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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Because you're seeing different currents at the two stub terminals, you
must be modeling it with wires, which should reflect reality quite well.
Look carefully at the currents along the stub and you'll find they're
not equal and opposite on the two conductors. Such a radiating stub *is*
very different from a coil. That shouldn't be surprising. I have a high
level of confidence that if you built the antenna just like you modeled
it, you would find the results to closely agree with the model.

Roy Lewallen, W7EL

Cecil Moore wrote:
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.

Roy Lewallen wrote:

Because you're seeing different currents at the two stub terminals, you
must be modeling it with wires, which should reflect reality quite well.
Look carefully at the currents along the stub and you'll find they're
not equal and opposite on the two conductors. Such a radiating stub *is*
very different from a coil.

Instead of a knee-jerk defense of your ideas, why don't you actually take
a look at the problem? Those stubs are vertical. EZNEC shows virtually zero
vertically polarized radiation. According to EZNEC, those stubs are radiating
a negligible amount, just like the lumped inductance. Why the 40% difference
in current between the two configurations? Is this a characteristic of NEC?
--
73, Cecil http://www.qsl.net/w5dxp



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On Mon, 03 Nov 2003 15:19:44 -0600, Cecil Moore
wrote:

Roy Lewallen wrote:

Because you're seeing different currents at the two stub terminals, you
must be modeling it with wires, which should reflect reality quite well.
Look carefully at the currents along the stub and you'll find they're
not equal and opposite on the two conductors. Such a radiating stub *is*
very different from a coil.

Instead of a knee-jerk defense of your ideas, why don't you actually take
a look at the problem?
Ah the quality of sneer review.

Those stubs are vertical. EZNEC shows virtually zero
vertically polarized radiation. According to EZNEC, those stubs are radiating
a negligible amount, just like the lumped inductance. Why the 40% difference
in current between the two configurations? Is this a characteristic of NEC?

Cecil, you have two stubs and they are driven antiphase (typical of a
doublet) and through symmetry would have equal antiphase currents when
compared to their opposites, but not necessarily equal currents within
their twin-pair of lines. The sum of ALL currents (and not just the
myopic view of one of two stubs) would suggest exactly what Roy has
offered.

This, of course, returns us to the question of what part of the
radiator radiates. Sadly, the convention of the current pulse (or
maxima, or other equivalent term) trips up discussion just in these
matters. ALL elements radiate, it is only in the far field where
their contributions negate, not literally within the structure.

73's
Richard Clark, KB7QHC

Are you referring to something you emailed me? If so, I haven't received
it. I'll be glad to look at it when I do.

Roy Lewallen, W7EL

Cecil Moore wrote:
Roy Lewallen wrote:

Because you're seeing different currents at the two stub terminals,
you must be modeling it with wires, which should reflect reality quite
well. Look carefully at the currents along the stub and you'll find
they're not equal and opposite on the two conductors. Such a radiating
stub *is* very different from a coil.


Instead of a knee-jerk defense of your ideas, why don't you actually take
a look at the problem? Those stubs are vertical. EZNEC shows virtually zero
vertically polarized radiation. According to EZNEC, those stubs are
radiating
a negligible amount, just like the lumped inductance. Why the 40%
difference
in current between the two configurations? Is this a characteristic of NEC?

On Mon, 03 Nov 2003 12:20:02 -0600, Cecil Moore
wrote:

|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.

If you use the ideal transmission line model, there is *zero*
difference between an ideal inductor and a transmission line stub.

You are comparing a mess of wire with a ideal lumped inductor. Apples
and oranges.

If you really want to model this stuff accurately take a few hundred
$K out of your next retirement check and buy a high frequency
structure simulator.

www.hfss.com

Wes Stewart wrote:
You are comparing a mess of wire with a ideal lumped inductor. Apples
and oranges.

Not my fault. Why does EZNEC treat these two inductances so differently?

If you really want to model this stuff accurately take a few hundred
$K out of your next retirement check and buy a high frequency
structure simulator.

No thanks, Wes. I can live without that. But let me ask you a question.
If there is a one degree delay through one foot of copper wire, how
can there be a zero delay through 20 feet of wire coiled into a one
foot coil? Given the pressure on the electrons, one would expect
20 times the delay through the coil as through the one foot wire.
--
73, Cecil http://www.qsl.net/w5dxp



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