m. :-) The coil is almost all of the necessary 1/4WL.

Will this vary from antenna to antenna? I would think so.

Of course. It is all capable of being calculated.

Is it your opinion that the modeling
we now see with these antennas and coils is quite flawed?

The antenna current reported by EZNEC is inaccurate because of
simplified assumptions. EZNEC assumes that the current doesn't
change through the single point inductive load. Therefore, EZNEC
cannot be used to prove that the current doesn't change.
--
73, Cecil http://www.qsl.net/w5dxp



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The "simplified assumptions" made by EZNEC (and NEC in general) are the
same ones you'll find in any circuit analysis or electromagnetics text.
EZNEC includes a model of a lumped inductor (or "load"), which is
accurately represented. It also includes an accurate model of a straight
conductor which has physical length. If you could build an antenna from
straight conductors and lumped inductors, the result would be very close
to EZNEC's predictions.

EZNEC does not have a model of a coil which has physical length. Neither
the straight wire model nor the lumped inductor model can or should be
expected to behave exactly like a coil which has physical length. As
I've mentioned before, a useful approximation can be made by inserting
one or more lumped inductor models into a model wire. I don't have any
measurements to assess the accuracy of that approximation, however.

Roy Lewallen, W7EL

Cecil Moore wrote:

The antenna current reported by EZNEC is inaccurate because of
simplified assumptions. EZNEC assumes that the current doesn't
change through the single point inductive load. Therefore, EZNEC
cannot be used to prove that the current doesn't change.

Roy Lewallen wrote:
If you could build an antenna from
straight conductors and lumped inductors, the result would be very close
to EZNEC's predictions.

Hard to prove since lumped inductors are impossible in reality. Why
does EZNEC show so much difference between lumped inductors and stub
inductors? The difference in coils Vs stubs in reality is virtually
nill.
--
73, Cecil http://www.qsl.net/w5dxp



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On Mon, 03 Nov 2003 09:26:05 -0600, Cecil Moore
wrote:

|Roy Lewallen wrote:
| If you could build an antenna from
| straight conductors and lumped inductors, the result would be very close
| to EZNEC's predictions.
|
|Hard to prove since lumped inductors are impossible in reality. Why
|does EZNEC show so much difference between lumped inductors and stub
|inductors?

I see no such difference in my model.

|The difference in coils Vs stubs in reality is virtually
|nill.

Wes Stewart wrote:
On Mon, 03 Nov 2003 09:26:05 -0600, Cecil Moore
wrote:

|Roy Lewallen wrote:
| If you could build an antenna from
| straight conductors and lumped inductors, the result would be very close
| to EZNEC's predictions.
|
|Hard to prove since lumped inductors are impossible in reality. Why
|does EZNEC show so much difference between lumped inductors and stub
|inductors?

I see no such difference in my model.

There shouldn't be a lot of difference. I have modeled two short dipoles,
one loaded with a lumped inductive reactance and one modeled with the
same reactance using an inductive stub. EZNEC reports the following:

Inductance lumped j335 10'stub

current in segment just before the coil .8374 amp .8384 amp

current in segment just after the coil .7971 amp .5642 amp

The relative difference just before the coil is quite small, 0.12%.

The relative difference just after the coil is quite large, 41.28%.

There just cannot be that amount of difference between a coil and a
stub.
--
73, Cecil http://www.qsl.net/w5dxp



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A radiating stub does act differently than a lumped inductor, in both
modeling and reality. EZNEC should reflect this difference accurately.
If you're aware of a situation where you think it doesn't, please email
me the models illustrating the difficulty.

If you model a stub using a transmission line model, it should behave
exactly the same as a lossless lumped inductor at a given frequency.
However, it's an accurate model of reality only if the real stub has
exactly equal and opposite currents on the two conductors. That is, it's
an entirely non-radiating stub.

Roy Lewallen, W7EL

Cecil Moore wrote:
Roy Lewallen wrote:

If you could build an antenna from straight conductors and lumped
inductors, the result would be very close to EZNEC's predictions.


Hard to prove since lumped inductors are impossible in reality. Why
does EZNEC show so much difference between lumped inductors and stub
inductors? The difference in coils Vs stubs in reality is virtually
nill.

Roy Lewallen wrote:

A radiating stub does act differently than a lumped inductor, in both
modeling and reality. EZNEC should reflect this difference accurately.
If you're aware of a situation where you think it doesn't, please email
me the models illustrating the difficulty.

I have already done that but I just sent them to you again.

If you model a stub using a transmission line model, it should behave
exactly the same as a lossless lumped inductor at a given frequency.
However, it's an accurate model of reality only if the real stub has
exactly equal and opposite currents on the two conductors. That is, it's
an entirely non-radiating stub.

The difference in current between the two configurations that I sent to
you means the vertical stubs are radiating better than the horizontal
antenna which is unlikely since EZNEC doesn't show any vertical radiation.
--
73, Cecil http://www.qsl.net/w5dxp



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